Chapter 42: Gas Exchange 1.Why is gas exchange important? -Aerobic organisms need O 2 for oxidative phosphorylation (making ATP) -CO 2 from citric acid

Chapter 42: Gas Exchange

1. Why is gas exchange important?- Aerobic organisms need O2 for oxidative phosphorylation (making ATP)- CO2 from citric acid cycle must be removed

Figure 42.19 The role of gas exchange in bioenergetics

Organismal level

Cellular level

Circulatory system

Cellular respiration ATPEnergy-richmoleculesfrom food

Respiratorysurface

Respiratorymedium(air or water)

O2CO2



2. How have gas exchange systems changed as animals evolved? - Small, thin organisms – diffusion directly across skin- Larger organisms need a larger surface area (gills, trachea or lungs)

Figure 42.20 Diversity in the structure of gills, external body surfaces functioning in gas exchange

(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.

(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.

(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.

GillsGills

Gill

Parapodia

Gills

Tube foot

Coelom




3. How have fish gills evolved for maximal gas exchange?

Figure 42.21 The structure and function of fish gills

Gill arch

Water flow Operculum

Gill arch

Blood vessel

Gillfilaments

Oxygen-poorblood

Oxygen-richblood

Water flowover lamellaeshowing % O2

Blood flowthrough capillariesin lamellaeshowing % O2

Lamella

Countercurrent exchange

100%

40%

70%

15%

90%

60%

30% 5%

At best, concurrent exchange would give blood O2 of 50%.Fish expend lots of energy ventilating – forcing water across gills to get O2.




3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?

- Too dry for gills large surface area- External gas exchange will not occur

5. What adaptations do land animals have?- Internal exchange- Tracheal systems with many openings (spiracles)

Figure 42.22 Tracheal systems

Spiracle

Tracheae

Air sacsAirsac

Body cell

Air

Trachea

Tracheole

Tracheoles Mitochondria

MyofibrilsBody wall

2.5 µm

(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.

(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.


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?

- Internal exchange- Tracheal systems with many openings (spiracles)- Lungs in spiders, terrestrial snails & vertebrates

- 1 location for opening- Dense net of capillaries

6. What is the flow of air in our respiratory system?Nostrils nasal cavity pharynx larynx tracheaBronchi bronchioles alveoli

Branch 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 The mammalian respiratory system

- Mostly lined with cilia & thin layer of mucus


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?6. What is the flow of air in our respiratory system?7. What is the difference between positive & negative breathing?

- Positive – tongue pushes air down into lungs – frogs- Negative – air pulled down into lungs - us

Figure 42.24 Negative pressure breathing

Rib cage expands asrib muscles contract

Rib cage gets smaller asrib muscles relax

Air inhaled Air exhaled

INHALATIONDiaphragm contracts

(moves down)

EXHALATIONDiaphragm relaxes

(moves up)

Diaphragm

Lung

Thoracic cavity expands & air is forced into nose.

Muscles relax & thoracic cavity gets smallerand air is forced out of nose.


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?6. What is the flow of air in our respiratory system?7. What is the difference between positive & negative breathing?8. How is breathing controlled? (oxygen homeostasis)

- Medulla oblongata & pons- O2 sensors in aorta & carotids & CO2 sensors in carotids

PonsBreathing control centers Medulla

oblongata

Diaphragm

Carotidarteries

Aorta

Cerebrospinalfluid

Rib muscles

The control center in themedulla sets the basic

rhythm, and a control centerin the pons moderates it,

smoothing out thetransitions between

inhalations and exhalations.

Nerve impulses trigger muscle contraction. Nerves

from a breathing control centerin the medulla oblongata of the

brain send impulses to thediaphragm and rib muscles, stimulating them to contract

and causing inhalation.

In a person at rest, these nerve impulses result in

about 10 to 14 inhalationsper minute. Between

inhalations, the musclesrelax and the person exhales.

The medulla’s control center alsohelps regulate blood CO2 level. Sensorsin the medulla detect changes in the pH (reflecting CO2 concentration) of the blood and cerebrospinal fluid bathing the surface of the brain.

Nerve impulses relay changes in CO2 and O2 concentrations. Other sensors in the walls of the aortaand carotid arteries in the neck detect changes in blood pH andsend nerve impulses to the medulla. In response, the medulla’s breathingcontrol center alters the rate anddepth of breathing, increasing bothto dispose of excess CO2 or decreasingboth if CO2 levels are depressed.

The sensors in the aorta andcarotid arteries also detect changesin O2 levels in the blood and signal the medulla to increase the breathing rate when levels become very low.

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3

6

5

4

Figure 42.26 Automatic control of breathing


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?6. What is the flow of air in our respiratory system?7. What is the difference between positive & negative breathing?8. How is breathing controlled? (oxygen homeostasis)9. How are gases exchanged across selectively permeable membranes?

