Structural and BiomechanicalProperties of the Exchange Tissue

of the Avian LungJOHN N MAINA

Department of Zoology University of Johannesburg Kingsway JohannesburgSouth Africa

ABSTRACTThe blood capillaries (BC) and the air capillaries (ACs) are the termi-

nal gas exchange units of the avian lung The minuscule structures areastonishingly strong It is only recently that the morphologies and the bio-mechanical properties of the BCs and the ACs were investigated Regard-ing size and shape the BCs and the ACs differ remarkably While theywere previously claimed to be tubular (cylindrical) in shape the ACs arerather rotund structures which interconnect across short narrow passage-ways Atypical of those in other tissues the BCs in the exchange tissue ofthe avian lung comprise of distinct segments which are about as long asthey are wide and which are coupled in three-dimensions The thin blood-gas barrier (BGB) which separates the ACs from the BCs is peculiarlystrong The causes of the strengths of the ACs and the BCs in general andthe BGB in particular are varied and controversial Here the recent mor-phological and physiological findings on the structure biomechanical prop-erties and the strengths of the respiratory units of the avian lung and theBGB have been critically examined Also in light of the new morphologi-cal findings of the ACs and the BCs the functional model which is cur-rently in use to assess the gas exchange efficiency of the avian lungshould be revised and the inappropriateness of the terms lsquoblood capillaryrsquoand lsquoair capillaryrsquo for the gas exchange units of the avian lung is pointedout Anat Rec 2981673ndash1688 2015 VC 2015 Wiley Periodicals Inc

Key words birds lung air capillaries blood capillariesblood-gas barrier strength

In the last decade many studies analyzed the relationshipbetween tissue composition microstructure and macro-physiology showing that the lung physiological behaviorreflects both the mechanical properties of tissue individ-ual components and its complex structural organization

Faffe and Zin (2009)

INTRODUCTION

Among the organs and organ-systems which havebeen longest and continuously scientifically studied theavian respiratory system (the lung-air sac system)stands out Recorded literature dates back to four-and-half centuries (Coitier 1573 cited in Campana 1875)The accounts have formed the basis of the currentunderstanding of the structure and function of the avianrespiratory system However many important aspects of

This article includes AR WOW Videos which can be viewedat httpbcovemerdey47b1 httpbcovemehp35pl8q httpbcovemey82m8sfh httpbcovemewejluuik

Additional Supporting Information may be found in the onlineversion of this article

Grant sponsor National Research Foundation (NRF) of SouthAfrica

Correspondence to JN Maina Department of Zoology Uni-versity of Johannesburg Auckland Park Campus Johannes-burg 2006 South Africa E-mail jmainaujacza

Received 4 November 2014 Revised 29 January 2015Accepted 20 February 2015

DOI 101002ar23162Published online 9 April 2015 in Wiley Online Library(wileyonlinelibrarycom)

THE ANATOMICAL RECORD 2981673ndash1688 (2015)

VVC 2015 WILEY PERIODICALS INC

its biology still remain unresolved (Maina 2005 20062011) Debates and controversies are indelibly written inthe accounts of research on the avian respiratory sys-tem Some of the notable examples are (a) the claimthat the path followed by the inspired air in the avianlung was controlled by anatomical valves (sphincters)(Dotterweich 1930 Vos 1934) was after long and futilesearch debunked after it was incontrovertibly deter-mined that such structures do not exist anywhere in theairways (Fedde 1980 Brackenburry 1987 Maina2005) (b) while the mechanism of ldquoinspiratory aerody-namic valvingrdquo where inspired air flows through theintrapulmonary primary bronchus (IPPB) to the caudalair sacs (CASs) completely by-passing the openings ofthe medioventral secondary bronchi is now reasonablywell-understood (Banzett et al 1987 1991 Kuethe1988 Wang et al 1988 1992 Maina et al 2009) prac-tically nothing is known on the process of ldquoexpiratoryaerodynamic valvingrdquo (Brown et al 1995) where expiredair is directed into the mediodorsal secondary bronchiinstead of exiting the way it comes in ie through theIPPB and (c) the intense debate which reigned in the1960s and 1970s over existence or nonexistence of acounter-current gas exchange system in the avian lung(Schmidt-Nielsen 1971) was only settled after an ele-gant experiment conducted by Scheid and Piiper (1972)which after reversing the direction of the air flow in thelung showed that the partial pressure of oxygen in arte-rial blood did not change

The remark by Donald Farner (1970) some four-and-half decades ago that ldquohistorically the avian respiratorysystem is highly ranked among the controversial organ-systemsrdquo is in many areas of the biology of the gasexchanger as valid today as it was back then This appliesto the forms and the biomechanical properties of the ter-minal respiratory units of avian lung the blood capillaries(BCs) and the air capillaries (ACs) These aspects haverecently attracted intense interest (West et al 20062007 2007b Maina 2007a 2007b Maina et al 2010Makanya et al 2011 Maina and Sikiru 2013 Maina andJimoh 2012) and enthusiastic debate (Maina 2007a2007b 2008 West et al 2007 2007b) This account dwellson the following (a) the current state-of-the-art under-standing of the morphologies and the three-dimensional(3D) arrangement of the BCs and the ACs (b) the possiblecauses of the remarkable strengths of the BCs the ACsand the blood-gas barrier (BGB) and (c) the inappropri-ateness of the terms ldquoair capillaryrdquo and ldquoblood capillaryrdquoin regard to the shape and the arrangement of the termi-nal gas exchange units of the avian lung

STRUCTURE OF THE BLOOD- AND AIRCAPILLARIES OF THE AVIAN LUNG

Blood Capillaries

The word ldquocapillaryrdquo was first used in the 14th Cen-tury (Merriam-Webster Dictionary wwwmerriam-webstercomdictionary) Etymologically it derives fromthe Latin word ldquocapillarisrdquo which means ldquoa hair of theheadrdquo (caput for head and pillus for hair) While the sim-ilarity between a BC and that of a hair is only cursoryas the hair is not hollow it nonetheless underscores thethinness of a BC In the Free Dictionary (httpwwwthe-freedictionarycom) a capillary is defined as ldquoa structure

which is long and slender and has a very small innerdiameterrdquo In the English Language the word capillaryhas been retained in related words like ldquocapillaceousrdquomeaning ldquohair- or thread-likerdquo ldquocapillamentrdquo meaning ldquoahair-like fibrerdquo or ldquoa filamentrdquo and ldquocapillitiumrdquo meaningldquofilamentous matterrdquo (The Shorter Oxford English Dic-tionary 6th Edition) In many histology text books BCsare depicted termed and described as ldquotubesrdquo ldquonarrowcanalsrdquo ldquotubules of uniform diameterrdquo and ldquothinnestvesselsrdquo (eg Trautmann and Fiebiger 1957 Banks1986 Dellmann and Brown 1987 Leeson et al 1988Fawcett 1998 Gartner et al 2011 Young et al 2013)

The BCs were first directly examined in animal tis-sues by Leonardo da Vinci (1452ndash1519) (see review byClayton and Philo 2012) and later described by the Ital-ian anatomist Marcello Malpighi (1628ndash1695) afterexamining the frog lung by a light microscope (Malpighi1661 see a recent review of this by West 2013)

Air Capillaries

According to Schulze (1908) visualization of a networkof tiny air spaces in the gas exchange tissue of the avianlung was first made by Rainey (1849) Picturing them to beldquotiny tubulesrdquo he called them ldquointercapillary air passagesrdquoEberth (1863) described the air spaces in the gas exchangetissue of the avian lung as ldquoterminal air canalsrdquo In thenow classical study of the lung of Apteryx and the duckHuxley (1882) called the air spaces ldquointercellular passagesof Raineyrdquo clearly in recognition of Raineyrsquos pioneeringwork Without attempting to structurally differentiate thetwo parts Huxley (1882) called the outermost (terminal)parts of the ldquointercapillary passages of Raineyrdquo theldquointercellular passagesrdquo Schulze (1908) supposed thatEberth (1863) was the first person to declare that theldquoterminal canalsrdquo ended as blind expansions

Even after the thorough work of Fischer (1905) whichshowed that the ACs anastomosed copiously controversycontinued Investigators like Krause (1922) and Cover(1953) contended that morphologically the air spaceswere ldquostraight tubulesrdquo From recent three-dimensional(3D) computer reconstruction of the ACs and the BCsfrom serial sections studies (Woodward and Maina2005 2008 Maina and Woodward 2009) it was shownthat it is impossible to determine the shapes and thearrangement of the ACs and the BCs by means of simpletechnologies like a hand-held magnifying lens and acompound microscope the only tools which were avail-able to the early investigators Furthermore examina-tion of gross- andor histological ie sectionalpreparations of the avian lung gas exchange tissue can-not explicate the form and the geometries of the ACsand the BCs Even at higher magnifications because oftheir small sizes and complex arrangement (see andcompare Figs 1ndash12) it is impossible to obtain a proper3D conception of the ACs and the BCs For example attransmission electron microscope level of magnificationin cross-section the ACs and the BCs appear as a com-plex network of tubes (Figs 13ndash15)

The term ldquoair capillaryrdquo was in all probability coinedafter insightful deduction that for gas exchange to occurin the exchange tissue of the avian lung BCs mustrelate with equally small air spaces which resembled theBCs both in size and shape The term ldquoalveolusrdquo

1674 MAINA

(ldquoalveolusrdquomdashLatin word for ldquolittle cavityrdquo) was mostprobably avoided because the much larger air spaceshad already been observed and described by Malpighi(1661) Because he easily and clearly observed theldquoalveolirdquo of the frog lung compared with those of thelungs of animals like sheep and other mammals Mal-pighi termed the ldquoinstructiverdquo frog the ldquomicroscope ofnaturerdquo We now know that the main reason why the airspaces in the frog lung were easier to visualize isbecause they are much larger compared with the alveoliof the mammalian lung (Tenney and Remmers 1963Hermida et al 1998 Kuehne and Junqueira 2000Hyde et al 2004 Knust et al 2009)

Unlike the terminal respiratory units of the mamma-lian lung the alveoli which have been counted in thelungs of some mammalian species by eg Pinkerton andJoad (2000) Ochs et al (2004) Hyde et al (2004) andKnust et al (2009) the ACs and the BCs have not been

counted in the exchange tissue of the lung of any birdThis is mainly because of the following reasons (a)unlike the alveoli the ACs are not discrete respiratoryunits like the alveoli but are rather rotund intercon-nected structures (Figs 5ndash10 11 12 Supporting Infor-mation Video 1) strictly an AC comprises of therounded part and the associated narrow passagewaysand (b) while the BCs comprise of interconnected seg-ments they are nevertheless contiguous tubes Regard-ing the gas exchange function of the avian lung thenumbers of ACs and BCs are not meaningful parameterssince they poorly indicate the available respiratory sur-face area the part of the wall which separates adjacentACs and where BCs adjoin do not participate in gasexchange Moreover the actual respiratory surface areain the avian lung can be directly and accurately deter-mined by stereological methods (Maina 1989 20052006 Maina et al 1989)

Fig 1ndash4 Fig 1 A scanning electron micrograph of a critical-pointdried preparation showing the exchange tissue of the lung of thedomestic fowl Gallus gallus variant domesticus and blood capillaries(BC) originating from an arteriole (At) The BCs entwine with the aircapillaries (AC) Fig 2 A confocal microscope image of an area similarto that of Fig 1 showing an arteriole (At) giving rise to blood capillaries(BC) which interlock with the air capillaries (AC) Fig 3 A scanning

electron micrograph of a latex cast of the blood capillaries (BC) of thelung of the domestic fowl Gallus gallus variant domesticus The boun-daries of some of the air capillaries are shown with dashed circlesFig 4 Three-dimensional computer reconstruction of the blood capil-laries (BC) of the lung of the muscovy duck Cairina moschata Theextents of the air capillaries are shown with dashed circles

THE AC AND THE BC OF THE AVIAN LUNG 1675

SHAPES SIZES AND ARRANGEMENT OF THEAIR- AND THE BLOOD CAPILLARIES

The BCs and the ACs constitute as much as 90 ofthe volume of the gas exchange tissue (Maina 1989

2005 Maina et al 1989) Dependent on body mass andthe species of bird the BCs are 3 to 10 mm wide(Duncker 1972 West et al 1977 Abdalla 1989 Wood-ward and Maina 2005 Maina and Woodward 2009)while the ACs are 3 to 20 mm in diameter (Bargmann

Fig 5ndash10 Fig 5ndash7 Scanning electron micrographs of a latex castpreparations of the lung of the domestic fowl Gallus gallus variantdomesticus showing the air capillaries (AC) which interconnect acrossnarrow passageways (stars) The air capillaries intertwine with theblood capillaries (arrows) Fig 8ndash10 Three-dimensional computer

reconstructions of the air capillaries (AC) of the exchange tissue of thelung of the domestic fowl Gallus gallus variant domesticus which areconnected by narrow passageways (stars) The spaces between theair capillaries (asterisks) are occupied by the blood capillaries

1676 MAINA

and Knoop 1961 MacDonald 1970 Akester 1970aWest et al 1977 Duncker and Geurountert 1989 Mainaand Nathaniel 2001 Woodward and Maina 2005 2008Maina and Woodward 2009) The smallest (narrowest)alveoli of a mammalian lung are 35 mm in diameter inan unnamed species of bat (Tenney and Remmers 1963)

Many investigators have remarked on the complexityof the shapes and the arrangement of the ACs and theBCs of the exchange tissue of the avian lung For exam-ple King (1966) noted that ldquothe exchange area is formedby a complex network of anastomosing tubules the ACswhich intimately interlock with BCsrdquo King and Molony(1971) described the ACs as ldquofine anastomosing tubuleswhich are intimately interlocked with BCsrdquo West et al(1977) stated that the exchange tissue of the avian lungcomprises of ldquoa dense apparently random 3D sponge ofBCs and ACsrdquo McLelland (1989) observed that the gasexchange tissue was formed by ldquoan anastomosing 3Dnetwork which is intimately interlaced with a BCnetworkrdquo Duncker (1989) noted that the ACs and theBCs form ldquoa sinusoidal 3D networkrdquo Klika et al (1997)described the exchange tissue of the avian lung as con-sisting of ldquoanastomosing ACs interlaced with numerousBC networkrdquo Scheuermann et al (1997) noted that theexchange tissue comprised of ldquoa complex network of fineanastomosing tubules that were intimately interlockedwith a network of BCsrdquo Bellairs and Osmond (1998)bluntly stated that the exchange tissue appeared like ldquoasponge in structurerdquo Nasu (2005) stated that theexchange tissue comprised of ldquovascular capillaries form-ing a 3D network as they intermingle with the ACsrdquoand Makanya et al (2007) stated that the exchange tis-

sue comprised of ldquoa dexteriously crafted BC networkthat interfaces with the equally complex AC labyrinthrdquo

