exchttt~ i - · pdf fileexchange behwen an axolotl or any other animal and its surroundings...

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KEY CONCEPTS

I.... Figure 42.1 How does a feathery fringe help thisanimal survive'?

42.1 Circulatory systems link exchange surfaceswith cells throughout the body

42.2 Coordinated cycles of heart contraction drivedouble circulation in mammals

42.3 Patterns of blood pressure and flow reflect thestructure and arrangement of blood vessels

42.4 Blood components function in exchange,transport, and defense

42.5 Gas exchange occurs across specializedrespiratory surfaces

42.6 Breathing ventilates the lungs42.7 Adaptations for gas exchange include pigments

that bind and transport gases

The animal in Figure 42.1 may look like a creaturefrom a science fiction film, but it's actually an axolotl, a salamander native to shallow ponds in central

Mexico. The feathery red appendages jutting out from thehead of this albino adult are gills. Although external gills are

uncommon in adult animals, they help satisfy the need sharedby all animals to exchange substances with their environment.

Exchange behwen an axolotl or any other animal and itssurroundings ultimately occurs at the cellular level. The resources that animal cells require, such as nutrients and oxygen(02), enter the cytoplasm by crossing the plasma membrane.Metabolic by-products, such as carbon dioxide (C02), exit thecell by crossing the same membrane. In unicellular organisms,exchange occurs directly with the external environment. Formost multicellular organisms, however, direct exchange between every cell and the environment is not possible. Instead,these organisms rely on specialized systems that carry out ex-

898

change with the environment and that transport materials between sites of exchange and the rest of the body.

The reddish color and branching structure of the axolotl'sgills reflect the intimate association between exchange andtransport. Tiny blood vessels lie close to the surface ofeach fil·ament in the gills. Across this surface, there is a net diffusionof02 from the surrounding water into the blood and ofe02

from the blood into the water. The short distances involved allow diffusion to be rapid. Pumping of the axolotl's heart propels the oxygen-rich blood from the gill filaments to all othertissues of the body. There, more short-range exchange occurs,involving nutrients and O2 as well as CO2 and other wastes.

Because internal transport and gas exchange are functionally related in most animals, not just axolotls, we will examineboth circulatory and respiratory systems in this chapter. Wewill explore the remarkable variation in form and organization

of these systems by considering examples from a number ofspecies. We will also highlight the roles of circulatory and respiratory systems in maintaining homeostasis under a range ofphysiological and environmental stresses.

r~~;~':I:~o~~~~tems linkexchange surfaces with cellsthroughout the body

The molecular trade that animals carry out with their environment-gaining O 2 and nutrients while shedding CO2 andother waste products-must ultimately involve every cell inthe body. As you learned in Chapter 7, small, nonpolar molecules such as O 2 and CO2 can move between cells and theirimmediate surroundings by diffusion. But diffusion is veryslow for distances of more than a few millimeters. That's because the time it takes for a substance to diffuse from one

place to another is proportional to the square of the distance.For example, if it takes 1 second for a given quantity ofglucoseto diffuse 100 11m, it will take 100 seconds for the same quantity to diffuse I mm, and almost 3 hours to diffuse I em. This

relationship between diffusion time and distance places a sub

stantial constraint on the body plan of any animal.Given that diffusion is rapid only over small distances, how

does each cell of an animal participate in exchange? Naturalselection has resulted in two general solutions to this problem. TIle first solution is a body size and shape that keep manyor all cells in direct contact with the environment. Each cellcan thus exchange materials directly with the surroundingmedium. This type of body plan is found only in certain invertebrates, including sponges, cnidarians, and flatworms.The second solution, found in all other animals, is a circulatory system that moves fluid between each cell's immediatesurroundings and the tissues where exchange with the environment occurs.

Gastrovascular CavitiesLet's begin by looking at animals that lack a distinct circulatory system. In hydras and other cnidarians, a central gas

trovascular cavity functions both in digestion and in thedistribution of substances throughout the body. As wasshown for a hydra in Figure 41.8, a single opening maintainscontinuity between the fluid inside the cavity and the wateroutside. As a result, both the inner and outer tissue layers arebathed by fluid. Only the cells of the inner layer have direct access to nutrients, but since the body wall is a mere two cellsthick, the nutrients must diffuse only a short distance to reachthe cells of the outer layer. Thin branches of a hydra's gas-

Circularcanal

trovascular cavity extend into the animal's tentacles. Somecnidarians, such as jellies, have gastrovascular cavities with amuch more elaborate branching pattern (Figure 42.2a).

Planarians and most other flatworms also survive without

a circulatory system. Their combination of a gastrovascularcavity and a flat body is well suited for exchange with the environment (Figure 42.2b). A flat body optimizes diffusional

exchange by increasing surface area and minimizing diffusiondistances.