- Simple diffusion

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 Loading and unloading of respiratory gases


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?6. What is the flow of air in our respiratory system?7. What is the difference between positive & negative breathing?8. How is breathing controlled? (oxygen homeostasis)9. How are gases exchanged?10. How is the O2 carried in the blood?

- By hemoglobin in RBCs

Figure 42.28 Hemoglobin loading and unloading O2

Heme group Iron atom

O2 loadedin lungs

O2 unloadedIn tissues

Polypeptide chain

O2

O2

1 RBC has 250 million Hb molecules X 4 O2 molecules =

1 billion O2 per RBC X 25 trillion RBC per person =

1 billion O2 per RBC

2.5 x 1022 O2 total

Cooperativity works in loading & unloading of O2.RBC do not have a nucleus so more room for Hb.


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?6. What is the flow of air in our respiratory system?7. What is the difference between positive & negative breathing?8. How is breathing controlled? (oxygen homeostasis)9. How are gases exchanged?10. How is the O2 carried in the blood?11. How is O2 dumped from hemoglobin?

Figure 42.29 Dissociation curves for hemoglobin

O2 unloaded fromhemoglobinduring normalmetabolism

O2 reserve that canbe unloaded fromhemoglobin totissues with highmetabolism

Tissues duringexercise

Tissuesat rest

100

80

60

40

20

0

100

80

60

40

20

0

100806040200

100806040200

Lungs

PO2 (mm Hg)

PO2 (mm Hg)

O2 s

atur

atio

n of

hem

oglo

bin

(%)

O2 s

atur

atio

n of

hem

oglo

bin

(%)

Bohr shift:Additional O2

released from hemoglobin at lower pH(higher CO2

concentration)

pH 7.4

pH 7.2

(a) PO2 and Hemoglobin Dissociation at 37°C and ph 7.4

(b) pH and Hemoglobin Dissociation


1. Why is gas exchange important?2. How have gas exchange systems changed as animals evolved? 3. How have fish gills evolved for maximal gas exchange?4. Why don’t gills work on land?5. What adaptations do land animals have?6. What is the flow of air in our respiratory system?7. What is the difference between positive & negative breathing?8. How is breathing controlled? (oxygen homeostasis)9. How are gases exchanged?10. How is the O2 carried in the blood?11. How is O2 dumped from hemoglobin?12. How does CO2 travel from tissues to lungs?

- Most dissolved in plasma as bicarbonate ion

Figure 42.30 Carbon dioxide transport in the blood

Tissue cell

CO2Interstitialfluid

CO2 producedCO2 transportfrom tissues

CO2

CO2

Blood plasmawithin capillary Capillary

wall

H2O

Redbloodcell

HbCarbonic acidH2CO3

HCO3–

H++Bicarbonate

HCO3–

Hemoglobinpicks up

CO2 and H+

HCO3–

HCO3– H++

H2CO3Hb

Hemoglobinreleases

CO2 and H+

CO2 transportto lungs

H2O

CO2

CO2

CO2

CO2

Alveolar space in lung

2

1

34

5 6

7

8

9

10

11

To lungs

Carbon dioxide produced bybody tissues diffuses into the interstitial fluid and the plasma.

Over 90% of the CO2 diffuses into red blood cells, leaving only 7%in the plasma as dissolved CO2.

Some CO2 is picked up and transported by hemoglobin.

However, most CO2 reacts with water in red blood cells, forming carbonic acid (H2CO3), a reaction catalyzed bycarbonic anhydrase contained. Withinred blood cells.

Carbonic acid dissociates into a biocarbonate ion (HCO3

–) and a hydrogen ion (H+).

Hemoglobin binds most of the H+ from H2CO3 preventing the H+ from acidifying the blood and thuspreventing the Bohr shift.

CO2 diffuses into the alveolarspace, from which it is expelledduring exhalation. The reductionof CO2 concentration in the plasmadrives the breakdown of H2CO3 Into CO2 and water in the red bloodcells (see step 9), a reversal of the reaction that occurs in the tissues (see step 4).

Most of the HCO3– diffuse

into the plasma where it is carried in the bloodstream to the lungs.

In the HCO3– diffuse

from the plasma red blood cells, combining with H+ released from hemoglobin and forming H2CO3.

Carbonic acid is converted back into CO2 and water.

CO2 formed from H2CO3 is unloadedfrom hemoglobin and diffuses into the interstitial fluid.

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Documents

Chapter 42: Gas Exchange 1.Why is gas exchange important? -Aerobic organisms need O 2 for oxidative phosphorylation (making ATP) -CO 2 from citric acid