The observations cited above are vague and lackdetails they were largely based on perceptions ratherthan on matter-of-fact empiric data Taking advantage ofthe recent increase in computing power and availabilityof robust soft ware the shapes and the spatial arrange-ment of the ACs and the BCs were recently determinedby 3D computer reconstruction of serial sections of lungsof several species of birds by Woodward and Maina(2005 2008) and Maina and Woodward (2009) It wasobserved that the BCs consist of conspicuous segmentswhich are about as long as they are wide and which areinterconnected in 3D (Figs 3 and 4) Their morphologi-cal property differs from that of the BCs of many organsand tissues (eg the skeletal muscle) which are charac-teristically longer than they are wide The ACs arerather rotund structures which are interconnected byshort narrow passageways (Figs 5ndash11) However whilemost of the ACs connect interestingly some ACs arecompletely isolated (Woodward and Maina 2005) (Fig12) The role of such ACs is unclear since they do notreceive inspired air Epithelial-epithelial cells connec-tions (E-ECCs) separate the ACs a BGB separates theACs from the BCs (Figs 13ndash15) and in some areasBCs lie side to side (Figs 13 14 and 16) In such casesthe BC endothelial cells lie on a common basementmembrane (Figs 16 and 17) Unlike in the mammalianlung where thick (supporting side) and thin (gasexchanging) sides of the interalveolar septum occur (Fig15 insert) in the avian lung the BGB is fairly uniformin thickness (Figs 16 insert) On closer examinationhowever sproradic attenuations exist in the BGB of theavian lung (Maina and King 1982) (Fig 16) It was sug-gested by Weibel and Knight (1964) that corrugation ofthe BGB in the lung enhances the diffusing capacity ofthe lung for oxygen Sporadic thinning and thickening ofthe BGB allows very thin parts which enhance gasexchange to form while the thicker parts ensure themechanical integrity of the barrier The BGB of theavian lung is more corrugated than those of other air-breathing vertebrates (Maina and King 1982)

Contrary to the observation made by West et al (1977)that the ACs and the BCs are mirror images recent find-ings show that morphologically regarding both shape andsize the respiratory units differ considerably (Woodwardand Maina 2005 2008 Maina and Woodward 2009)(Figs 18 and 19) In 3D space they entwine very closely(Figs 20 and 21 Supporting Information Video 2 Video 3and Video 4) The disposition was described by West et al(2006 2010) as a ldquohoneycomb-likerdquo arrangement (Fig 13)Without lucidly describing their form Baier (1896) andMacDonald (1970) remarked that the BCs are ldquomoreprofuserdquo than the ACs

For a long time it was assumed that the ACs werenarrow straight (nonbranching) and blind-endingtubules which run outwards from the parabronchiallumen and that the BCs are similar passageways whichrun inwards from the periphery of the parabronchusparallel to and in contact with the ACs The arrange-ment has been widely used to model gas exchange effi-ciency of the avian lung (Crank and Gallanger 1978Scheid 1979 Powell and Scheid 1989) Although biolog-ical models are mathematical simplifications of highlycomplex systems (Maina 2005) a good model must

Fig 11ndash12 Fig 11 Three-dimensional serial section reconstructionshowing different views of air capillaries (AC) which connect throughnarrow passageways (stars) in the exchange tissue of the muscovyduck Cairina moschata Fig 12 Three-dimensional serial sectionreconstruction showing different views of an isolated air capillary (AC)in the exchange tissue of the lung of the muscovy duck Cairinamoschata

THE AC AND THE BC OF THE AVIAN LUNG 1677

and Knoop 1961 MacDonald 1970 Akester 1970aWest et al 1977 Duncker and Geurountert 1989 Mainaand Nathaniel 2001 Woodward and Maina 2005 2008Maina and Woodward 2009) The smallest (narrowest)alveoli of a mammalian lung are 35 mm in diameter inan unnamed species of bat (Tenney and Remmers 1963)

Many investigators have remarked on the complexityof the shapes and the arrangement of the ACs and theBCs of the exchange tissue of the avian lung For exam-ple King (1966) noted that ldquothe exchange area is formedby a complex network of anastomosing tubules the ACswhich intimately interlock with BCsrdquo King and Molony(1971) described the ACs as ldquofine anastomosing tubuleswhich are intimately interlocked with BCsrdquo West et al(1977) stated that the exchange tissue of the avian lungcomprises of ldquoa dense apparently random 3D sponge ofBCs and ACsrdquo McLelland (1989) observed that the gasexchange tissue was formed by ldquoan anastomosing 3Dnetwork which is intimately interlaced with a BCnetworkrdquo Duncker (1989) noted that the ACs and theBCs form ldquoa sinusoidal 3D networkrdquo Klika et al (1997)described the exchange tissue of the avian lung as con-sisting of ldquoanastomosing ACs interlaced with numerousBC networkrdquo Scheuermann et al (1997) noted that theexchange tissue comprised of ldquoa complex network of fineanastomosing tubules that were intimately interlockedwith a network of BCsrdquo Bellairs and Osmond (1998)bluntly stated that the exchange tissue appeared like ldquoasponge in structurerdquo Nasu (2005) stated that theexchange tissue comprised of ldquovascular capillaries form-ing a 3D network as they intermingle with the ACsrdquoand Makanya et al (2007) stated that the exchange tis-

sue comprised of ldquoa dexteriously crafted BC networkthat interfaces with the equally complex AC labyrinthrdquo

The observations cited above are vague and lackdetails they were largely based on perceptions ratherthan on matter-of-fact empiric data Taking advantage ofthe recent increase in computing power and availabilityof robust soft ware the shapes and the spatial arrange-ment of the ACs and the BCs were recently determinedby 3D computer reconstruction of serial sections of lungsof several species of birds by Woodward and Maina(2005 2008) and Maina and Woodward (2009) It wasobserved that the BCs consist of conspicuous segmentswhich are about as long as they are wide and which areinterconnected in 3D (Figs 3 and 4) Their morphologi-cal property differs from that of the BCs of many organsand tissues (eg the skeletal muscle) which are charac-teristically longer than they are wide The ACs arerather rotund structures which are interconnected byshort narrow passageways (Figs 5ndash11) However whilemost of the ACs connect interestingly some ACs arecompletely isolated (Woodward and Maina 2005) (Fig12) The role of such ACs is unclear since they do notreceive inspired air Epithelial-epithelial cells connec-tions (E-ECCs) separate the ACs a BGB separates theACs from the BCs (Figs 13ndash15) and in some areasBCs lie side to side (Figs 13 14 and 16) In such casesthe BC endothelial cells lie on a common basementmembrane (Figs 16 and 17) Unlike in the mammalianlung where thick (supporting side) and thin (gasexchanging) sides of the interalveolar septum occur (Fig15 insert) in the avian lung the BGB is fairly uniformin thickness (Figs 16 insert) On closer examinationhowever sproradic attenuations exist in the BGB of theavian lung (Maina and King 1982) (Fig 16) It was sug-gested by Weibel and Knight (1964) that corrugation ofthe BGB in the lung enhances the diffusing capacity ofthe lung for oxygen Sporadic thinning and thickening ofthe BGB allows very thin parts which enhance gasexchange to form while the thicker parts ensure themechanical integrity of the barrier The BGB of theavian lung is more corrugated than those of other air-breathing vertebrates (Maina and King 1982)

Contrary to the observation made by West et al (1977)that the ACs and the BCs are mirror images recent find-ings show that morphologically regarding both shape andsize the respiratory units differ considerably (Woodwardand Maina 2005 2008 Maina and Woodward 2009)(Figs 18 and 19) In 3D space they entwine very closely(Figs 20 and 21 Supporting Information Video 2 Video 3and Video 4) The disposition was described by West et al(2006 2010) as a ldquohoneycomb-likerdquo arrangement (Fig 13)Without lucidly describing their form Baier (1896) andMacDonald (1970) remarked that the BCs are ldquomoreprofuserdquo than the ACs

For a long time it was assumed that the ACs werenarrow straight (nonbranching) and blind-endingtubules which run outwards from the parabronchiallumen and that the BCs are similar passageways whichrun inwards from the periphery of the parabronchusparallel to and in contact with the ACs The arrange-ment has been widely used to model gas exchange effi-ciency of the avian lung (Crank and Gallanger 1978Scheid 1979 Powell and Scheid 1989) Although biolog-ical models are mathematical simplifications of highlycomplex systems (Maina 2005) a good model must

Fig 11ndash12 Fig 11 Three-dimensional serial section reconstructionshowing different views of air capillaries (AC) which connect throughnarrow passageways (stars) in the exchange tissue of the muscovyduck Cairina moschata Fig 12 Three-dimensional serial sectionreconstruction showing different views of an isolated air capillary (AC)in the exchange tissue of the lung of the muscovy duck Cairinamoschata

THE AC AND THE BC OF THE AVIAN LUNG 1677

identify the parts andor properties which are mostimportant to understanding functional design Consider-ing the shape the arrangement and the configuration ofthe ACs and BCs the ldquotube-on-tuberdquo model is overlysimplistic and anatomically utterly incorrect It shouldbe revised to incorporate most if not all of the recentlydetermined morphological properties

For the avian lung the very small sizes of the ACsmay greatly contribute to the high gas exchange effi-ciency In the mammalian lung Gehr et al (1981) specu-lated that the partial pressure gradient of oxygen(DPo2) ie the driving force for oxygen from air to capil-lary blood across the BGB must increase with decreas-ing body mass The investigators supposed that the

diffusion distance for oxygen molecules in the alveolarair phase ie from the front of inspired air to the gasexchange surface is longer in the larger animals whichhave larger (wider) alveoli compared with small animalswith smaller alveoli By extension this would mean thatfor birds which have very narrow terminal respiratoryunits the ACs the driving pressure ie the drivingforce for oxygen across the BGB is very high

STRENGTHS OF THE AIR- AND THE BLOODCAPILLARIES

For remarkably very small structures the ACs andthe BCs are unexpectedly very strong In the lung of a

Fig 13ndash16 Figs 13 14 Transmission electron micrographs of theexchange tissue of the lung of the herring gull Larus ridibundus show-ing the network of air capillaries (AC) and blood capillaries (BC)Er 5 erythrocytes stars 5 blood capillary-blood capillary connectionsarrows 5 air capillary-air capillary connections circles 5 blood-gas bar-rier Fig 15 Transverse section of the blood capillaries (BC) in the gasexchange tissue of the lung of the domestic fowl Gallus gallus variantdomesticus Arrows 5 epithelial-epithelial cells connections AC 5 aircapillaries circles 5 blood-gas barrier Er 5 erythrocytes Interalveolarseptum of the lung of the vervet monkey Cercopithecus aethiops

showing a thick (supporting) side (star) and a thin (gas exchanging)side (circle) (insert) BC 5 blood capillary Al 5 alveolus Fig 16 Trans-verse section of two adjacent blood capillaries (BC) in the gasexchange tissue of the lung of the domestic fowl Gallus gallus variantdomesticus which are separated by a fused basement membrane(star) which is lined by endothelial cells Boxes 5 blood-gas barrierEr 5 erythrocytes AC 5 air capillary A blood-gas barrier of the lung ofthe mallard duck Anser anser showing that it is fairly uniform in thick-ness (insert) Circle 5 site where an erythrocyte (Er) is pressing on theblood-gas barrier BC 5 blood capillary AC 5 air capillary

1678 MAINA

duck Macklem et al (1979) observed that the ACsremained open after the parabronchial exchange tissuewas compressed at a pressure of 2 kPa In the same spe-cies of bird Powell et al (1985) noted that doubling theblood flow to one lung (by temporarily occluding the pul-monary artery to one lung) caused doubling of blood flowto the lung and generated a twofold increase in pulmo-nary vascular resistance (PVR) that showed that pulmo-nary recruitment and distension of the BCs did notoccur ie the BCs were noncompliant or rigid LaterWatson et al (2007) observed that the diameters of theBCs increased by only 13 when the pressure insidethem was raised from 0 to 25 kPa and increasing thepressure outside the BC relative to that inside to 35kPa did not change their diameter In comparison forequal pressure changes in the lungs of dogs and catsthe mean diameters of the BCs respectively increased by125 and 128 and at the higher pressure (35 kPa)the BCs totally collapsed (Watson et al 2007) For thelung of the domestic fowl West et al (2007 2007b) noted

near-linear increase in pressure with flow rate and thatthe BCs remained open after the pressure in the ACswas raised to well-above that in the BCs this showedthat the BCs behaved like near-rigid tubes which resistdistension and compression The behavior is in completecontrast to that of the BCs of the mammalian lungwhere increases in either the pulmonary arterial pres-sure or pulmonary venous pressures results in largedrops in the PVR which is largely caused by recruitmentand distension of the BCs (Hardy and Campbell 1953Borst et al 1956 Lloyd and Wright 1960 Roos et al1961 Glazier et al 1969) This shows that unlike theBCs of the avian lung which are rigid those of the mam-malian lung are flexible (compliant)

PUTATIVE BASES OF THE STRENGTHS OFTHE AIR- AND THE BLOOD CAPILLARIES

The strengths of the ACs and the BCs of the avianlung are intriguing A cursory look at the exchange

Fig 17 Fig 17 Diagram showing the four principal forces which acton the blood-gas barrier (BGB) and the epithelial-epithelial cells con-nections (E-ECCs) of the exchange tissue of the avian lung A Blood inthe blood capillary is under pressure from the contractions of the heartmuscle the outward push creates circumferential tension (intramuralpressure) which acts directly on the BGB and indirectly on the E-ECCsB Intermolecular forces at the air-liquid interface on the surfaces of the

air capillaries creates surface tension which exerts an inward pull onthe wall C Intrapulmonary pressure exerts an outward push or pro-vides external support to the BGB and the E-ECCs D Depending onthe body posture the weight of lung tissue and that of blood in theblood capillaries (through gravity) tense the BGB directly and the E-ECCs indirectly A network of blood capillaries (BC) and air capillaries(AC) showing their interconnections Er 5 erythrocytes (insert)

