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Circulation and gas exchange
• Overview
• Circulation
• Gas exchange
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Overview
• Overview: Trading with the Environment
• Every organism must exchange materials with its environment
– And this exchange ultimately occurs at the cellular level
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• In unicellular organisms
– These exchanges occur directly with the environment
• For most of the cells making up multicellular organisms
– Direct exchange with the environment is not possible
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• The feathery gills projecting from a salmon
– Are an example of a specialized exchange system found in animals
Figure 42.1
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Circulation
• Overview
• Circulatory systems
– Open and closed
– Circulation in invertebrates
– Circulation in vertebrates
• Mammalian circulation system
– The Pathway
– The Pumping
– The Movement
– The Exchange
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Circulation
• Diffusion alone is not adequate for transporting substances over long distances in animals
– Time for a substance to diffuse is proportional to the square of the distance
• Transport systems
– Functionally connect the organs of exchange with the body cells
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• Most complex animals have internal transport systems
– That circulate fluid, providing a lifeline between the aqueous environment of living cells and the exchange organs, such as lungs, that exchange chemicals with the outside environment
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Gastrovascular Cavities
• Simple animals, such as cnidarians
– Have a body wall only two cells thick that encloses a gastrovascular cavity
• The gastrovascular cavity
– Functions in both digestion and distribution of substances throughout the body
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• Some cnidarians, such as jellies
– Have elaborate gastrovascular cavities
Figure 42.2
Circularcanal
Radial canal
5 cmMouth
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Open and Closed Circulatory Systems
• More complex animals
– Have one of two types of circulatory systems: open or closed
• Both of these types of systems have three basic components
– A circulatory fluid (blood)
– A set of tubes (blood vessels)
– A muscular pump (the heart)
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• In insects, other arthropods, and most molluscs
– Blood bathes the organs directly in an open circulatory system
Heart
Hemolymph in sinusessurrounding ograns
Anterior vessel
Tubular heart
Lateral vessels
Ostia
(a) An open circulatory systemFigure 42.3a
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• In a closed circulatory system
– Blood is confined to vessels and is distinct from the interstitial fluid
Figure 42.3b
Interstitialfluid
Heart
Small branch vessels in each organ
Dorsal vessel(main heart)
Ventral vesselsAuxiliary hearts
(b) A closed circulatory system
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• Open systems
– Less energetically costly
– Can function as a hydrostatic skeleton
• Closed systems
– Are more efficient at transporting circulatory fluids to tissues and cells
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Survey of Vertebrate Circulation
• Humans and other vertebrates have a closed circulatory system
– Often called the cardiovascular system
• Blood flows in a closed cardiovascular system
– Consisting of blood vessels and a two- to four-chambered heart
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• Arteries carry blood to capillaries
– The sites of chemical exchange between the blood and interstitial fluid
• Veins
– Return blood from capillaries to the heart
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FISHES AMPHIBIANS REPTILES (EXCEPT BIRDS) MAMMALS AND BIRDS
Systemic capillaries Systemic capillaries Systemic capillaries Systemic capillaries
Lung capillaries Lung capillariesLung and skin capillariesGill capillaries
Right Left Right Left Right Left Systemic
circuitSystemic
circuit
Pulmocutaneouscircuit
Pulmonarycircuit
Pulmonarycircuit
SystemiccirculationVein
Atrium (A)
Heart:ventricle (V)
Artery Gillcirculation
A
V VV VV
A A A AALeft Systemicaorta
Right systemicaorta
Figure 42.4
• Vertebrate circulatory systems
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Fishes
• A fish heart has two main chambers
– One ventricle and one atrium
• Blood pumped from the ventricle
– Travels to the gills, where it picks up O2 and disposes of CO2
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Amphibians
• Frogs and other amphibians
– Have a three-chambered heart, with two atria and one ventricle
• The ventricle pumps blood into a forked artery
– That splits the ventricle’s output into the pulmocutaneous circuit and the systemic circuit
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Reptiles (Except Birds)
• Reptiles have double circulation
– With a pulmonary circuit (lungs) and a systemic circuit
• Turtles, snakes, and lizards
– Have a three-chambered heart
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Mammals and Birds
• In all mammals and birds
– The ventricle is completely divided into separate right and left chambers
• The left side of the heart pumps and receives only oxygen-rich blood
– While the right side receives and pumps only oxygen-poor blood
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• A powerful four-chambered heart
– Was an essential adaptation of the endothermic way of life characteristic of mammals and birds
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Mammalian Circulation: The Pathway
• The mammalian cardiovascular system
Pulmonary vein
Right atrium
Right ventricle
Posteriorvena cava Capillaries of
abdominal organsand hind limbs
Aorta
Left ventricle
Left atriumPulmonary vein
Pulmonaryartery
Capillariesof left lung
Capillaries ofhead and forelimbs
Anteriorvena cava
Pulmonaryartery
Capillariesof right lung
Aorta
Figure 42.5
1
10
11
5
4
6
2
9
33
7
8
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• Heart valves
– Dictate a one-way flow of blood through the heart
• Blood begins its flow
– With the right ventricle pumping blood to the lungs
• In the lungs
