Chapter 42Chapter 42

Circulation and Gas Exchange

Salmon gills

Trading with the Environment

• Every organism must exchange materials with its environment (this exchange ultimately occurs at the cellular level)

• In unicellular organisms

– These exchanges occur directly with the environment

• In multicellular organisms

– For most of the cells making up a multicellular organism direct exchange with the environment is not possible (hence the need for the circulatory and respiratory systems)

Circulatory System

Invertebrate Circulation

• The wide range of invertebrate body size and form is paralleled by a great diversity in circulatory systems

• Gastrovascular Cavities

• Open Circulatory System

• Closed Circulatory System

Gastrovascular Cavities

• Simple cnidarians have a body wall only two cells thick that encloses a gastrovascular cavity

• Functions in both digestion and distribution of substances throughout the body

• Some cnidarians, such as jellies have elaborate gastrovascular cavities

Circularcanal

Radial canal5 cm

Mouth

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 or hemolymph)

– A set of tubes (blood vessels)

– A muscular pump (the heart)

Open Circulatory System

• 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 system

Blood (hemolymph) returns to the heart through ostia which have valves that close during heart contraction

In an open circulatory system blood and interstitial fluid are the same (hemolymph)

Closed Circulatory System

• In a closed circulatory system blood is confined to vessels and is distinct from the interstitial fluid

• Closed systems are more efficient at transporting circulatory fluids to tissues and cells

Interstitialfluid

Heart

Small branch vessels in each organ

Dorsal vessel(main heart)

Ventral vesselsAuxiliary hearts

(b) A closed circulatory system

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

• Arteries carry blood to capillaries

– The sites of chemical exchange between the blood and interstitial fluid

• Veins return blood from capillaries to the heart

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

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 (double circuit)

Reptiles (Except Birds)

• Reptiles also have double circulation

– With a pulmonary circuit (lungs) and a systemic circuit

• Turtles, snakes, and lizards

– Have a three-chambered heart (with a septum partially dividing the single ventricle)

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

• A powerful four-chambered heart was an essential adaptation of the endothermic way of life characteristic of mammals and birds (it more efficiently delivers oxygenated blood to tissues)

Vertebrate circulatory systems

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

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

1

10

11

5

4

6

2

9

33

7

8

Mammalian Cardiovascular System

Trace the path of blood flow

The Mammalian Heart: A Closer Look

Aorta

Pulmonaryveins

Semilunarvalve

Atrioventricularvalve

Left ventricleRight ventricle

Anterior vena cava

Pulmonary artery

Semilunarvalve

Atrioventricularvalve

Posterior vena cava

Pulmonaryveins

Right atrium

Pulmonaryartery

Leftatrium

Parts List:

• Atria

• Ventricles

• Atrioventricular valves

• Semilunar valves

• Aorta

• Pulmonary arteries and veins

• Vena cavae

Cardiac Cycle

• Systole: The contraction, or pumping, phase of the cycle

• Diastole: The relaxation, or filling, phase of the cycle

• Heart rate: The number of beats per minute

• Cardiac output: The volume of blood pumped into the systemic circulation per minute

Semilunarvalvesclosed

AV valvesopen

AV valvesclosed

Semilunarvalvesopen

Atrial and ventricular diastole

1

Atrial systole; ventricular diastole

2

Ventricular systole; atrial diastole

3

0.1 sec

0.3 sec0.4 sec

Heart Excitation

• The sinoatrial (SA) node, or pacemaker sets the rate and timing at which all cardiac muscle cells contract

– Electrical impulses from the SA node spread rapidly through the Atria causing them to contract in unison

– The pacemaker is influenced by nerves, hormones, body temperature, and exercise

• Impulses from the SA node travel to the atrioventricular (AV) node

• At the AV node, the impulses are delayed slightly then relayed and amplified

– They travel via the Purkinje fibers and the ventricles contract in unison

Electrical Excitation

Figure 42.8

SA node(pacemaker)