THE AC AND THE BC OF THE AVIAN LUNG 1679

tissue of the avian lung and the very small respiratoryunits which largely comprise it the ACs and the BCsinstinctively leads to the conclusion made by Klika et al(1997) that the ldquoavian air capillaries are delicate struc-tures compared with the mammalian pulmonaryalveolusrdquo Unlike for the parenchyma (gas exchange tis-sue) of the mammalian lung where a dedicated support-ing fibroskeleton framework comprising of collagen andelastic tissue exists in the interalveolar septum (Ryanet al 1969 Weibel 1984) (Fig 15 insert) in theexchange tissue of the avian lung such parts do not exist(Figs 13ndash16) Compared with that of the lungs of otherair-breathing vertebrates (Meban 1980 Gehr et al1981) the BGB of the avian lung is relatively muchthinner and more uniform in thickness (Maina 19892005 Maina et al 1989 Maina and West 2005) (Figs13ndash16)

From published literature the reported causes of thestrengths of the ACs and the BCs are (a) the so-called

ldquohoneycomb-likerdquo arrangement (West et al 2006 2010)which confers ldquomechanical interdependencerdquo (Chen1997) between the BCs and the ACs (Figs 1ndash4 13 1420 and 21) (b) presence of epithelial cell bridges (alsocalled retinaculae struts bridges and cross-braces)which separate the ACs while connecting the BCs(Scheuermann et al 1997 Klika et al 1997 Maina2005 West et al 2006 2007 2007b 2010 Watson et al2008) (Figs 13 and 15) (c) presence of a trilamilar sub-stance (TLS) a type of surfactant which is unique to theavian lung (Tyler and Pangborn 1964 Petrik and Rie-del 1968 Scheuermann et al 2000) and which servesas an intricate intercapillary anastomosing skeletal sup-porting system (Klika et al 1997) (d) possible existenceof a tensegrity (tension integrity) system (Fuller 1961Ingber 1998) which in the compact and rigid lung(Maina 2005) tension and compression forces are effi-ciently absorbed and dissipated by well-organized con-nective tissue elements like smooth muscle collagen-

Fig 18ndash21 Figs 18 19 Schematic diagrams showing the morphol-ogies of the blood capillaries (BCs) and the air capillaries (AC) respec-tively The blood capillaries comprise of segments which interconnectthree-dimensionally and are about as long as they are wide while theair capillaries are rather globular structures which connect by narrow

passageways (stars) Figs 20 21 Schematic three-dimensional dia-grams showing how the air capillaries (AC) and the blood capillaries(BC) entwine in the exchange tissue of the avian lungStars 5 connecting passageways

1680 MAINA

and elastic tissue fibers (Maina 2007a 2007b Mainaet al 2010) (Figs 22ndash25) and (e) presence of the strongtype-IV collagen (Stromberg and Wiederhielm 1969Maina and West 2005 Jimoh and Maina 2012) in thebasement membranes of the BGB and the E-ECCs(Maina and Jimoh in press) (Figs 25) It is unclearwhether the isolated ACs which were identified byWoodward and Maina (2005) (Fig 12) play a role instrengthening the air spaces in the manner which aninflated ball or balloon resists compression better com-pared with a less inflated or a punctured one

Departing from the early claims of its absence (Millerand Bondurant 1961) it is now irrefutable that a sur-factant lining exists on the surface of the ACs of the

avian lung (Pattle 1978 Corral 1995) Secreted by thetype-II (granular) pneumocytes which are located in theatria and the infundibula (Maina 2005) on fixed lungtissue the surfactant appears as a very thin electrondense layer which covers the epithelial cells of the ACs(Fig 25 insert) The composition and concentration ofthe surfactant in the avian lung per unit respiratorysurface area is equivalent to that of the mammalianlung (Fujiwara et al 1970) Two forms of surfactantnamely the lamellated osmiophilic bodies (LOBs) andthe TLS occurs only on the respiratory surface of theavian lung The LOBs resemble those of other vertebratelungs and are secreted by the type II epithelial cells(Akester 1970b) while the TLS is secreted by the type-I

Fig 22ndash25 Fig 22 Scanning electron micrograph of alkali (sodiumhydroxide KOH) digested tissue preparation of the gas exchange tis-sue of the lung of the domestic fowl Gallus gallus variant domesticusshowing collagen fibres (circles) in the wall of the blood capillaries (apart of which is outlined with a dashed cylinder) AC 5 air capillary(boundary encircled) Er 5 erythrocyte Undigested lung tissue showinga blood capillary (BC) with intact blood-gas barrier (insert)Er 5 erythrocyte AC 5 air capillary Fig 23 Transmission electronmicrograph of alkali digested tissue preparation of the gas exchangetissue of the lung of the domestic fowl Gallus gallus variant domesti-cus showing collagen fibres (circles) in the walls of the blood capilla-ries (BC) and the air capillaries (AC) A transmission electronmicrograph of an area similar to that shown in Fig 23 showing aircapillaries (AC) and blood capillaries (BC) Stars 5 tissue barriers sepa-

rating the respiratory units Fig 24 A transmission electron micro-graph showing presence of collagen (circles) in the blood-gas barrierof the lung of the domestic fowl Gallus gallus variant domesticusAC 5 air capillary BC 5 blood capillary Er 5 erythrocyte Fig 25Transmission electron micrograph of the blood-gas barrier of the lungof the domestic fowl Gallus gallus variant domesticus showing type IVcollagen which has been labelled with immunogold (arrows)EC 5 endothelium BM 5 basement membrane Ep 5 epitheliumAC 5 air capillary BC 5 blood capillary Transmission electron micro-graph showing undigested blood-gas barrier which comprises of anepithelial cell (EC) a basement membrane (BM) and an endothelialcell (EC) (insert) AC 5 air capillary BC 5 blood capillaryarrows 5 collagen fibres

THE AC AND THE BC OF THE AVIAN LUNG 1681

(squamous) respiratort cells (Klika et al 1997 Scheuer-mann et al 1997) In the rigid avian lung (Jones et al1985) where the ACs are very strong (Macklem et al1979 West et al 2006 2010) existence of surfactant amolecular factor which evolved in the air-breathing ver-tebrates to stabilize the narrow terminal respiratory

units (Pattle 1976 Maina 1998 Clements and Long2010) is paradoxical Fedde (1980) suggested that thesurfactant may be a substance which was carried overduring the evolution of the avian lung from the reptilianone In the avian lung the surfactant however appearsto be involved in preventing exudation of blood plasma

Fig 26ndash31 Figs 26ndash28 Failures of the epithelial-epithelial cells con-nections (stars) in the gas exchange tissue of the lung of the domesticfowl Gallus gallus variant domesticus Figs 29ndash31 Failures of theblood-gas barrier (arrows Fig 29 dashed area Fig 30 star Fig 31)in the gas exchange tissue of the lung of the domestic fowl Gallus

gallus variant domesticus Figs 26 27 29 and 30 are scanning elec-tron micrographs while Figs 28 and 31 are transmission electronmicrographs BC 5 blood capillary AC 5 air capillary Er 5 erythrocyteWBC 5 white blood cell asterisk (Fig 30) a flap which has come fromthe failed area of the blood-gas barrier

1682 MAINA

onto the respiratory surface and therefore keeping therespiratory surface dry The TLS has been associatedwith functions like coagulation of blood which may oozethrough the BGB hydration of the surfactant andabsorption of fluid which may accumulate on the surfaceof the ACs (Pattle 1978) Without offering direct evi-dence for it Klika et al (1997) attributed the stability ofthe ACs to presence of the TLS although interestinglyvery little of it exists on the respiratory surface of theadult avian lung (McLelland 1989)

STRENGTHS OF THE BGB AND THE E-ECCsOF THE EXCHANGE TISSUE OF AVIAN LUNG

In the field of engineering structural failure isdefined as loss of load carrying capacity from damage ordeformation of a component a member or a completestructure following overloading or functioning beyondthe highest load-tolerating capacity or strength thresh-old (Feld and Carper 1997 Stephens and Fuchs 2001)One of the perplexing properties of the functional designof the avian lung is that although the BGB is approxi-mately three times thinner than that of a mammal ofequivalent body mass (Maina 1989 2005 Maina et al1989) it tolerates much higher intramural blood pres-sure (Seymour and Blaylock 2000) which is generatedby large hearts with large stroke volumes and cardiacoutputs (Hartmann 1955 Berger and Hart 1974) TheBGB the E-ECCs and the blood-capillary-blood capil-lary connections are largely subjected to four forces (Fig17) These are (a) the intramural pressure which ema-nates from the contractions of the heart muscle (b) sur-face tension which arises from the interactions of themolecules of the water film which lines the surface ofthe AC (c) depending on the posture of the body pos-ture the weight of the lung tissue and that of the bloodin the BCs and (d) the intrapulmonary pressure

Direct estimation of the strengths of the BGB and theE-ECCs in the exchange tissue of the avian lung has

only been directly determined in the domestic fowl Gal-lus gallus variant domesticus (Maina and Sikiru 2013Maina and Jimoh in press) When these forces exceedthose which the tissue components of the BGB and theE-ECCs can tolerate the structures fail (break) (Figs26ndash31) The failures of the BGB and the E-ECCs in thedifferent regions of the lung which are supplied withblood by the four branches of the pulmonary artery (PA)were quantified under different exercise intensities (run-ning on a treadmill) (Maina and Jimoh in press) andperfusion at different pressures (Maina and Sikiru2013) It was observed that (a) the BGB also fails inresting ie nonstressed birds (b) breaks of the BGB-and the E-ECCs increased with increasing exerciseintensities (Fig 32) and perfusion pressures (Fig 33) (c)the numbers of E-ECCs breaks surpassed those of theBGB (Figs 32 and 33) and (d) the numbers of BGB- andE-ECCs breaks in the different regions of the lung corre-lated with the sizes (diameters) of the branches of thePA and the angles at which the branches originatedfrom it The failure of the E-ECCs occurred at an aver-age intramural pressure of 290 kPa while that of theBGB happened at a higher one of 339 kPa (Maina andSikiru 2013) this showed that the BGB is stronger thanthe E-ECCs In the lungs of the rabbit- the dog- andthe horse the BGBs fail at intramural pressures respec-tively of 533 960 and 1386 kPa (West et al 19911993 West and Mathieu 1992 Birks et al 1994Mathieu-Costello et al 1995) While these pressures arehigher than that at which the BGB in the domestic fowl(chicken) lung fails when the pressures are standar-dized (= divided) with the thicknesses of the basementmembrane of the BGB (the component which mostly con-tributes to the strength of the BGB) (Crouch et al 1997West and Mathieu-Costello 1999 Maina and West2005 West 2009) which is 0045 lm thick in the chickenlung (Watson et al 2007) compared with the muchthicker ones of 0174 0319 and 0386 lm in the rabbitthe dog and the horse lungs respectively (Birks et al1994) the tension at which the chickenrsquos pulmonary

Fig 32 Fig 32 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing exercise intensities and at allthe exercise intensities the numbers of E-ECCs breaks exceeded theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

Fig 33 Fig 33 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing intramural pressures and atall intramural pressures the numbers of E-ECCs breaks surpassed theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

THE AC AND THE BC OF THE AVIAN LUNG 1683

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

duck Macklem et al (1979) observed that the ACsremained open after the parabronchial exchange tissuewas compressed at a pressure of 2 kPa In the same spe-cies of bird Powell et al (1985) noted that doubling theblood flow to one lung (by temporarily occluding the pul-monary artery to one lung) caused doubling of blood flowto the lung and generated a twofold increase in pulmo-nary vascular resistance (PVR) that showed that pulmo-nary recruitment and distension of the BCs did notoccur ie the BCs were noncompliant or rigid LaterWatson et al (2007) observed that the diameters of theBCs increased by only 13 when the pressure insidethem was raised from 0 to 25 kPa and increasing thepressure outside the BC relative to that inside to 35kPa did not change their diameter In comparison forequal pressure changes in the lungs of dogs and catsthe mean diameters of the BCs respectively increased by125 and 128 and at the higher pressure (35 kPa)the BCs totally collapsed (Watson et al 2007) For thelung of the domestic fowl West et al (2007 2007b) noted

near-linear increase in pressure with flow rate and thatthe BCs remained open after the pressure in the ACswas raised to well-above that in the BCs this showedthat the BCs behaved like near-rigid tubes which resistdistension and compression The behavior is in completecontrast to that of the BCs of the mammalian lungwhere increases in either the pulmonary arterial pres-sure or pulmonary venous pressures results in largedrops in the PVR which is largely caused by recruitmentand distension of the BCs (Hardy and Campbell 1953Borst et al 1956 Lloyd and Wright 1960 Roos et al1961 Glazier et al 1969) This shows that unlike theBCs of the avian lung which are rigid those of the mam-malian lung are flexible (compliant)

PUTATIVE BASES OF THE STRENGTHS OFTHE AIR- AND THE BLOOD CAPILLARIES

The strengths of the ACs and the BCs of the avianlung are intriguing A cursory look at the exchange

Fig 17 Fig 17 Diagram showing the four principal forces which acton the blood-gas barrier (BGB) and the epithelial-epithelial cells con-nections (E-ECCs) of the exchange tissue of the avian lung A Blood inthe blood capillary is under pressure from the contractions of the heartmuscle the outward push creates circumferential tension (intramuralpressure) which acts directly on the BGB and indirectly on the E-ECCsB Intermolecular forces at the air-liquid interface on the surfaces of the

air capillaries creates surface tension which exerts an inward pull onthe wall C Intrapulmonary pressure exerts an outward push or pro-vides external support to the BGB and the E-ECCs D Depending onthe body posture the weight of lung tissue and that of blood in theblood capillaries (through gravity) tense the BGB directly and the E-ECCs indirectly A network of blood capillaries (BC) and air capillaries(AC) showing their interconnections Er 5 erythrocytes (insert)