– The blood loads O2 and unloads CO2
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• Oxygen-rich blood from the lungs
– Enters the heart at the left atrium and is pumped to the body tissues by the left ventricle
• Blood returns to the heart
– Through the right atrium
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Mammalian Circulation: The Pumping
• The heart contracts and relaxes
– In a rhythmic cycle called the cardiac cycle
• The contraction, or pumping, phase of the cycle
– Is called systole
• The relaxation, or filling, phase of the cycle
– Is called diastole
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• The heart rate, also called the pulse
– Is the number of beats per minute
• The cardiac output
– Is the volume of blood pumped into the systemic circulation per minute
• It depends on the heart rate and stroke volume
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• Some cardiac muscle cells are self-excitable
– Meaning they contract without any signal from the nervous system
• A region of the heart called the sinoatrial (SA) node, or pacemaker
– Sets the rate and timing at which all cardiac muscle cells contract
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• The SA node generates electrical impulses that spread rapidly through the wall of the atria
• Purkinje fibers conduct the signals to the apex of the heart and throughout ventricular walls
• This stimulates the ventricles to contract from the apex toward the atria, driving blood to the arteries
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• The pacemaker is influenced by
– Nerves, hormones, body temperature, and exercise
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Mammalian Circulation: The Movement
• Physical principles govern blood circulation
• The same physical principles that govern the movement of water in plumbing systems
– Also influence the functioning of animal circulatory systems
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• The “infrastructure” of the circulatory system
– Is its network of blood vessels
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• All blood vessels
– Are built of similar tissues
– Have three similar layers
Figure 42.9
Artery Vein
100 µm
Artery Vein
ArterioleVenule
Connectivetissue
Smoothmuscle
Endothelium
Connectivetissue
Smoothmuscle
EndotheliumValve
Endothelium
Basementmembrane
Capillary
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• Structural differences in arteries, veins, and capillaries
– Correlate with their different functions
• Capillaries have very thin walls
– To enhance exchange
• Arteries have thicker walls
– To accommodate the high pressure of blood pumped from the heart
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• In the thinner-walled veins
– Blood flows back to the heart mainly as a result of muscle action
Figure 42.10
Direction of blood flowin vein (toward heart)
Valve (open)
Skeletal muscle
Valve (closed)
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Blood Flow Velocity
• Physical laws governing the movement of fluids through pipes
– Influence blood flow and blood pressure
• Blood travels over a thousand times faster in the aorta than in capillaries
• Law of continuity
– Fluid flows through narrower segments of the pipe faster than it flows through wider segments
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• The velocity of blood flow varies in the circulatory system
– And is slowest in the capillary beds as a result of the large total cross-sectional area
Figure 42.11
5,0004,0003,0002,0001,000
0
Aor
ta
Art
erie
s
Art
erio
les
Cap
illar
ies
Ven
ules
Vei
ns
Ven
ae c
avae
Pre
ssur
e (m
m H
g)V
eloc
ity (
cm/s
ec)
Are
a (c
m2)
Systolicpressure
Diastolicpressure
50403020100
120100806040200
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Blood Pressure
• Fluids exert a force called hydrostatic pressure
– Fluids flow from areas of high to low pressure
• Blood pressure
– Is the hydrostatic pressure that blood exerts against the wall of a vessel; it is much greater in the arteries when the heart contracts during systole
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• Surge of pressure felt during pulse is due to narrow openings of arterioles impeding the exit of blood from arteries, the peripheral resistance
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• Blood pressure is determined partly by cardiac output
– And partly by peripheral resistance due to variable constriction of the arterioles
• Nerve impulses, hormones and other signals control the arteriole wall muscles
– Stress, both physical and emotional can raise blood pressure by triggering nervous and hormonal responses that will constrict blood vessels
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• Cardiac output is adjusted in concert with changes in peripheral resistance
– During heavy exercise, arterioles in working muscles dilate admitting greater flow of oxygen and decreasing peripheral resistance
– At the same time cardiac output increases
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Capillary Function
• Capillaries in major organs are usually filled to capacity
– But in many other sites, the blood supply varies
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• Two mechanisms
– Regulate the distribution of blood in capillary beds
• In one mechanism
– Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel
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• In a second mechanism
– Precapillary sphincters control the flow of blood between arterioles and venules
Figure 42.13 a–c
Precapillary sphincters Thoroughfarechannel
ArterioleCapillaries
Venule(a) Sphincters relaxed
(b) Sphincters contractedVenuleArteriole
(c) Capillaries and larger vessels (SEM)
20 m
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Mammalian Circulation: The Exchange
• The critical exchange of substances between the blood and interstitial fluid
– Takes place across the thin endothelial walls of the capillaries
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• The difference between blood pressure and osmotic pressure
– Drives fluids out of capillaries at the arteriole end and into capillaries at the venule end
At the arterial end of acapillary, blood pressure is
greater than osmotic pressure,and fluid flows out of the
capillary into the interstitial fluid.