AV node Bundlebranches

Heartapex

Purkinjefibers

2 Signals are delayedat AV node.

1 Pacemaker generates wave of signals to contract.

3 Signals passto heart apex.

4 Signals spreadThroughoutventricles.

ECG

Blood Vessels

• Structure

– Endothelium

– Smooth Muscle

– Connective tissue

Artery Vein

100 µm

Artery Vein

ArterioleVenule

Connectivetissue

Smoothmuscle

Endothelium

Connectivetissue

Smoothmuscle

EndotheliumValve

Endothelium

Basementmembrane

Capillary

Arteries and Veins

• Arteries have thicker walls to accommodate the high pressure of blood pumped from the heart

• In the thinner-walled veins blood flows back to the heart mainly as a result of muscle action. Veins have valves.

Direction of blood flowin vein (toward heart)

Valve (open)

Skeletal muscle

Valve (closed)

Blood Flow Velocity

• The velocity of blood flow varies in the circulatory system

– And is slowest in the capillary beds as a result of the high resistance and large total cross-sectional area

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

Blood Pressure

• Blood pressure

– The hydrostatic pressure that blood exerts against the wall of a vessel

– Blood pressure is determined by

• cardiac output

• peripheral resistance due to variable constriction of the arterioles

• Systolic pressure

– Is the pressure in the arteries during ventricular systole

– Is the highest pressure in the arteries

• Diastolic pressure

– Is the pressure in the arteries during diastole

– Is lower than systolic pressure

Blood Pressure Measurement

Artery

Rubber cuffinflatedwith air

Arteryclosed

120 120

Pressurein cuff above 120

Pressurein cuff below 120

Pressurein cuff below 70

Sounds audible instethoscope

Sounds stop

Blood pressurereading: 120/70

A typical blood pressure reading for a 20-year-oldis 120/70. The units for these numbers are mm of mercury (Hg); a blood pressure of 120 is a force that can support a column of mercury 120 mm high.

1

A sphygmomanometer, an inflatable cuff attached to apressure gauge, measures blood pressure in an artery.The cuff is wrapped around the upper arm and inflated until the pressure closes the artery, so that no blood flows past the cuff. When this occurs, the pressure exerted by the cuff exceeds the pressure in the artery.

2 A stethoscope is used to listen for sounds of blood flow below the cuff. If the artery is closed, there is no pulse below the cuff. The cuff is gradually deflated until blood begins to flow into the forearm, and sounds from blood pulsing into the artery below the cuff can be heard with the stethoscope. This occurs when the blood pressure is greater than the pressure exerted by the cuff. The pressure at this point is the systolic pressure.

3

The cuff is loosened further until the blood flows freely through the artery and the sounds below the cuff disappear. The pressure at this point is the diastolic pressure remaining in the artery when the heart is relaxed.

4

70

Blood Flow Through Capillary Beds

• Capillaries in major organs are usually filled to capacity, but in many other sites, the blood supply varies

– Amount of bloodflow through capillaries is regulated by

• Diameter of arteriole leading to the capillaries

• Precapillary sphincters

Precapillary Sphincters

• Precapillary sphincters control the flow of blood between arterioles and venules

Precapillary sphincters Thoroughfarechannel

ArterioleCapillaries

Venule(a) Sphincters relaxed

(b) Sphincters contractedVenuleArteriole

(c) Capillaries and larger vessels (SEM)

20 m

•The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries

• 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.