THE AC AND THE BC OF THE AVIAN LUNG 1679

tissue of the avian lung and the very small respiratoryunits which largely comprise it the ACs and the BCsinstinctively leads to the conclusion made by Klika et al(1997) that the ldquoavian air capillaries are delicate struc-tures compared with the mammalian pulmonaryalveolusrdquo Unlike for the parenchyma (gas exchange tis-sue) of the mammalian lung where a dedicated support-ing fibroskeleton framework comprising of collagen andelastic tissue exists in the interalveolar septum (Ryanet al 1969 Weibel 1984) (Fig 15 insert) in theexchange tissue of the avian lung such parts do not exist(Figs 13ndash16) Compared with that of the lungs of otherair-breathing vertebrates (Meban 1980 Gehr et al1981) the BGB of the avian lung is relatively muchthinner and more uniform in thickness (Maina 19892005 Maina et al 1989 Maina and West 2005) (Figs13ndash16)

From published literature the reported causes of thestrengths of the ACs and the BCs are (a) the so-called

ldquohoneycomb-likerdquo arrangement (West et al 2006 2010)which confers ldquomechanical interdependencerdquo (Chen1997) between the BCs and the ACs (Figs 1ndash4 13 1420 and 21) (b) presence of epithelial cell bridges (alsocalled retinaculae struts bridges and cross-braces)which separate the ACs while connecting the BCs(Scheuermann et al 1997 Klika et al 1997 Maina2005 West et al 2006 2007 2007b 2010 Watson et al2008) (Figs 13 and 15) (c) presence of a trilamilar sub-stance (TLS) a type of surfactant which is unique to theavian lung (Tyler and Pangborn 1964 Petrik and Rie-del 1968 Scheuermann et al 2000) and which servesas an intricate intercapillary anastomosing skeletal sup-porting system (Klika et al 1997) (d) possible existenceof a tensegrity (tension integrity) system (Fuller 1961Ingber 1998) which in the compact and rigid lung(Maina 2005) tension and compression forces are effi-ciently absorbed and dissipated by well-organized con-nective tissue elements like smooth muscle collagen-

Fig 18ndash21 Figs 18 19 Schematic diagrams showing the morphol-ogies of the blood capillaries (BCs) and the air capillaries (AC) respec-tively The blood capillaries comprise of segments which interconnectthree-dimensionally and are about as long as they are wide while theair capillaries are rather globular structures which connect by narrow

passageways (stars) Figs 20 21 Schematic three-dimensional dia-grams showing how the air capillaries (AC) and the blood capillaries(BC) entwine in the exchange tissue of the avian lungStars 5 connecting passageways

1680 MAINA

and elastic tissue fibers (Maina 2007a 2007b Mainaet al 2010) (Figs 22ndash25) and (e) presence of the strongtype-IV collagen (Stromberg and Wiederhielm 1969Maina and West 2005 Jimoh and Maina 2012) in thebasement membranes of the BGB and the E-ECCs(Maina and Jimoh in press) (Figs 25) It is unclearwhether the isolated ACs which were identified byWoodward and Maina (2005) (Fig 12) play a role instrengthening the air spaces in the manner which aninflated ball or balloon resists compression better com-pared with a less inflated or a punctured one

Departing from the early claims of its absence (Millerand Bondurant 1961) it is now irrefutable that a sur-factant lining exists on the surface of the ACs of the

avian lung (Pattle 1978 Corral 1995) Secreted by thetype-II (granular) pneumocytes which are located in theatria and the infundibula (Maina 2005) on fixed lungtissue the surfactant appears as a very thin electrondense layer which covers the epithelial cells of the ACs(Fig 25 insert) The composition and concentration ofthe surfactant in the avian lung per unit respiratorysurface area is equivalent to that of the mammalianlung (Fujiwara et al 1970) Two forms of surfactantnamely the lamellated osmiophilic bodies (LOBs) andthe TLS occurs only on the respiratory surface of theavian lung The LOBs resemble those of other vertebratelungs and are secreted by the type II epithelial cells(Akester 1970b) while the TLS is secreted by the type-I

Fig 22ndash25 Fig 22 Scanning electron micrograph of alkali (sodiumhydroxide KOH) digested tissue preparation of the gas exchange tis-sue of the lung of the domestic fowl Gallus gallus variant domesticusshowing collagen fibres (circles) in the wall of the blood capillaries (apart of which is outlined with a dashed cylinder) AC 5 air capillary(boundary encircled) Er 5 erythrocyte Undigested lung tissue showinga blood capillary (BC) with intact blood-gas barrier (insert)Er 5 erythrocyte AC 5 air capillary Fig 23 Transmission electronmicrograph of alkali digested tissue preparation of the gas exchangetissue of the lung of the domestic fowl Gallus gallus variant domesti-cus showing collagen fibres (circles) in the walls of the blood capilla-ries (BC) and the air capillaries (AC) A transmission electronmicrograph of an area similar to that shown in Fig 23 showing aircapillaries (AC) and blood capillaries (BC) Stars 5 tissue barriers sepa-

rating the respiratory units Fig 24 A transmission electron micro-graph showing presence of collagen (circles) in the blood-gas barrierof the lung of the domestic fowl Gallus gallus variant domesticusAC 5 air capillary BC 5 blood capillary Er 5 erythrocyte Fig 25Transmission electron micrograph of the blood-gas barrier of the lungof the domestic fowl Gallus gallus variant domesticus showing type IVcollagen which has been labelled with immunogold (arrows)EC 5 endothelium BM 5 basement membrane Ep 5 epitheliumAC 5 air capillary BC 5 blood capillary Transmission electron micro-graph showing undigested blood-gas barrier which comprises of anepithelial cell (EC) a basement membrane (BM) and an endothelialcell (EC) (insert) AC 5 air capillary BC 5 blood capillaryarrows 5 collagen fibres

THE AC AND THE BC OF THE AVIAN LUNG 1681

(squamous) respiratort cells (Klika et al 1997 Scheuer-mann et al 1997) In the rigid avian lung (Jones et al1985) where the ACs are very strong (Macklem et al1979 West et al 2006 2010) existence of surfactant amolecular factor which evolved in the air-breathing ver-tebrates to stabilize the narrow terminal respiratory

units (Pattle 1976 Maina 1998 Clements and Long2010) is paradoxical Fedde (1980) suggested that thesurfactant may be a substance which was carried overduring the evolution of the avian lung from the reptilianone In the avian lung the surfactant however appearsto be involved in preventing exudation of blood plasma

Fig 26ndash31 Figs 26ndash28 Failures of the epithelial-epithelial cells con-nections (stars) in the gas exchange tissue of the lung of the domesticfowl Gallus gallus variant domesticus Figs 29ndash31 Failures of theblood-gas barrier (arrows Fig 29 dashed area Fig 30 star Fig 31)in the gas exchange tissue of the lung of the domestic fowl Gallus

gallus variant domesticus Figs 26 27 29 and 30 are scanning elec-tron micrographs while Figs 28 and 31 are transmission electronmicrographs BC 5 blood capillary AC 5 air capillary Er 5 erythrocyteWBC 5 white blood cell asterisk (Fig 30) a flap which has come fromthe failed area of the blood-gas barrier

1682 MAINA

onto the respiratory surface and therefore keeping therespiratory surface dry The TLS has been associatedwith functions like coagulation of blood which may oozethrough the BGB hydration of the surfactant andabsorption of fluid which may accumulate on the surfaceof the ACs (Pattle 1978) Without offering direct evi-dence for it Klika et al (1997) attributed the stability ofthe ACs to presence of the TLS although interestinglyvery little of it exists on the respiratory surface of theadult avian lung (McLelland 1989)

STRENGTHS OF THE BGB AND THE E-ECCsOF THE EXCHANGE TISSUE OF AVIAN LUNG

In the field of engineering structural failure isdefined as loss of load carrying capacity from damage ordeformation of a component a member or a completestructure following overloading or functioning beyondthe highest load-tolerating capacity or strength thresh-old (Feld and Carper 1997 Stephens and Fuchs 2001)One of the perplexing properties of the functional designof the avian lung is that although the BGB is approxi-mately three times thinner than that of a mammal ofequivalent body mass (Maina 1989 2005 Maina et al1989) it tolerates much higher intramural blood pres-sure (Seymour and Blaylock 2000) which is generatedby large hearts with large stroke volumes and cardiacoutputs (Hartmann 1955 Berger and Hart 1974) TheBGB the E-ECCs and the blood-capillary-blood capil-lary connections are largely subjected to four forces (Fig17) These are (a) the intramural pressure which ema-nates from the contractions of the heart muscle (b) sur-face tension which arises from the interactions of themolecules of the water film which lines the surface ofthe AC (c) depending on the posture of the body pos-ture the weight of the lung tissue and that of the bloodin the BCs and (d) the intrapulmonary pressure

Direct estimation of the strengths of the BGB and theE-ECCs in the exchange tissue of the avian lung has

only been directly determined in the domestic fowl Gal-lus gallus variant domesticus (Maina and Sikiru 2013Maina and Jimoh in press) When these forces exceedthose which the tissue components of the BGB and theE-ECCs can tolerate the structures fail (break) (Figs26ndash31) The failures of the BGB and the E-ECCs in thedifferent regions of the lung which are supplied withblood by the four branches of the pulmonary artery (PA)were quantified under different exercise intensities (run-ning on a treadmill) (Maina and Jimoh in press) andperfusion at different pressures (Maina and Sikiru2013) It was observed that (a) the BGB also fails inresting ie nonstressed birds (b) breaks of the BGB-and the E-ECCs increased with increasing exerciseintensities (Fig 32) and perfusion pressures (Fig 33) (c)the numbers of E-ECCs breaks surpassed those of theBGB (Figs 32 and 33) and (d) the numbers of BGB- andE-ECCs breaks in the different regions of the lung corre-lated with the sizes (diameters) of the branches of thePA and the angles at which the branches originatedfrom it The failure of the E-ECCs occurred at an aver-age intramural pressure of 290 kPa while that of theBGB happened at a higher one of 339 kPa (Maina andSikiru 2013) this showed that the BGB is stronger thanthe E-ECCs In the lungs of the rabbit- the dog- andthe horse the BGBs fail at intramural pressures respec-tively of 533 960 and 1386 kPa (West et al 19911993 West and Mathieu 1992 Birks et al 1994Mathieu-Costello et al 1995) While these pressures arehigher than that at which the BGB in the domestic fowl(chicken) lung fails when the pressures are standar-dized (= divided) with the thicknesses of the basementmembrane of the BGB (the component which mostly con-tributes to the strength of the BGB) (Crouch et al 1997West and Mathieu-Costello 1999 Maina and West2005 West 2009) which is 0045 lm thick in the chickenlung (Watson et al 2007) compared with the muchthicker ones of 0174 0319 and 0386 lm in the rabbitthe dog and the horse lungs respectively (Birks et al1994) the tension at which the chickenrsquos pulmonary

Fig 32 Fig 32 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing exercise intensities and at allthe exercise intensities the numbers of E-ECCs breaks exceeded theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

Fig 33 Fig 33 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing intramural pressures and atall intramural pressures the numbers of E-ECCs breaks surpassed theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

THE AC AND THE BC OF THE AVIAN LUNG 1683

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

tissue of the avian lung and the very small respiratoryunits which largely comprise it the ACs and the BCsinstinctively leads to the conclusion made by Klika et al(1997) that the ldquoavian air capillaries are delicate struc-tures compared with the mammalian pulmonaryalveolusrdquo Unlike for the parenchyma (gas exchange tis-sue) of the mammalian lung where a dedicated support-ing fibroskeleton framework comprising of collagen andelastic tissue exists in the interalveolar septum (Ryanet al 1969 Weibel 1984) (Fig 15 insert) in theexchange tissue of the avian lung such parts do not exist(Figs 13ndash16) Compared with that of the lungs of otherair-breathing vertebrates (Meban 1980 Gehr et al1981) the BGB of the avian lung is relatively muchthinner and more uniform in thickness (Maina 19892005 Maina et al 1989 Maina and West 2005) (Figs13ndash16)

From published literature the reported causes of thestrengths of the ACs and the BCs are (a) the so-called

ldquohoneycomb-likerdquo arrangement (West et al 2006 2010)which confers ldquomechanical interdependencerdquo (Chen1997) between the BCs and the ACs (Figs 1ndash4 13 1420 and 21) (b) presence of epithelial cell bridges (alsocalled retinaculae struts bridges and cross-braces)which separate the ACs while connecting the BCs(Scheuermann et al 1997 Klika et al 1997 Maina2005 West et al 2006 2007 2007b 2010 Watson et al2008) (Figs 13 and 15) (c) presence of a trilamilar sub-stance (TLS) a type of surfactant which is unique to theavian lung (Tyler and Pangborn 1964 Petrik and Rie-del 1968 Scheuermann et al 2000) and which servesas an intricate intercapillary anastomosing skeletal sup-porting system (Klika et al 1997) (d) possible existenceof a tensegrity (tension integrity) system (Fuller 1961Ingber 1998) which in the compact and rigid lung(Maina 2005) tension and compression forces are effi-ciently absorbed and dissipated by well-organized con-nective tissue elements like smooth muscle collagen-

Fig 18ndash21 Figs 18 19 Schematic diagrams showing the morphol-ogies of the blood capillaries (BCs) and the air capillaries (AC) respec-tively The blood capillaries comprise of segments which interconnectthree-dimensionally and are about as long as they are wide while theair capillaries are rather globular structures which connect by narrow

passageways (stars) Figs 20 21 Schematic three-dimensional dia-grams showing how the air capillaries (AC) and the blood capillaries(BC) entwine in the exchange tissue of the avian lungStars 5 connecting passageways

1680 MAINA

and elastic tissue fibers (Maina 2007a 2007b Mainaet al 2010) (Figs 22ndash25) and (e) presence of the strongtype-IV collagen (Stromberg and Wiederhielm 1969Maina and West 2005 Jimoh and Maina 2012) in thebasement membranes of the BGB and the E-ECCs(Maina and Jimoh in press) (Figs 25) It is unclearwhether the isolated ACs which were identified byWoodward and Maina (2005) (Fig 12) play a role instrengthening the air spaces in the manner which aninflated ball or balloon resists compression better com-pared with a less inflated or a punctured one

Departing from the early claims of its absence (Millerand Bondurant 1961) it is now irrefutable that a sur-factant lining exists on the surface of the ACs of the

avian lung (Pattle 1978 Corral 1995) Secreted by thetype-II (granular) pneumocytes which are located in theatria and the infundibula (Maina 2005) on fixed lungtissue the surfactant appears as a very thin electrondense layer which covers the epithelial cells of the ACs(Fig 25 insert) The composition and concentration ofthe surfactant in the avian lung per unit respiratorysurface area is equivalent to that of the mammalianlung (Fujiwara et al 1970) Two forms of surfactantnamely the lamellated osmiophilic bodies (LOBs) andthe TLS occurs only on the respiratory surface of theavian lung The LOBs resemble those of other vertebratelungs and are secreted by the type II epithelial cells(Akester 1970b) while the TLS is secreted by the type-I