Open and Closed Circulatory Systems

For animals ,,~th many cell layers, diffusion distances are too

great for adequate exchange of nutrients and wastes by a gastrovascular cavity. In these organisms, a circulatory system minimizes the distances that substances must diffuse to enter orleave a cell. By transporting fluid throughout the body, the circulatory system functionally connects the aqueous environment ofthe body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes. In mammals, for example, O 2 frominhaled air diffuses across only two layers ofcells in the lungs before reaching the blood. The circulatory system, powered by theheart, then carries the oxygen-rich blood to all parts ofthe body.As the blood streams throughout the body tissues in tiny blood

vessels, O2 in the blood again diffuses only a short distance before entering the interstitial fluid that directly bathes the cells.

A circulatory system has three basic components: a circulatory fluid, a set of interconnecting tubes, and a muscularpump, the heart. The heart powers circulation by using metabolic energy to elevate the hydrostatic pressure of the circulatory fluid, which then flows through a circuit of vessels andback to the heart.

\ \MocthPharynx

I 2 mm I

(a) The moon jelly Aurelia, a cnidarian. The Jelly is viewed here from its underside (oral surface),The mouth leads to an elaborate gastrovascular cavity that consists of radial arms (canals)leading to and from a circular canal Ciliated cells lining the canals C1fculate fluid within thecavity as indicated by the arrows.

... figure 42.2 Internal transport in gastrovascular cavities.-'mU '14 Suppose a gastrQvascular cavity were open at two ends, with fluid entering oneend and leaving the other How would this affect the gastrovascular cavity's function)

(b) The planarian Dugesia, a flatworm. Themouth and pharynx on the ventral side leadto the highly branched gastrovascular cavity.stained dark brown in this specimen (LM),

CHAPTER FORTY·TWO Circulation and Gas Exchange 899

•

Hemolymph in sinusessurrounding organs

I•

IHeart

t•I

(b) A closed circulatory system. Closed circulatory systems circulateblood entirely within vessels, so the blood is distinct from theinterstitial fluid. Chemical exchange occurs between the blood andthe interstitial fluid, as well as between the interstitial fluid and bodycells In an earthworm. the dorsal vessel functIOns as the main heart.pumping blood forward by peristalsis. Near the worm's anterior end,five pairs of vessels loop around the digestive tract and function asauxiliary hearts.

Tubular heart

(a) An open circulatory system. In an open circulatory system, such asthat of agrasshopper, the circulatory fluid, called hemolymph. is thesame as interstitial fluid. The heart pumps hemolymph throughvessels into sinuses. fluid-filled spaces where materials are exchangedbetween the hemolymph and cells Hemolymph returns to the heartthrough pores, which are equipped with valves that close when theheart contracts,

.... Figure 42.3 Open and closed circulatory systems.

Auxiliary hearts Ventral vessels

Arthropods and most mollusks have an open circulatorysystem, in which the circulatory fluid bathes the organs di·rectly (Figure 42,3a). In these animals, the circulatory fluid,

called hemolymph, is also the interstitial fluid. Contraction ofone or more hearts pumps the hemolymph through the circulatory vessels into interconnected sinuses, spaces surroundingthe organs. Within the sinuses, chemical exchange occurs between the hemolymph and body cells. Relaxation of the heartdraws hemolymph back in through pores, and body movements help circulate the hemolymph by periodically squeezingthe sinuses. The open circulatory system of larger crustaceans,such as lobsters and crabs, includes a more extensive system ofvessels as well as an accessory pump.

In a closed circulatory system, blood is confined to vessels and is distinct from the interstitial fluid (Figure 42.3b).

One or more hearts pump blood into large vessels that branchinto smaller ones coursing through the organs. Materials areexchanged between the smallest vessels and the interstitialfluid bathing the cells. Annelids (including earthworms),cephalopods (including squids and octopuses), and all vertebrates have closed circulatory systems.

The fact that both open and closed circulatory systems arewidespread among animals suggests that there are advantages

900 UNIT SEVEN Animal Form and Function

to each system. The lower hydrostatic pressures associatedwith open circulatory systems make them less costly thanclosed systems in terms of energy expenditure. In some invertebrates, open circulatory systems serve additional functions.For example, in spiders, the hydrostatic pressure generated bythe open circulatory system provides the force used to extend

the animal's legs.The benefits of closed circulatory systems include rela

tively high blood pressures, which enable the effective delivery of~ and nutrients to the cells of larger and more activeanimals. Among the molluscs, for instance, closed circulatorysystems are found in the largest and most active species, thesquids and octopuses. Closed systems are also particularlywell suited to regulating the distribution of blood to differentorgans, as you'll learn later in this chapter. In examiningclosed circulatory systems in more detail, we will focus on thevertebrates.

Organization of VertebrateCirculatory Systems

The closed circulatory system ofhumans and othervertebratesis often called the cardiovascular system. Blood circulates to

and from the heart through an amazingly extensive network ofvessels: The total length of blood vessels in an average humanadult is twice Earth's circumference at the equator!