Fluid Return by the Lymphatic System

• The lymphatic system

– Functions

• Returns fluid to the body from the capillary beds

• Aids in body defense

– Fluid reenters the circulation

• directly at the venous end of the capillary bed

• indirectly through the lymphatic system

Blood

• Blood is a connective tissue

• Blood consists of

– several kinds of cells

– suspended in a liquid matrix called plasma

• The cellular elements

– Occupy about 45% of the volume of blood in man

Plasma

• Blood plasma is about 90% water

• Many solutes are found in plasma

– Inorganic salts in the form of dissolved ions, sometimes referred to as electrolytes

– Plasma proteins

• Plasma proteins influence blood pH, osmotic pressure, and viscosity

• Function in lipid transport, immunity, and blood clotting

Composition of mammalian plasma

Plasma 55%

Constituent Major functions

Water Solvent forcarrying othersubstances

SodiumPotassiumCalciumMagnesiumChlorideBicarbonate

Osmotic balancepH buffering, andregulation of membranepermeability

Albumin

Fibringen

Immunoglobulins(antibodies)

Plasma proteins

Ions (blood electrolytes

Osmotic balance,pH buffering

Substances transported by bloodNutrients (such as glucose, fatty acids, vitamins)Waste products of metabolismRespiratory gases (O2 and CO2)Hormones

Defense

Separatedbloodelements

Clotting

Cellular Elements

1. Erythrocytes (Red blood cells)

• Transport oxygen

2. Leukocytes (White blood cells)

• Function in defense

• Neutrophils

• Lymphocytes

• Eosinophils

• Monocytes

• Basophils

• Thrombocytes (platelets)

– Are fragments of cells that are involved in clotting

Cellular elements 45%

Cell type Numberper L (mm3) of blood

Functions

Erythrocytes(red blood cells) 5–6 million Transport oxygen

and help transportcarbon dioxide

Leukocytes(white blood cells)

5,000–10,000 Defense andimmunity

Eosinophil

Basophil

Platelets

NeutrophilMonocyte

Lymphocyte

250,000400,000

Blood clotting

Cellular elements of mammalian blood

Separatedbloodelements

Stem cells

• Erythrocytes, leukocytes, and platelets all develop from a common source

– A single population of cells called pluripotent stem cells in the red marrow of bones

B cells T cells

Lymphoidstem cells

Pluripotent stem cells(in bone marrow)

Myeloidstem cells

Erythrocytes

Platelets Monocytes

Neutrophils

Eosinophils

Basophils

Lymphocytes

Blood cells need to be constantly replaced throughout life

Blood clotting• When the endothelium of a blood vessel is damaged the clotting

mechanism begins

• A cascade of reactions converts fibrinogen to fibrin, forming a clot

Plateletplug

Collagen fibers

Platelet releases chemicalsthat make nearby platelets sticky

Clotting factors from:PlateletsDamaged cellsPlasma (factors include calcium, vitamin K)

Prothrombin Thrombin

Fibrinogen Fibrin5 µm

Fibrin clotRed blood cell

The clotting process begins when the endothelium of a vessel is damaged, exposing connective tissue in the vessel wall to blood. Plateletsadhere to collagen fibers in the connective tissue and release a substance thatmakes nearby platelets sticky.

1 The platelets form a plug that providesemergency protectionagainst blood loss.

2 This seal is reinforced by a clot of fibrin when vessel damage is severe. Fibrin is formed via amultistep process: Clotting factors released fromthe clumped platelets or damaged cells mix withclotting factors in the plasma, forming an activation cascade that converts a plasma proteincalled prothrombin to its active form, thrombin.Thrombin itself is an enzyme that catalyzes the final step of the clotting process, the conversion of fibrinogen to fibrin. The threads of fibrin become interwoven into a patch (see colorized SEM).

3

Cardiovascular Disease

• Cardiovascular diseases

– Disorders of the heart and the blood vessels

– Account for more than half the deaths in the United States

Atherosclerosis

• Atherosclerosis: Is caused by the buildup of cholesterol within arteries

(a) Normal artery (b) Partly clogged artery50 µm 250 µm

Smooth muscleConnective tissue Endothelium Plaque

Cardiovascular Diseases

• Hypertension, or high blood pressure

– Promotes atherosclerosis and increases the risk of heart attack and stroke

• Heart attack (Myocardial Infarction)

– Is the death of cardiac muscle tissue resulting from blockage of one or more coronary arteries