Fig 22ndash25 Fig 22 Scanning electron micrograph of alkali (sodiumhydroxide KOH) digested tissue preparation of the gas exchange tis-sue of the lung of the domestic fowl Gallus gallus variant domesticusshowing collagen fibres (circles) in the wall of the blood capillaries (apart of which is outlined with a dashed cylinder) AC 5 air capillary(boundary encircled) Er 5 erythrocyte Undigested lung tissue showinga blood capillary (BC) with intact blood-gas barrier (insert)Er 5 erythrocyte AC 5 air capillary Fig 23 Transmission electronmicrograph of alkali digested tissue preparation of the gas exchangetissue of the lung of the domestic fowl Gallus gallus variant domesti-cus showing collagen fibres (circles) in the walls of the blood capilla-ries (BC) and the air capillaries (AC) A transmission electronmicrograph of an area similar to that shown in Fig 23 showing aircapillaries (AC) and blood capillaries (BC) Stars 5 tissue barriers sepa-

rating the respiratory units Fig 24 A transmission electron micro-graph showing presence of collagen (circles) in the blood-gas barrierof the lung of the domestic fowl Gallus gallus variant domesticusAC 5 air capillary BC 5 blood capillary Er 5 erythrocyte Fig 25Transmission electron micrograph of the blood-gas barrier of the lungof the domestic fowl Gallus gallus variant domesticus showing type IVcollagen which has been labelled with immunogold (arrows)EC 5 endothelium BM 5 basement membrane Ep 5 epitheliumAC 5 air capillary BC 5 blood capillary Transmission electron micro-graph showing undigested blood-gas barrier which comprises of anepithelial cell (EC) a basement membrane (BM) and an endothelialcell (EC) (insert) AC 5 air capillary BC 5 blood capillaryarrows 5 collagen fibres

THE AC AND THE BC OF THE AVIAN LUNG 1681

(squamous) respiratort cells (Klika et al 1997 Scheuer-mann et al 1997) In the rigid avian lung (Jones et al1985) where the ACs are very strong (Macklem et al1979 West et al 2006 2010) existence of surfactant amolecular factor which evolved in the air-breathing ver-tebrates to stabilize the narrow terminal respiratory

units (Pattle 1976 Maina 1998 Clements and Long2010) is paradoxical Fedde (1980) suggested that thesurfactant may be a substance which was carried overduring the evolution of the avian lung from the reptilianone In the avian lung the surfactant however appearsto be involved in preventing exudation of blood plasma

Fig 26ndash31 Figs 26ndash28 Failures of the epithelial-epithelial cells con-nections (stars) in the gas exchange tissue of the lung of the domesticfowl Gallus gallus variant domesticus Figs 29ndash31 Failures of theblood-gas barrier (arrows Fig 29 dashed area Fig 30 star Fig 31)in the gas exchange tissue of the lung of the domestic fowl Gallus

gallus variant domesticus Figs 26 27 29 and 30 are scanning elec-tron micrographs while Figs 28 and 31 are transmission electronmicrographs BC 5 blood capillary AC 5 air capillary Er 5 erythrocyteWBC 5 white blood cell asterisk (Fig 30) a flap which has come fromthe failed area of the blood-gas barrier

1682 MAINA

onto the respiratory surface and therefore keeping therespiratory surface dry The TLS has been associatedwith functions like coagulation of blood which may oozethrough the BGB hydration of the surfactant andabsorption of fluid which may accumulate on the surfaceof the ACs (Pattle 1978) Without offering direct evi-dence for it Klika et al (1997) attributed the stability ofthe ACs to presence of the TLS although interestinglyvery little of it exists on the respiratory surface of theadult avian lung (McLelland 1989)

STRENGTHS OF THE BGB AND THE E-ECCsOF THE EXCHANGE TISSUE OF AVIAN LUNG

In the field of engineering structural failure isdefined as loss of load carrying capacity from damage ordeformation of a component a member or a completestructure following overloading or functioning beyondthe highest load-tolerating capacity or strength thresh-old (Feld and Carper 1997 Stephens and Fuchs 2001)One of the perplexing properties of the functional designof the avian lung is that although the BGB is approxi-mately three times thinner than that of a mammal ofequivalent body mass (Maina 1989 2005 Maina et al1989) it tolerates much higher intramural blood pres-sure (Seymour and Blaylock 2000) which is generatedby large hearts with large stroke volumes and cardiacoutputs (Hartmann 1955 Berger and Hart 1974) TheBGB the E-ECCs and the blood-capillary-blood capil-lary connections are largely subjected to four forces (Fig17) These are (a) the intramural pressure which ema-nates from the contractions of the heart muscle (b) sur-face tension which arises from the interactions of themolecules of the water film which lines the surface ofthe AC (c) depending on the posture of the body pos-ture the weight of the lung tissue and that of the bloodin the BCs and (d) the intrapulmonary pressure

Direct estimation of the strengths of the BGB and theE-ECCs in the exchange tissue of the avian lung has

only been directly determined in the domestic fowl Gal-lus gallus variant domesticus (Maina and Sikiru 2013Maina and Jimoh in press) When these forces exceedthose which the tissue components of the BGB and theE-ECCs can tolerate the structures fail (break) (Figs26ndash31) The failures of the BGB and the E-ECCs in thedifferent regions of the lung which are supplied withblood by the four branches of the pulmonary artery (PA)were quantified under different exercise intensities (run-ning on a treadmill) (Maina and Jimoh in press) andperfusion at different pressures (Maina and Sikiru2013) It was observed that (a) the BGB also fails inresting ie nonstressed birds (b) breaks of the BGB-and the E-ECCs increased with increasing exerciseintensities (Fig 32) and perfusion pressures (Fig 33) (c)the numbers of E-ECCs breaks surpassed those of theBGB (Figs 32 and 33) and (d) the numbers of BGB- andE-ECCs breaks in the different regions of the lung corre-lated with the sizes (diameters) of the branches of thePA and the angles at which the branches originatedfrom it The failure of the E-ECCs occurred at an aver-age intramural pressure of 290 kPa while that of theBGB happened at a higher one of 339 kPa (Maina andSikiru 2013) this showed that the BGB is stronger thanthe E-ECCs In the lungs of the rabbit- the dog- andthe horse the BGBs fail at intramural pressures respec-tively of 533 960 and 1386 kPa (West et al 19911993 West and Mathieu 1992 Birks et al 1994Mathieu-Costello et al 1995) While these pressures arehigher than that at which the BGB in the domestic fowl(chicken) lung fails when the pressures are standar-dized (= divided) with the thicknesses of the basementmembrane of the BGB (the component which mostly con-tributes to the strength of the BGB) (Crouch et al 1997West and Mathieu-Costello 1999 Maina and West2005 West 2009) which is 0045 lm thick in the chickenlung (Watson et al 2007) compared with the muchthicker ones of 0174 0319 and 0386 lm in the rabbitthe dog and the horse lungs respectively (Birks et al1994) the tension at which the chickenrsquos pulmonary

Fig 32 Fig 32 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing exercise intensities and at allthe exercise intensities the numbers of E-ECCs breaks exceeded theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

Fig 33 Fig 33 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing intramural pressures and atall intramural pressures the numbers of E-ECCs breaks surpassed theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

THE AC AND THE BC OF THE AVIAN LUNG 1683

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

and elastic tissue fibers (Maina 2007a 2007b Mainaet al 2010) (Figs 22ndash25) and (e) presence of the strongtype-IV collagen (Stromberg and Wiederhielm 1969Maina and West 2005 Jimoh and Maina 2012) in thebasement membranes of the BGB and the E-ECCs(Maina and Jimoh in press) (Figs 25) It is unclearwhether the isolated ACs which were identified byWoodward and Maina (2005) (Fig 12) play a role instrengthening the air spaces in the manner which aninflated ball or balloon resists compression better com-pared with a less inflated or a punctured one

Departing from the early claims of its absence (Millerand Bondurant 1961) it is now irrefutable that a sur-factant lining exists on the surface of the ACs of the

avian lung (Pattle 1978 Corral 1995) Secreted by thetype-II (granular) pneumocytes which are located in theatria and the infundibula (Maina 2005) on fixed lungtissue the surfactant appears as a very thin electrondense layer which covers the epithelial cells of the ACs(Fig 25 insert) The composition and concentration ofthe surfactant in the avian lung per unit respiratorysurface area is equivalent to that of the mammalianlung (Fujiwara et al 1970) Two forms of surfactantnamely the lamellated osmiophilic bodies (LOBs) andthe TLS occurs only on the respiratory surface of theavian lung The LOBs resemble those of other vertebratelungs and are secreted by the type II epithelial cells(Akester 1970b) while the TLS is secreted by the type-I

Fig 22ndash25 Fig 22 Scanning electron micrograph of alkali (sodiumhydroxide KOH) digested tissue preparation of the gas exchange tis-sue of the lung of the domestic fowl Gallus gallus variant domesticusshowing collagen fibres (circles) in the wall of the blood capillaries (apart of which is outlined with a dashed cylinder) AC 5 air capillary(boundary encircled) Er 5 erythrocyte Undigested lung tissue showinga blood capillary (BC) with intact blood-gas barrier (insert)Er 5 erythrocyte AC 5 air capillary Fig 23 Transmission electronmicrograph of alkali digested tissue preparation of the gas exchangetissue of the lung of the domestic fowl Gallus gallus variant domesti-cus showing collagen fibres (circles) in the walls of the blood capilla-ries (BC) and the air capillaries (AC) A transmission electronmicrograph of an area similar to that shown in Fig 23 showing aircapillaries (AC) and blood capillaries (BC) Stars 5 tissue barriers sepa-

rating the respiratory units Fig 24 A transmission electron micro-graph showing presence of collagen (circles) in the blood-gas barrierof the lung of the domestic fowl Gallus gallus variant domesticusAC 5 air capillary BC 5 blood capillary Er 5 erythrocyte Fig 25Transmission electron micrograph of the blood-gas barrier of the lungof the domestic fowl Gallus gallus variant domesticus showing type IVcollagen which has been labelled with immunogold (arrows)EC 5 endothelium BM 5 basement membrane Ep 5 epitheliumAC 5 air capillary BC 5 blood capillary Transmission electron micro-graph showing undigested blood-gas barrier which comprises of anepithelial cell (EC) a basement membrane (BM) and an endothelialcell (EC) (insert) AC 5 air capillary BC 5 blood capillaryarrows 5 collagen fibres

THE AC AND THE BC OF THE AVIAN LUNG 1681

(squamous) respiratort cells (Klika et al 1997 Scheuer-mann et al 1997) In the rigid avian lung (Jones et al1985) where the ACs are very strong (Macklem et al1979 West et al 2006 2010) existence of surfactant amolecular factor which evolved in the air-breathing ver-tebrates to stabilize the narrow terminal respiratory

units (Pattle 1976 Maina 1998 Clements and Long2010) is paradoxical Fedde (1980) suggested that thesurfactant may be a substance which was carried overduring the evolution of the avian lung from the reptilianone In the avian lung the surfactant however appearsto be involved in preventing exudation of blood plasma

Fig 26ndash31 Figs 26ndash28 Failures of the epithelial-epithelial cells con-nections (stars) in the gas exchange tissue of the lung of the domesticfowl Gallus gallus variant domesticus Figs 29ndash31 Failures of theblood-gas barrier (arrows Fig 29 dashed area Fig 30 star Fig 31)in the gas exchange tissue of the lung of the domestic fowl Gallus

gallus variant domesticus Figs 26 27 29 and 30 are scanning elec-tron micrographs while Figs 28 and 31 are transmission electronmicrographs BC 5 blood capillary AC 5 air capillary Er 5 erythrocyteWBC 5 white blood cell asterisk (Fig 30) a flap which has come fromthe failed area of the blood-gas barrier

1682 MAINA

onto the respiratory surface and therefore keeping therespiratory surface dry The TLS has been associatedwith functions like coagulation of blood which may oozethrough the BGB hydration of the surfactant andabsorption of fluid which may accumulate on the surfaceof the ACs (Pattle 1978) Without offering direct evi-dence for it Klika et al (1997) attributed the stability ofthe ACs to presence of the TLS although interestinglyvery little of it exists on the respiratory surface of theadult avian lung (McLelland 1989)

STRENGTHS OF THE BGB AND THE E-ECCsOF THE EXCHANGE TISSUE OF AVIAN LUNG

In the field of engineering structural failure isdefined as loss of load carrying capacity from damage ordeformation of a component a member or a completestructure following overloading or functioning beyondthe highest load-tolerating capacity or strength thresh-old (Feld and Carper 1997 Stephens and Fuchs 2001)One of the perplexing properties of the functional designof the avian lung is that although the BGB is approxi-mately three times thinner than that of a mammal ofequivalent body mass (Maina 1989 2005 Maina et al1989) it tolerates much higher intramural blood pres-sure (Seymour and Blaylock 2000) which is generatedby large hearts with large stroke volumes and cardiacoutputs (Hartmann 1955 Berger and Hart 1974) TheBGB the E-ECCs and the blood-capillary-blood capil-lary connections are largely subjected to four forces (Fig17) These are (a) the intramural pressure which ema-nates from the contractions of the heart muscle (b) sur-face tension which arises from the interactions of themolecules of the water film which lines the surface ofthe AC (c) depending on the posture of the body pos-ture the weight of the lung tissue and that of the bloodin the BCs and (d) the intrapulmonary pressure