Capillary Redbloodcell
15 m
Tissue cell INTERSTITIAL FLUID
CapillaryNet fluidmovement out
Net fluidmovement in
Direction of blood flow
Blood pressureOsmotic pressure
Inward flow
Outward flow
Pre
ssur
e
Arterial end of capillary Venule end
At the venule end of a capillary, blood pressure is less than osmotic pressure, and fluid flows from the interstitial fluid into the capillary.
Figure 42.14
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Fluid Return by the Lymphatic System
• The lymphatic system
– Returns fluid to the body from the capillary beds
– Aids in body defense
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• Fluid reenters the circulation
– Directly at the venous end of the capillary bed and indirectly through the lymphatic system
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Gas exchange
• Respiratory surfaces
• Respiratory adaptations for aquatic animals
• Respiratory adaptations for terrestrial animals
• Breathing across vertebrates
• Diffusion of gases
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• Gas exchange occurs across specialized respiratory surfaces
• Gas exchange
– Supplies oxygen for cellular respiration and disposes of carbon dioxide
Figure 42.19
Organismal level
Cellular level
Circulatory system
Cellular respiration ATPEnergy-richmoleculesfrom food
Respiratorysurface
Respiratorymedium(air of water)
O2 CO2
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• Animals require large, moist respiratory surfaces for the adequate diffusion of respiratory gases
– Between their cells and the respiratory medium, either air or water
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• For simple animals, plasma membrane of every cell is close enough to outside environment for gases to diffuse in and out
• Most animals, the bulk of the body lacks direct access to the respiratory medium
– Some animals like earthworms and amphibians use entire outer skin as a respiratory organ, how?
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Gills in Aquatic Animals
• Gills are outfoldings of the body surface
– Specialized for gas exchange
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• In some invertebrates
– The gills have a simple shape and are distributed over much of the body
(a) Sea star. The gills of a sea star are simple tubular projections of the skin. The hollow core of each gillis an extension of the coelom(body cavity). Gas exchangeoccurs by diffusion across thegill surfaces, and fluid in thecoelom circulates in and out ofthe gills, aiding gas transport. The surfaces of a sea star’s tube feet also function in gas exchange.
Gills
Tube foot
Coelom
Figure 42.20a
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• Many segmented worms have flaplike gills
– That extend from each segment of their body
Figure 42.20b
(b) Marine worm. Many polychaetes (marine worms of the phylum Annelida) have a pair of flattened appendages called parapodia on each body segment. The parapodia serve as gillsand also function incrawling and swimming.
Gill
Parapodia
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• The gills of clams, crayfish, and many other animals
– Are restricted to a local body region
Figure 42.20c, d
(d) Crayfish. Crayfish and other crustaceanshave long, feathery gills covered by the exoskeleton. Specialized body appendagesdrive water over the gill surfaces.
(c) Scallop. The gills of a scallop are long, flattened plates that project from themain body mass inside the hard shell.Cilia on the gills circulate water around the gill surfaces.