• Stroke

– Is the death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head

• Aneurysms

– Dilation of blood vessel walls

Gas Exchange

Gas Exchange

• Gas exchange

– Supplies oxygen for cellular respiration and disposes of carbon dioxide

• Animals require large, moist respiratory surfaces for the adequate diffusion of respiratory gases between their cells and the respiratory medium, either air or water

Gills in Aquatic Animals

• Gills are outfoldings of the body surface

– Specialized for gas exchange

Invertebrates-1

• 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

Invertebrates-2

• Many segmented worms have flaplike gills

– That extend from each segment of their body(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

Invertebrates-3

• The gills of clams, crayfish, and many other animals are restricted to a local body region

(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

Fish Gills

• 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

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

Countercurrent flow of blood and water maintains a concentration gradient down which oxygen diffuses from water to blood over entire length of capillary.

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

• 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.

2.5 µm

Air

Lungs

Lungs are found in

• most terrestrial vertebrates (amphibians have relatively small lungs in general and some lack them entirely. The skin of amphibians supplements gas exchange in the lungs)

• spiders

• land snails

• a few fish (lungfish)

Mammalian Respiratory Systems: A Closer Look• A system of branching ducts conveys air to the lungs

• air inhaled via the nostrils passes through the pharynx into the trachea, bronchi, bronchioles, and dead-end alveoli, where gas exchange occurs 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

Ventilation

• Breathing ventilates the lungs

– The alternate inhalation and exhalation of air

• An amphibian such as a frog

– Ventilates its lungs by positive pressure breathing, which forces air down the trachea

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

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

Control of Breathing in Humans

• The main breathing control centers

– Are located in two regions of the brain, the medulla oblongata and the pons

PonsBreathing control centers Medulla

oblongata

Diaphragm

Carotidarteries

Aorta

Cerebrospinalfluid

Rib muscles

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 also helps regulate blood CO2 level. Sensors in 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 control center in themedulla sets the basic

rhythm, and a control centerin the pons moderates it,

smoothing out thetransitions between

inhalations and exhalations.

1

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.

2

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.

6

5

3

4

Control of Breathing in Humans

• The centers in the medulla

– Regulate the rate and depth of breathing in response to pH changes in blood and in the cerebrospinal fluid

– The medulla adjusts breathing rate and depth to match metabolic demands

• Sensors in the aorta and carotid arteries

– Monitor O2 and CO2 concentrations in the blood

– Exert secondary control over breathing

Gas Transport

• The metabolic demands of many organisms

– Require that the blood transport large quantities of O2 and CO2

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

• A gas always diffuses from a region of higher partial pressure to a region of lower partial pressure

• In the lungs and in the tissues O2 and CO2 diffuse from where their partial pressures are higher to where they are lower

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

Respiratory Pigments

• Respiratory pigments

– Are proteins that transport oxygen

– They greatly increase the amount of oxygen that blood can carry

– The respiratory pigment of almost all vertebrates is the protein hemoglobin, contained in the erythrocytes

• Like all respiratory pigments

– Hemoglobin must reversibly bind O2, loading O2 in the lungs and unloading it in other parts of the body

Heme group Iron atom

O2 loadedin lungs

O2 unloadedIn tissues

Polypeptide chain

O2

O2

Figure 42.28

• Hemoglobin also helps transport CO2 and assists in buffering

• Carbon dioxide from respiring cells

– Diffuses into the blood plasma and then into erythrocytes and is ultimately released in the lungs

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.

1

2

3

4

5

6

7

8

9

10

11

The Ultimate Endurance Runner

• The extreme O2 consumption of the antelope-like pronghorn

– Underlies its ability to run at high speed over long distances

Diving Mammals

• Deep-diving air breathers

– are able to store large amounts of oxygen in blood and muscle (increased amounts of myoglobin)

– Often have a large blood volume

– Have adaptations to conserve oxygen and use it slowly