Direct estimation of the strengths of the BGB and theE-ECCs in the exchange tissue of the avian lung has

only been directly determined in the domestic fowl Gal-lus gallus variant domesticus (Maina and Sikiru 2013Maina and Jimoh in press) When these forces exceedthose which the tissue components of the BGB and theE-ECCs can tolerate the structures fail (break) (Figs26ndash31) The failures of the BGB and the E-ECCs in thedifferent regions of the lung which are supplied withblood by the four branches of the pulmonary artery (PA)were quantified under different exercise intensities (run-ning on a treadmill) (Maina and Jimoh in press) andperfusion at different pressures (Maina and Sikiru2013) It was observed that (a) the BGB also fails inresting ie nonstressed birds (b) breaks of the BGB-and the E-ECCs increased with increasing exerciseintensities (Fig 32) and perfusion pressures (Fig 33) (c)the numbers of E-ECCs breaks surpassed those of theBGB (Figs 32 and 33) and (d) the numbers of BGB- andE-ECCs breaks in the different regions of the lung corre-lated with the sizes (diameters) of the branches of thePA and the angles at which the branches originatedfrom it The failure of the E-ECCs occurred at an aver-age intramural pressure of 290 kPa while that of theBGB happened at a higher one of 339 kPa (Maina andSikiru 2013) this showed that the BGB is stronger thanthe E-ECCs In the lungs of the rabbit- the dog- andthe horse the BGBs fail at intramural pressures respec-tively of 533 960 and 1386 kPa (West et al 19911993 West and Mathieu 1992 Birks et al 1994Mathieu-Costello et al 1995) While these pressures arehigher than that at which the BGB in the domestic fowl(chicken) lung fails when the pressures are standar-dized (= divided) with the thicknesses of the basementmembrane of the BGB (the component which mostly con-tributes to the strength of the BGB) (Crouch et al 1997West and Mathieu-Costello 1999 Maina and West2005 West 2009) which is 0045 lm thick in the chickenlung (Watson et al 2007) compared with the muchthicker ones of 0174 0319 and 0386 lm in the rabbitthe dog and the horse lungs respectively (Birks et al1994) the tension at which the chickenrsquos pulmonary

Fig 32 Fig 32 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing exercise intensities and at allthe exercise intensities the numbers of E-ECCs breaks exceeded theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

Fig 33 Fig 33 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing intramural pressures and atall intramural pressures the numbers of E-ECCs breaks surpassed theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

THE AC AND THE BC OF THE AVIAN LUNG 1683

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

(squamous) respiratort cells (Klika et al 1997 Scheuer-mann et al 1997) In the rigid avian lung (Jones et al1985) where the ACs are very strong (Macklem et al1979 West et al 2006 2010) existence of surfactant amolecular factor which evolved in the air-breathing ver-tebrates to stabilize the narrow terminal respiratory

units (Pattle 1976 Maina 1998 Clements and Long2010) is paradoxical Fedde (1980) suggested that thesurfactant may be a substance which was carried overduring the evolution of the avian lung from the reptilianone In the avian lung the surfactant however appearsto be involved in preventing exudation of blood plasma

Fig 26ndash31 Figs 26ndash28 Failures of the epithelial-epithelial cells con-nections (stars) in the gas exchange tissue of the lung of the domesticfowl Gallus gallus variant domesticus Figs 29ndash31 Failures of theblood-gas barrier (arrows Fig 29 dashed area Fig 30 star Fig 31)in the gas exchange tissue of the lung of the domestic fowl Gallus

gallus variant domesticus Figs 26 27 29 and 30 are scanning elec-tron micrographs while Figs 28 and 31 are transmission electronmicrographs BC 5 blood capillary AC 5 air capillary Er 5 erythrocyteWBC 5 white blood cell asterisk (Fig 30) a flap which has come fromthe failed area of the blood-gas barrier

1682 MAINA

onto the respiratory surface and therefore keeping therespiratory surface dry The TLS has been associatedwith functions like coagulation of blood which may oozethrough the BGB hydration of the surfactant andabsorption of fluid which may accumulate on the surfaceof the ACs (Pattle 1978) Without offering direct evi-dence for it Klika et al (1997) attributed the stability ofthe ACs to presence of the TLS although interestinglyvery little of it exists on the respiratory surface of theadult avian lung (McLelland 1989)

STRENGTHS OF THE BGB AND THE E-ECCsOF THE EXCHANGE TISSUE OF AVIAN LUNG

In the field of engineering structural failure isdefined as loss of load carrying capacity from damage ordeformation of a component a member or a completestructure following overloading or functioning beyondthe highest load-tolerating capacity or strength thresh-old (Feld and Carper 1997 Stephens and Fuchs 2001)One of the perplexing properties of the functional designof the avian lung is that although the BGB is approxi-mately three times thinner than that of a mammal ofequivalent body mass (Maina 1989 2005 Maina et al1989) it tolerates much higher intramural blood pres-sure (Seymour and Blaylock 2000) which is generatedby large hearts with large stroke volumes and cardiacoutputs (Hartmann 1955 Berger and Hart 1974) TheBGB the E-ECCs and the blood-capillary-blood capil-lary connections are largely subjected to four forces (Fig17) These are (a) the intramural pressure which ema-nates from the contractions of the heart muscle (b) sur-face tension which arises from the interactions of themolecules of the water film which lines the surface ofthe AC (c) depending on the posture of the body pos-ture the weight of the lung tissue and that of the bloodin the BCs and (d) the intrapulmonary pressure

Direct estimation of the strengths of the BGB and theE-ECCs in the exchange tissue of the avian lung has

only been directly determined in the domestic fowl Gal-lus gallus variant domesticus (Maina and Sikiru 2013Maina and Jimoh in press) When these forces exceedthose which the tissue components of the BGB and theE-ECCs can tolerate the structures fail (break) (Figs26ndash31) The failures of the BGB and the E-ECCs in thedifferent regions of the lung which are supplied withblood by the four branches of the pulmonary artery (PA)were quantified under different exercise intensities (run-ning on a treadmill) (Maina and Jimoh in press) andperfusion at different pressures (Maina and Sikiru2013) It was observed that (a) the BGB also fails inresting ie nonstressed birds (b) breaks of the BGB-and the E-ECCs increased with increasing exerciseintensities (Fig 32) and perfusion pressures (Fig 33) (c)the numbers of E-ECCs breaks surpassed those of theBGB (Figs 32 and 33) and (d) the numbers of BGB- andE-ECCs breaks in the different regions of the lung corre-lated with the sizes (diameters) of the branches of thePA and the angles at which the branches originatedfrom it The failure of the E-ECCs occurred at an aver-age intramural pressure of 290 kPa while that of theBGB happened at a higher one of 339 kPa (Maina andSikiru 2013) this showed that the BGB is stronger thanthe E-ECCs In the lungs of the rabbit- the dog- andthe horse the BGBs fail at intramural pressures respec-tively of 533 960 and 1386 kPa (West et al 19911993 West and Mathieu 1992 Birks et al 1994Mathieu-Costello et al 1995) While these pressures arehigher than that at which the BGB in the domestic fowl(chicken) lung fails when the pressures are standar-dized (= divided) with the thicknesses of the basementmembrane of the BGB (the component which mostly con-tributes to the strength of the BGB) (Crouch et al 1997West and Mathieu-Costello 1999 Maina and West2005 West 2009) which is 0045 lm thick in the chickenlung (Watson et al 2007) compared with the muchthicker ones of 0174 0319 and 0386 lm in the rabbitthe dog and the horse lungs respectively (Birks et al1994) the tension at which the chickenrsquos pulmonary

Fig 32 Fig 32 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing exercise intensities and at allthe exercise intensities the numbers of E-ECCs breaks exceeded theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

Fig 33 Fig 33 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing intramural pressures and atall intramural pressures the numbers of E-ECCs breaks surpassed theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

THE AC AND THE BC OF THE AVIAN LUNG 1683

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

onto the respiratory surface and therefore keeping therespiratory surface dry The TLS has been associatedwith functions like coagulation of blood which may oozethrough the BGB hydration of the surfactant andabsorption of fluid which may accumulate on the surfaceof the ACs (Pattle 1978) Without offering direct evi-dence for it Klika et al (1997) attributed the stability ofthe ACs to presence of the TLS although interestinglyvery little of it exists on the respiratory surface of theadult avian lung (McLelland 1989)

STRENGTHS OF THE BGB AND THE E-ECCsOF THE EXCHANGE TISSUE OF AVIAN LUNG

In the field of engineering structural failure isdefined as loss of load carrying capacity from damage ordeformation of a component a member or a completestructure following overloading or functioning beyondthe highest load-tolerating capacity or strength thresh-old (Feld and Carper 1997 Stephens and Fuchs 2001)One of the perplexing properties of the functional designof the avian lung is that although the BGB is approxi-mately three times thinner than that of a mammal ofequivalent body mass (Maina 1989 2005 Maina et al1989) it tolerates much higher intramural blood pres-sure (Seymour and Blaylock 2000) which is generatedby large hearts with large stroke volumes and cardiacoutputs (Hartmann 1955 Berger and Hart 1974) TheBGB the E-ECCs and the blood-capillary-blood capil-lary connections are largely subjected to four forces (Fig17) These are (a) the intramural pressure which ema-nates from the contractions of the heart muscle (b) sur-face tension which arises from the interactions of themolecules of the water film which lines the surface ofthe AC (c) depending on the posture of the body pos-ture the weight of the lung tissue and that of the bloodin the BCs and (d) the intrapulmonary pressure

Direct estimation of the strengths of the BGB and theE-ECCs in the exchange tissue of the avian lung has

only been directly determined in the domestic fowl Gal-lus gallus variant domesticus (Maina and Sikiru 2013Maina and Jimoh in press) When these forces exceedthose which the tissue components of the BGB and theE-ECCs can tolerate the structures fail (break) (Figs26ndash31) The failures of the BGB and the E-ECCs in thedifferent regions of the lung which are supplied withblood by the four branches of the pulmonary artery (PA)were quantified under different exercise intensities (run-ning on a treadmill) (Maina and Jimoh in press) andperfusion at different pressures (Maina and Sikiru2013) It was observed that (a) the BGB also fails inresting ie nonstressed birds (b) breaks of the BGB-and the E-ECCs increased with increasing exerciseintensities (Fig 32) and perfusion pressures (Fig 33) (c)the numbers of E-ECCs breaks surpassed those of theBGB (Figs 32 and 33) and (d) the numbers of BGB- andE-ECCs breaks in the different regions of the lung corre-lated with the sizes (diameters) of the branches of thePA and the angles at which the branches originatedfrom it The failure of the E-ECCs occurred at an aver-age intramural pressure of 290 kPa while that of theBGB happened at a higher one of 339 kPa (Maina andSikiru 2013) this showed that the BGB is stronger thanthe E-ECCs In the lungs of the rabbit- the dog- andthe horse the BGBs fail at intramural pressures respec-tively of 533 960 and 1386 kPa (West et al 19911993 West and Mathieu 1992 Birks et al 1994Mathieu-Costello et al 1995) While these pressures arehigher than that at which the BGB in the domestic fowl(chicken) lung fails when the pressures are standar-dized (= divided) with the thicknesses of the basementmembrane of the BGB (the component which mostly con-tributes to the strength of the BGB) (Crouch et al 1997West and Mathieu-Costello 1999 Maina and West2005 West 2009) which is 0045 lm thick in the chickenlung (Watson et al 2007) compared with the muchthicker ones of 0174 0319 and 0386 lm in the rabbitthe dog and the horse lungs respectively (Birks et al1994) the tension at which the chickenrsquos pulmonary

Fig 32 Fig 32 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing exercise intensities and at allthe exercise intensities the numbers of E-ECCs breaks exceeded theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

Fig 33 Fig 33 Relative numbers of blood-gas barrier (BGB) breaksand epithelial-epithelial cells connections (E-ECCs) breaks The num-bers of breaks increased with increasing intramural pressures and atall intramural pressures the numbers of E-ECCs breaks surpassed theBGB ones This shows that the E-ECCs are weaker than the BGBBars 6 SE of the mean

THE AC AND THE BC OF THE AVIAN LUNG 1683

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

BGB fails is 28 times higher than that for the rabbitand the dog and 24 times that of the horse

CRITIQUE OF THE CURRENT RESEARCHMETHODS AND FUTURE DIRECTIONS

The strengths of the ACs and the BCs have beendetermined indirectly For the ACs it was inferred fromcompression of the parabronchial exchange tissue(Macklem et al 1979) for the BCs it was deduced afterdoubling the flow of blood to one of the lungs (Powellet al 1985) and for the ACs and the BCs the propertywas appreciated after perfusing the lung at differenthead pressures and assessing the morphologies of thegas exchange units (West et al 2007 2007b Watsonet al 2008) For the last technique in the final stage ofperfusion the lung was fixed with glutaraldehyde formicroscopic examination The effect of fixation on thestrengths of the ACs and the BCs has not been deter-mined It has only been assumed that the biophysical

properties of fixed structural components of theexchange tissue namely the BGB the E-ECCs and theblood capillary-blood capillary connections (Fig 17) donot change on fixation ie they remain the same asthey were in their in-life state This may not be correctIt is known that fixatives change the properties of tis-sues they cause them to harden and shrink (Barnard1976 Mathieu et al 1978 Lee et al 1982) Wheremicroscopic examination of lung tissue at very high lev-els of magnification is not necessary unfixed cryo-frozensections can be viewed under the light microscope

In the mammalian lung perfusion can be performedunder predetermined intrapulmonary pressure (IP) egat the functional residual capacity (FRC) (Fu et al1992) by inflating or deflating the lung through theonly opening the trachea In birds in the course ofapproaching the heart cannulating the pulmonarytrunk and perfusing the lungs it is inevitable that oneor the other of the very delicate air sacs which spreadextensively in the coelomic cavity will be damaged It is

Fig 34ndash37 Fig 34 Latex cast of the lung of the vervet monkey Cercopithecus aethiops showing anacinus with a cluster of alveoli (Al) Fig 35 Latex cast of an alveolus Arrows interalveolar pores Fig 36An alveolus showing a surface which is lined by blood capillaries (stars) Circles interalveolar pores Fig37 Close-up of blood capillaries on the surface of an alveolus (stars)

1684 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

therefore impractical to adjust the IP in the avian respi-ratory system When an air sac is punctured the IP islost and the balance of forces on the two sides of theBGB is offset The effect of loss of the IP on the strengthof the BGB and the E-ECCs is presently unknown How-ever because the avian lung is practically rigid (Joneset al 1985) since it is firmly attached to the vertebraethe ribs and the horizontal septum (King and McLel-land 1984 Maina 2005) the loss of balance of forcesacross the BGB may not significantly affect its strength