Gills
Gills
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• The effectiveness of gas exchange in some gills, including those of fishes
– Is increased by ventilation and countercurrent flow of blood and water
Countercurrent exchange
Figure 42.21
Gill arch
Water flow Operculum
Gill arch
Blood vessel
Gillfilaments
Oxygen-poorblood
Oxygen-richblood
Water flowover lamellaeshowing % O2
Blood flowthrough capillariesin lamellaeshowing % O2
Lamella
100%
40%
70%
15%
90%
60%
30% 5%
O2
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Figure 42.22a
Tracheae
Air sacs
Spiracle
(a) The respiratory system of an insect consists of branched internaltubes that deliver air directly to body cells. Rings of chitin reinforcethe largest tubes, called tracheae, keeping them from collapsing. Enlarged portions of tracheae form air sacs near organs that require a large supply of oxygen. Air enters the tracheae through openings called spiracles on the insect’s body surface and passes into smaller tubes called tracheoles. The tracheoles are closed and contain fluid(blue-gray). When the animal is active and is using more O2, most ofthe fluid is withdrawn into the body. This increases the surface area of air in contact with cells.
Tracheal Systems in Insects
• The tracheal system of insects
– Consists of tiny branching tubes that penetrate the body
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• The tracheal tubes
– Supply O2 directly to body cells
Airsac
Body cell
Trachea
Tracheole
TracheolesMitochondria
Myofibrils
Body wall
(b) This micrograph shows crosssections of tracheoles in a tinypiece of insect flight muscle (TEM).Each of the numerous mitochondriain the muscle cells lies within about5 µm of a tracheole.
Figure 42.22b 2.5 µm
Air
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Lungs
• Spiders, land snails, and most terrestrial vertebrates
– Have internal lungs
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Mammalian Respiratory Systems: A Closer Look
• A system of branching ducts
– Conveys air to the lungsBranch from the pulmonary vein (oxygen-rich blood) Terminal bronchiole
Branch from thepulmonaryartery(oxygen-poor blood)
Alveoli
Colorized SEMSEM
50 µ
m
50 µ
m
Heart
Left lung
Nasalcavity
Pharynx
Larynx
Diaphragm
Bronchiole
Bronchus
Right lung
Trachea
Esophagus
Figure 42.23
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• Breathing ventilates the lungs
• The process that ventilates the lungs is breathing
– The alternate inhalation and exhalation of air
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How an Amphibian Breathes
• An amphibian such as a frog
– Ventilates its lungs by positive pressure breathing, which forces air down the trachea
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How a Mammal Breathes
• Mammals ventilate their lungs
– By negative pressure breathing, which pulls air into the lungs
Air inhaled Air exhaled
INHALATIONDiaphragm contracts
(moves down)
EXHALATIONDiaphragm relaxes
(moves up)
Diaphragm
Lung
Rib cage expands asrib muscles contract
Rib cage gets smaller asrib muscles relax
Figure 42.24
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• Lung volume increases
– As the rib muscles and diaphragm contract
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How a Bird Breathes
• Besides lungs, bird have eight or nine air sacs
– That function as bellows that keep air flowing through the lungs
INHALATIONAir sacs fill
EXHALATIONAir sacs empty; lungs fill
Anteriorair sacs
Trachea
Lungs LungsPosteriorair sacs
Air Air
1 mm
Air tubes(parabronchi)in lung
Figure 42.25
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• Air passes through the lungs
– In one direction only
• Every exhalation
– Completely renews the air in the lungs
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The Role of Partial Pressure Gradients
• Gases diffuse down pressure gradients
– In the lungs and other organs
• Diffusion of a gas
– Depends on differences in a quantity called partial pressure
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• A gas always diffuses from a region of higher partial pressure
– To a region of lower partial pressure
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• In the lungs and in the tissues
– O2 and CO2 diffuse from where their partial pressures are higher to where they are lower
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Inhaled air Exhaled air
160 0.2O2 CO2
O2 CO2
O2 CO2
O2 CO2 O2 CO2
O2 CO2 O2 CO2
O2 CO2
40 45
40 45
100 40
104 40
104 40
120 27
CO2O2
Alveolarepithelialcells
Pulmonaryarteries
Blood enteringalveolar
capillaries
Blood leavingtissue
capillaries
Blood enteringtissue
capillaries
Blood leaving
alveolar capillaries
CO2O2
Tissue capillaries
Heart
Alveolar capillaries
of lung
<40 >45
Tissue cells
Pulmonaryveins
Systemic arteriesSystemic
veinsO2
CO2
O2
CO 2
Alveolar spaces
12
43
Figure 42.27