The dynamics of the flow of a perfusate in the avianpulmonary vasculature when a bird is placed in a supineposition for a ventral approach and cannulation of theheart is unknown In the human lung (Nyren et al1999) observed that pulmonary perfusion is more uni-form in the prone than in the supine position The rheo-logical property of the fluids used to perfusefix the lungshould be as close to that of blood as possible Because itcontains suspended cells (mainly the red- and the whiteblood cells) blood is a non-Newtonian fluid Duringexperiments ideally heparinized autologous bloodshould be used to perfuse the lung However in mostcases the amount of blood may not be adequate If phos-phate buffered saline (PBS) has to be used a plasmaexpander like Dextran T70 should be used to increasethe viscosity of the PBS

In future other methods and techniques should beattempted to study and assess the strengths of the ACsand the BCs specifically the BGB These include the fol-lowing (a) creation of satisfactory 3D computer modelsof the ACs the BCs and the BGB which can be studiedby appropriate stress and tension engineering testingsoftwareprograms This technique was found to behighly informative in investigation the flow of air in thecomplicated airways of the avian lung by computationalfluid dynamics (CFD) (Maina et al 2009) and (b) directbiomechanical tests should be attempted on tissue cul-tured ie in vitro prepared satisfactory BGB prepara-tions of the avian lung Ultrastructurally normal BGB ofthe mammalian lung has been cultured by eg Her-manns et al (2004) Ehrhardt et al (2008) and Mahtoet al (2012)

CONCLUSIONS

The ACs are rather globular units which interconnectby short and narrow passageways Interestingly theshape of the globular part of an AC (Figs 11 and 12)resembles that of an alveolus (Figs 34 and 35) while theinterconnecting passageways are similar to the interal-veolar pores (Figs 35 and 36) Although the similaritymay be purely coincidental it is possible that the corre-sponding morphology may show a certain degree of evo-lutionary convergence in the design of the forms of theterminal respiratory units in the gas exchangers of theendothermic vertebrates The BCs of the avian lungcomprise of segments which are about as long as theyare wide and are coupled in 3D (Figs 1ndash4) They differfrom those of the mammalian lung where the BCslargely spread on the 2D surface of the interalveolar sep-tum (Meurouhlfeld et al 2010) (Figs 36 and 37) Such anarrangement was described by West et al (1977) as ldquoaflat sheet-like arrangementrdquo In terms of their biome-chanical properties the BCs and the ACs of the avianlung are very strong The origins of their strengths are

diverse and currently uncertain Further research isneeded to explain it The immunological demonstrationof presence of the exceptionally strong type IV collagenin the basement membrane of the BGB and the E-ECCsof the exchange tissue of the avian lung by Jimoh andMaina (2012) is the first direct and unequivocal demon-stration of one of the causes of the strength of the bloodcapillaries and the air capillaries With the new under-standing of the morphologies of the ACs and BCs theterms ldquoair capillaryrdquo and ldquoblood capillaryrdquo are inappro-priate They should be revised by giving them moreappropriately descriptive terms like ldquoavipulmosacculesrdquofor the air capillaries and avipulmoreteformis for theblood capillaries

ACKNOWLEDGEMENTS

The authors are grateful to the two anonymousreviewers whose comments and suggestions greatlyhelped improve the article

LITERATURE CITED

Abdalla MA 1989 The blood supply to the lung In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 281ndash306

Akester AR 1970a The comparative anatomy of the respiratorypathways in the domestic fowl (Gallus domesticus) pigeon(Columba livia) and domestic duck (Anas platyrhynchus) J Anat94487ndash505

Akester AR 1970b Osmiophilic inclusion bodies as the sources oflaminated membrane in the epithelial lining of avian tertiarybronchi J Anat 107189ndash190

Barnard T 1976 An empirical relashionship for formulation of glu-taraldehyde based fixatives based on measurements of cell vol-ume change J Ultrastr Res 54478

Baier M 1896 Beitreuroage zur kenntnis der anatomie und physiologieder atemwerkzeuge bei den veuroogeln Z Wiss Zool 61420ndash498

Banks WJ 1986 Applied veterinary histology 2nd ed BaltimoreWilliams and Wilkins

Banzett RB Nations CS Barnas JL Lehr JL Jones JH 1987Inspiratory aerodynamic valving in goose lungs depends on gasdensity and velocity Respir Physiol 70287ndash300

Banzett RB Nations CS Wang N Fredberg JJ Butler PJ 1991Pressure profiles show features essential to aerodynamic valvingin geese Respir Physiol 84295ndash309

Bargmann W Knoop A 1961 Elektronenmikroskopische untersu-chungen an der reptilien und vogellunge Z Zellforsch MikroskAnat 54541ndash548

Bellairs R Osmond M 1998 The atlas of chick development Lon-don Academic Press

Berger M Hart JS 1974 Physiology and energetics of flight InFarner DS King JR editors Avian biology Vol 4 New York Aca-demic Press p 415ndash477

Birks EK Mathieu-Costello O Fu Z Tyler WS West JB 1994Comparative aspects of the strength of pulmonary capillaries inrabbit dog and horse Respir Physiol 97235ndash246

Borst HG McGregor M Whittenberger JL Berglund E 1956 Influ-ence of pulmonary arterial and left arterial pressures on pulmo-nary vascular resistance Circ Res 4393ndash399

Brackenbury JH 1987 Ventilation of the lung-air sac system InSeller TJ editor Bird respiration Vol I Boca Raton CRC Pressp 39ndash79

Brown RE Kovacs CE Butler JP Wang N Lehr J Banzett RB1995 The avian lung is there an aerodynamic expiratory valveJ Exp Biol 1982349ndash2357

Campana A 1875 Anatomie de lrsquoappareil pneumatique-pulmonaireetc chez le poulet Paris Mason

THE AC AND THE BC OF THE AVIAN LUNG 1685

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

Chen WF 1997 Handbook of structural engineering Boca RatonFL CRC Press LLC

Clayton M Philo R 2012 Leonardo da Vinci anatomist the RoyalCollection Trust Rosewood Drive MA The University of ChicagoPress

Clement AM Long JA 2010 Air-breathing adaptation in a marinedevonian lungfish Biol Lett 6509ndash512

Coitier V 1573 Anatomica avium externum et internarum praeci-palium humani corporis partium tabulae arque anatomicae exer-citationes Nuremberg (Germany) p 1ndash183

Cover MS 1953 Gross and microscopic anatomy of the respiratorysystem of the turkey II The larynx trachea syrinx bronchi andlungs Am J Vet Res 14230ndash238

Crank WD Gallanger RR 1978 Theory of gas exchange in theavian parabronchus Respir Physiol 359ndash15

Crouch EC Martin GR Jerome BS Laurie GW 1997 Basementmembranes In Crystal RG West JB Weibel ER Barnes PJeditors The lung scientific foundations Vol I PhiladelphiaLippincott-Raven p 769ndash791

Corral JPD 1995 Anatomy and histology of the lung and air sacsof birds In LM Pastor editor Histology ultrastructure andimmunohistochemistry of the respiratory organs in non-mammalian vertebrtaes Murcia Spain Publicaciones de la Uni-versitatd de University of Murcia p 179ndash233

Dellmann HD Brown M 1987 Textbook of veterinary histology3rd ed Philadelphia Lea and Febiger

Dotterweich H 1930 Versuch eurouberden weg der atemluft in dervogellunge Zeitsch Vergl Physiol 11271ndash284

Duncker HR 1972 Structure of the avian lungs Respir Physiol 1444ndash63

Duncker HR 1989 Structural and functional integration across thereptile-bird transition locomotor and respiratory systems InWake DB Roth G editors Complex organismal functionsintegration and evolution in vertebrates New York Wiley p 147ndash169

Duncker HR Geurountert M 1989 The quantitative design of the avianrespiratory system- from hummingbird to mute swan In Nachti-gall W editor BIONA report 3 Fischer Stuttgart p 361ndash378

Eberth CJ 1863 euroUber den feineren bau der lunge Z Wiss Zool 12427ndash454

Ehrhardt C Laue M Kim KJ 2008 In vitro models of the alveolarepithelial barrier Drug Absorp Stud 7258ndash282

Faffe BS Walter AZ 2009 Lung parenchymal mechanics in healthand disease Physiol Rev 89759ndash775

Farner DS 1970 Some glimpses of comparative avian physiologyFed Proc 291649ndash1663

Fawcett DW 1998 Bloom and Fawcett a texbook of histology 12thed Philadelphia WB Saunders

Fedde MR 1980 The structure and gas flow pattern in the avianlung Poult Sci 592642ndash2653

Feld J Carper KL 1997 Construction failure New York WileyFischer G 1905 Vergleichende anatomische Untersuchungen eurouber

den Bronchialbaum der Veuroogel Zoologica 191ndash46Fu Z Costello ML Tsukimoto K Prediletto R Elliot AR Mathieu-

Costello O West JB 1992 High lung volume increases stress fail-ure in pulmonary capillaries J Appl Physiol 73123ndash133

Fujiwara T Adams FH Nozaki M Dermer GB 1970 Pulmonarysurfactant phospholipids from turkey lung comparison with rab-bit lung Am J Physiol 218218ndash225

Fuller B 1961 Tensegrity Portfolio Art News Annual 4112ndash127Gartner LP Hiatt JL Strum JM 2011 BRS cell biology and histol-

ogy Philadelphia PA Lippincott Williams and WilkinsGehr P Mwangi DK Amman A Maloiy GMO Taylor CR Weibel

ER 1981 Design of the mammalian respiratory system V Scal-ing morphometric diffusing capacity to body mass wild anddomestic animals Respir Physiol 4461ndash86

Glazier JB Hughes JMB Maloney JE West JB 1969 Measure-ments of capillary dimensions and blood volume in rapidly frozenlungs J Appl Physiol 2665ndash76

Haddy FG Campbell GS 1953 Pulmonary vascular resistance inanaesthetised dogs Am J Physiol 172747

Hartmann FA 1955 Heart weight in birds Condor 57221ndash238

Hermanns MI Unger RE Kehe K Peters K Kirkpatrick CJ 2004Lung epithelial cell lines in coculture with human pulmonarymicrovascular endothelial cells development of an alveolo-capillary barrier in vitro Lab Invest 84736ndash752

Hermida GN Fiorito LE Farıas A 1998 The lung of the commontoad Bufo arenum (Anura Bufonidae) A light and electronmicroscopy study Biocell 2219ndash26

Huxley TH 1882 on the respiratory organ of apteryx Proc ZoolSoc Lond 1882560ndash569

Hyde DM Tyler NK Putney LF Singh P Gundersen HJ 2004Total number and mean size of alveoli in mammalian lungestimated using fractionator sampling and unbiased estimatesof the euler characteristic of alveolar openings Anat Rec 277216ndash226

Ingber DE 1998 The architecture of life Sci Am 27818ndash57Jimoh SA Maina JN 2012 Immuno-localization of type-IV collagen

in the blood-gas barrier and the epithelialndashepithelial cell connec-tions of the avian lung Biol Lett 920120951 httpdxdoiorg101098rsbl20120951

Jones JH Effmann EL Schmidt-Nielsen K 1985 Lung volumechanges during respiration in ducks Respir Physiol 5915ndash25

King AS 1966 Structural and functional aspects of the avian lungand its air sacs Intern Rev Gen Exp Zool 2171ndash267

King AS McLelland 1984 Birds their structure and function Lon-don Bailliere Tindall

King AS Morony V 1971 The anatomy of respiration In Bell DJFreeman BM editors Physiology and biochemistry of the domes-tic fowl Vol I London Academic Press p 93ndash169

Klika E Scheuermann DW de Groodt-Lasseel MHA Bazantova ISwitka A 1997 Anchoring and support system of pulmonary gasexchange in four species of birds Acta Anat 15930ndash41

Knust J Ochs M Gundersen JG Nyengaard JR 2009 Stereologicalestimates of alveolar number and size and capillary length andsurface area in mice lungs Anat Rec 292113ndash122

Krause R 1922 Mikroskopische Anatomie der Wirbeltiere in Ein-zeldarstellungen 11 Veuroogel und Reptilien Berlin-Leipzig DeGruyter and Co

Kuehne B Junqueira LC 2000 Histology of the trachea and lungof Siphonops annulatus (Amphibia Gymnophiona) Rev BrasilBiol 60167ndash172

Kuethe DO 1988 Fluid mechanical valving of air flow in birdJ Exp Biol 1361ndash12

Lee RMKW McKenzie R Kobayashi K Garfield RE Forrest JBDaniel EE 1982 Effects of glutaraldehyde fixative osmolaritieson muscle cell volume and osmotic reactivity of the cells after fix-ation J Microsc 12577ndash78

Leeson TS Leeson CR Paparo AA 1988 Textatlas of histologyPhiladelphia Saunders

Lloyd TC Wright GW 1960 Pulmonary vascular resistance andvascular transmural gradient J Appl Physiol 15241ndash245

MacDonald JW 1970 Observations on the histology of the lung ofGallus domesticus Br Vet J 12689ndash93

Macklem PT Bouverot P Scheid P 1979 Measurement of the dis-tensibility of the parabronchus in duck lungs Respir Physiol 3823ndash35

Mahto SK Tenenbaum-Katan J Sznitman J 2012 Respiratoryphysiology on a chip Scientica 20121ndash12 httpdxdoiorg1060642012364054

Maina JN 1989 The morphometry of the avian lung In King ASMcLelland J editors Form and function in birds Vol4 LondonAcademic Press p 307ndash368

Maina JN 1998 The gas exchangers structure function and evo-lution of the respiratory processes Heidelberg Springer-Verlag

Maina JN 2005 The design of the lung-air sac system of birds evo-lution development structure and function Heidelberg SpringerVerlag

Maina JN 2006 Development structure and function of a novelrespiratory organ the lung-air sac system of birds to go whereno other vertebrate has gone Biol Rev 81545ndash579

Maina JN 2007a Spectacularly robust Tensegrity principleexplains the mechanical strength of the avian lung Respir Phys-iol Neurobiol 1551ndash10

1686 MAINA

Maina JN 2007b Minutialization at its extreme best The under-pinnings of the remarkable strengths of the air- and the bloodcapillaries of the avian lung a conundrum Respir Physiol Neuro-biol 159141ndash145

Maina JN 2008 Structure of the air- and blood capillaries of theavian lung and the debate regarding the basis of their astoundingstrengths In Morris S Vosloo A editors Proceedings of the 4thComparative Physiology and Biochemistry Conference in AfricaMaasai Mara Game Reserve Kenya Bologna Italy MedimondSrl via Maserati p 304ndash313

Maina JN 2011 Bioengineering aspects in the design of gasexchangers comparative evolutionary morphological functionaland molecular perspectives Heidelberg Springer-Verlag

Maina JN Jimoh SA 2012 Structural failures of the blood-gas bar-rier and the epithelial-epithelial cell connections in the differentvascular regions of the lung of the domestic fowl Gallus gallusvariant domesticus at rest and during exercise BioOpen 2267ndash276

Maina JN Jimoh SA Hosie M 2010 Implicit mechanistic role ofthe collagen- smooth muscle and elastic tissue components instrengthening the air- and the blood capillaries of the avian lungJ Anat 217597ndash608

Maina JN King AS 1982 The thickness of the avian blood-gas bar-rier qualitative and quantitative observations J Anat 134553ndash562

Maina JN King AS Settle G 1989 An allometric study of the pul-monary morphometric parameters in birds with mammalian com-parison Philos Trans R Soc Lond 3261ndash57

Maina JN Nathaniel C 2001 A qualitative and quantitative studyof the lung of an ostrich Struthio camelus J Exp Biol 2042313ndash2330

Maina JN Sikiru AJ 2013 Study of stress induced failure ofthe blood-gas barrier and the epithelial-epithelial cells connec-tions of the lung of the domestic fowl Gallus gallus variantdomesticus after vascular perfusion Biomed Eng Comp Biol 577ndash88

Maina JN Singh P Moss EA 2009 Inspiratory aerodynamic valv-ing occurs in the ostrich Struthio camelus lung computationalfluid dynamics study under resting unsteady state inhalationRespir Physiol Neurobiol 169262ndash270

Maina JN West JB 2005 Thin and strong The bioengineeringdilemma in the structural and functional design of the blood-gasbarrier Physiol Rev 85811ndash844

Maina JN Woodward JD 2009 Three-dimensional serial sectioncomputer reconstruction of the arrangement of the structuralcomponents of the parabronchus of the ostrich Struthio cameluslung Anat Rec 2921685ndash1698

Makanya AN El-Darawish Y Kavoi BM Djonov V 2011 Spatialand functional relationships between air conduits and blood capil-laries in the pulmonary gas exchange tissue of adult and develop-ing chickens Microsc Res Tech 74159ndash169

Makanya AN Hlushchuk R Baum O Velinov N Ochs M Djonov V2007 Microvascular endowment in the developing chickenembryo lung Am J Physiol Lung Cell Mol Physiol 292L1136ndashL1146

Malpighi M 1661 De pulmonibus Philos Trans R Soc Lond 16611ndash9

Mathieu O Claassen H Weibel ER 1978 Differential effect of glu-taraldehyde and buffer osmolarity on cell dimensions a study oflung tissue J Ultrastr Res 6320ndash34

Mathieu-Costello O Willford DC Fu Z Garden RM West JB 1995Pulmonary capillaries are more resistant to stress failure in dogsthan in rabbits J Appl Physiol 79908ndash917

Mclelland J 1989 Anatomy of the lungs and air sacs In King ASMcLelland J editors Form and function in birds Vol 4 LondonAcademic Press p 220ndash278

Meban C 1980 Thicknesses of the air-blood barriers in vertebratelungs J Anat 131299ndash307

Miller DN Bondurant S 1961 Surface characteristics of vertebratelung extracts J Appl Physiol 161075ndash1077

Meurouhlfeld C Weibel ER Hahn U Kummer W Nyengaard JR OchsM 2010 Is length an appropriate estimator to characterize pul-

monary alveolar capillaries A critical evaluation in the humanlung Anat Rec 2931270ndash1275

Nasu T 2005 Scanning electron microscopic study of the microarch-itecture of the vascular system in the pigeon lung J Vet Med Sci671071ndash1074

Nyren S Mure M Jacobsson H Larson SA Lindahl SGE 1999Pulmonary perfusion is more uniform in the prone than in thesupine position scintigraphy in healthy humans J Appl Physiol861135ndash1141

Ochs M Nyengaard JR Jung A Knudsen L Voigt M Wahlers TRichter J Gundersen HJG 2004 The number of alveoli in thehuman lung Am J Respir Crit Care Med 169120ndash124

Pattle RE 1976 The lung surfactant in the evolutionary tree InHughes GM editor Respiration of amphibious vertebrates Lon-don Academic Press p 233ndash255

Pattle RE 1978 Lung surfactant and lung lining in birds InPiiper J editor Respiratory function in birds adult and embry-onic Berlin Springer-Verlag p 23ndash32

Petrik P Reidel B 1968 A continupus osmiophilic bilaminar liningfilm at the respiratory surfaces of the avian lung Z F Zellforsch88204ndash219

Pinkerton KE Joad JP 2000 The mammalian respiratory systemand critical windows of exposure for childrenrsquos health EnvironHealth Perspect 108457ndash462

Powell FL Hastings RH Mazzone RW 1985 Pulmonary vascularresistance during unilateral pulmonary artery occlusion in ducksAm J Physiol 249R34ndashR43

Powell FL Scheid P 1989 Physiology of gas exchange in the avianrespiratory system In King AS McLelland J editors Form andfunction of the avian lung Vol 4 Academic Press London p393ndash437

Rainey G 1849 On the minute anatomy of the lung of the birdconsidered chiefly in relation to the structure with which the airis in contact whilst traversing the ultimate subdivisions of the airpassages Med Chir Trans 8247ndash58

Roos A Thomas LJ Nagel EL Prommas DC 1961 Pulmonary vas-cular resistance as determined by lung inflation and vascularpressures Am J Physiol 1677ndash84

Ryan SF Dumais C Ciannella A 1969 The structure of the interal-veolar septum of the mammalian lung Anat Rec 165467ndash483

Scheid P 1979 Mechanisms of gas exchange in bird lungs RevPhysiol Biochem Pharmacol 86137ndash186

Scheid P Piiper J 1972 Crosscurrrent gas exchange in the avianlungs effects of reversed parabronchial air flow in ducks RespirPhysiol 16304ndash312

Scheuermann DW Klika E De Groodt-Lasseel MH Bazantova ISwitka A 2000 Lamellar inclusions and trilaminar substance inthe parabronchial epithelium of the quail (Coturnix coturnix)Ann Anat 182221ndash233

Scheurmann DW Klika E Lasseel DG Bazantova I Switka A1997 An electron microscopic study of the parabronchial epithe-lium in the mature lung of four bird species Anat Rec 249213ndash225

Schmidt-Nielsen K 1971 How birds breathe Sci Am 22573ndash79Schulze FE 1908 Die lungen des afrikanisches strauszliges S-B Preus

Akad Wiss 1416ndash431Seymour RS Blaylock AJ 2000 The principle of laplace and scaling

of ventricular wall stress and blood pressure in mammals andbirds Physiol Biochem Zool 73389ndash405

Stephens RI Fuchs HO editors 2001 Metal fatigue in engineering2nd ed New York Wiley

Stromberg DD Wiederhielm CA 1969 Viscoelastic description of acollagenous tissue in simple elongation J Appl Physiol 26857ndash862 [PMC][5786422]

Tenney SM Remmers JE 1963 Comparative quantitative morphol-ogy of the mammalian lung diffusing area Nature (Lond) 19754ndash56

Trautmann A Fiebiger J 1957 The histology of the domestic ani-mals 2nd ed Ithaca NY Cornell University Press

Tyler WS Pangborn J 1964 Laminated membrane surface andosmiophilic inclusions in avian lung epithelium J Cell Biol 20157ndash164

THE AC AND THE BC OF THE AVIAN LUNG 1687

Vos HJ 1934 euroUber die wege der atemluft in der entenlunge ZeischVergl Physiol 21552ndash578

Wang N Banzett RB Butler JP Fredberg JJ 1988 Bird lung mod-els show that convective inertia effects inspiratory aerodynamicvalving Respir Physiol 73111ndash124

Wang N Banzett RB Nations CS Jenkins EA 1992 An aerodynamicvalve in the avian primary bronchus J Exp Biol 262441ndash445

Watson RR Fu Z West JB 2007 Morphometry of the extremelythin pulmonary blood-gas barrier in the chicken lung Am J Phys-iol Lung Cell Mol Physiol 292L769ndashL777

Watson RR Fu Z West JB 2008 Minimal distensibility of pulmo-nary capillaries in avian lungs compared with mammalian lungsRespir Physiol Neurobiol 160208ndash214

Weibel ER 1984 The pathway for oxygen structure and functionin the mammalian respiratory system Cambridge MA HarvardUniversity Press

Weibel ER Knight BW 1964 A morphometric study on the thick-ness of the pulmonary air-blood barrier J Cell Biol 21367ndash384

West JB 2009 Comparative physiology of the pulmonanary blood-gas barrier the unique avian solution Am J Physiol Regul IntegrComp Physiol 297R1625ndashR1634

West JB 2013 Marcello malphigi and the discovery of the pulmo-nary capillaries and alveoli Am J Physiol Lung Cell Mol Physiol304L383ndashL390

West JB Fu Z Deerinck T Mackey MR Obayashi JT EllismanMH 2010 Structure-function studies and air capillaries inchicken lung using 3D electron microscopy Respir Physiol Neuro-biol 170202

West JB Mathieu-Costello O 1992 Strength of pulmonary blood-gas barrier Respir Physiol 88141ndash148

West JB Mathieu-Costello O 1999 Structure strength failureand re-modeling of the pulmonary blood-gas barrier Annu RevPhysiol 61543ndash572

West JB Mathieu-Costello O Jones JH Birks EK Logerman RBPascoe JR Tyler WS 1993 Stress failure of pulmonary capillariesin racehorses with exercise-induced pulmonary haemorrhageJ Appl Physiol 751097ndash1109

West JB Tsukimoto K Mathieu-Costello O Prediletto R 1991 Stressfailure in pulmonary capillaries J Appl Physiol 701731ndash1742

West JB Watson RR Fu Z 2006 The honeycomb-like structure ofthe bird lung allows a uniquely thin blood-gas barrier RespirPhysiol Neurobiol 152115ndash118

West JB Watson RR Fu Z 2007 Support of pulmonary capillariesin avian lung Respir Physiol Neurobiol 159146 doi101016jresp200708006

West JB Watson RR Fu Z 2007b Major differences in the pulmo-nary circulation between birds and mammals Respir PhysiolNeurobiol 157382ndash390

West NH Bamford OS Jones DR 1977 A scanning electron micro-scope study of the microvasculature of the avian lung Cell TissRes 176553ndash564

Woodward JD Maina JN 2005 A 3-ddigital reconstruction of thecomponents of the gas exchange tissue of the lung of the muscovyduck Cairina moschata J Anat 206477ndash492

Woodward JD Maina JN 2008 Study of the structure of the air andblood capillaries of the gas exchange tissue of the avian lung by serialsection three-dimensional reconstruction J Microsc 23084ndash93

Young B Woodford B OrsquoDowd G 2013 Wheaterrsquos functional histol-ogy a text and colour atlas Philadelphia PA ElsevierChurchillLivingstone

1688 MAINA

Vos HJ 1934 euroUber die wege der atemluft in der entenlunge ZeischVergl Physiol 21552ndash578

Wang N Banzett RB Butler JP Fredberg JJ 1988 Bird lung mod-els show that convective inertia effects inspiratory aerodynamicvalving Respir Physiol 73111ndash124

Wang N Banzett RB Nations CS Jenkins EA 1992 An aerodynamicvalve in the avian primary bronchus J Exp Biol 262441ndash445

Watson RR Fu Z West JB 2007 Morphometry of the extremelythin pulmonary blood-gas barrier in the chicken lung Am J Phys-iol Lung Cell Mol Physiol 292L769ndashL777

Watson RR Fu Z West JB 2008 Minimal distensibility of pulmo-nary capillaries in avian lungs compared with mammalian lungsRespir Physiol Neurobiol 160208ndash214

Weibel ER 1984 The pathway for oxygen structure and functionin the mammalian respiratory system Cambridge MA HarvardUniversity Press

Weibel ER Knight BW 1964 A morphometric study on the thick-ness of the pulmonary air-blood barrier J Cell Biol 21367ndash384

West JB 2009 Comparative physiology of the pulmonanary blood-gas barrier the unique avian solution Am J Physiol Regul IntegrComp Physiol 297R1625ndashR1634

West JB 2013 Marcello malphigi and the discovery of the pulmo-nary capillaries and alveoli Am J Physiol Lung Cell Mol Physiol304L383ndashL390

West JB Fu Z Deerinck T Mackey MR Obayashi JT EllismanMH 2010 Structure-function studies and air capillaries inchicken lung using 3D electron microscopy Respir Physiol Neuro-biol 170202

West JB Mathieu-Costello O 1992 Strength of pulmonary blood-gas barrier Respir Physiol 88141ndash148

West JB Mathieu-Costello O 1999 Structure strength failureand re-modeling of the pulmonary blood-gas barrier Annu RevPhysiol 61543ndash572

West JB Mathieu-Costello O Jones JH Birks EK Logerman RBPascoe JR Tyler WS 1993 Stress failure of pulmonary capillariesin racehorses with exercise-induced pulmonary haemorrhageJ Appl Physiol 751097ndash1109

West JB Tsukimoto K Mathieu-Costello O Prediletto R 1991 Stressfailure in pulmonary capillaries J Appl Physiol 701731ndash1742

West JB Watson RR Fu Z 2006 The honeycomb-like structure ofthe bird lung allows a uniquely thin blood-gas barrier RespirPhysiol Neurobiol 152115ndash118

West JB Watson RR Fu Z 2007 Support of pulmonary capillariesin avian lung Respir Physiol Neurobiol 159146 doi101016jresp200708006

West JB Watson RR Fu Z 2007b Major differences in the pulmo-nary circulation between birds and mammals Respir PhysiolNeurobiol 157382ndash390

West NH Bamford OS Jones DR 1977 A scanning electron micro-scope study of the microvasculature of the avian lung Cell TissRes 176553ndash564

Woodward JD Maina JN 2005 A 3-ddigital reconstruction of thecomponents of the gas exchange tissue of the lung of the muscovyduck Cairina moschata J Anat 206477ndash492

Woodward JD Maina JN 2008 Study of the structure of the air andblood capillaries of the gas exchange tissue of the avian lung by serialsection three-dimensional reconstruction J Microsc 23084ndash93

Young B Woodford B OrsquoDowd G 2013 Wheaterrsquos functional histol-ogy a text and colour atlas Philadelphia PA ElsevierChurchillLivingstone

1688 MAINA