Arteries, veins, and capillaries are the three main types of

blood vessels. Within each type, blood flows in only one di

rection. Arteries carry blood away from the heart to organsthroughout the body. \'(fithin organs, arteries branch into

arterioles, small vessels that convey blood to the capillaries.Capillaries are microscopic vessels with very thin, porouswalls. Networks of these vessels, called capillary beds, infiltrate each tissue, passing within a few cell diameters of everycell in the body. Across the thin walls ofcapillaries, chemicals,including dissolved gases, are exchanged by diffusion between the blood and the interstitial fluid around the tissuecells. At their ~downstream" end, capillaries converge intovenules, and venu[es converge into veins, the vessels thatcarry blood back to the heart.

Arteries and veins are distinguished by the direction in

which they carry blood, not by the O2 content or other characteristics of the blood they contain. Arteries carry bloodfrom the heart toward capillaries, and veins return blood tothe heart from capillaries. There is one exception: the portalveins, which carry blood between pairs of capillary beds. Thehepatic portal vein, for example, carries blood from capillarybeds in the digestive system to capillary beds in the liver (seeChapter 41). From the liver, blood passes into the hepaticveins, which conduct blood toward the heart.

Natural selection has modified the cardiovascular systemsofdifferent vertebrates in accordance with their level of activ

ity. For example, animals with higher metabolic rates generally have more complex circulatory systems and morepowerful hearts than animals with lower metabolic rates.Similarly, within an animal, the complexity and number ofblood vessels in a particular organ correlate with that organ'smetabolic requirements.

The hearts ofall vertebrates contain two or more muscularchambers. The chambers that receive blood entering the heartare called atria (singular, atrium). The chambers responsiblefor pumping blood out of the heart are called ventricles. Thenumber of chambers and the extent to which they are separated from one another differ substantially among groups ofvertebrates, as we will discuss next. These important differ

ences reflect the close fit of form to function.

Single Circulation

In bony fishes, rays, and sharks, the heart consists oftwo cham

bers: an atrium and a ventricle. The blood passes through theheart once in each complete circuit, an arrangement calledsingle circulation (Figure 42.4). Blood entering the heart collects in the atrium before transfer to the ventricle. Contractionof the ventricle pumps blood to the gills, where there is a netdiffusion of O2 into the blood and of CO2 out of the blood. As

Artery

{

VentricleHeart

Atrium

Vein

Systemic capillaries

.. Figure 42.4 Single circulation in fishes. Fishes have a twochambered heart and a single circuit of blood flow,

blood leaves the gills, the capillaries converge into a vessel thatcarries oxygen-rich blood to capillary beds throughout thebody. Blood then returns to the heart.

In single circulation, blood that leaves the heart passesthrough two capillary beds before returning to the heart.When blood flows through a capillary bed, blood pressure

drops substantially, for reasons we will explain shortly. Thedrop in blood pressure in the gills ofa bony fish, ray, or sharklimits the rate of blood flow in the rest of the animal's body.As the animal swims, however, the contraction and relax

ation of its muscles help accelerate the relatively sluggishpace of circulation.

Double Circulation

As shown in Figure 42.5, on the next page, the circulatory systems of amphibians, reptiles, and mammals have two distinct

circuits, an arrangement called double circulation. Thepumps for the two circuits serve different tissues but are combined into a single organ, the heart. Having both pumps withina single heart simplifies coordination of the pumping cycles.

One pump, the right side ofthe heart, delivers oxygen-poorblood to the capillary beds of the gas exchange tissues, wherethere is a net movement of O2 into the blood and of CO2 outofthe blood. This part ofthe circulation is called a pulmonarycircuit if the capillary beds involved are all in the lungs, as inreptiles and mammals. It is called a pulmocutancous circuitif it includes capillaries in both the lungs and the skin, as in

many amphibians.After the oxygen-enriched blood leaves the gas exchange

tissues, it enters the other pump, the left side ofthe heart. Contraction of the heart propels this blood to capillary beds in organs and tissues throughout the body. Following the exchangeof O2 and C02> as well as nutrients and waste products, the


't' Figure 42.5

•• • Double Circulation in Vertebrates

Amphibians Reptiles (Except Birds) Mammals and BirdsAmphibians have athree-<:hambered heartand two circuits of bloodflow: pulmocutaneousand systemic.

Lizards, snakes, and turtleshave a thrfe-chamberedheart, with a septumpartial~ dividing thesingle ventricle. Incrocodilians, the septum is complete andthe heart is four-chambered.

Mammals andbirds have a fourchambered heartIn birds, the majO!"vessels near theheart are slightlydifferent than shown, but the panern ofdouble circulation is essentially the same.

Lung and skin capillaries lung capillaries Lung capillaries

VRight

Systemiccircuit

leftsystemicaorta

Rightsystemicaorta

Atrium (A)Atrium (Al41f-+

Ventricle (V)

Systemic capillaries Systemic capillaries Systemic capillaries

Syrtemic circuits include all body tissues except the primary gas exchange tissues. Note thatcirculatory systems are depicted as if the animal is facing you: The right side of the heart is shownon the left, and vice versa.

now oxygen-poor blood returns to the heart, completing thesystemic circuit.

Double circulation provides a vigorous flow of blood to thebrain, muscles, and other organs because the heart repressurizes the blood destined for these tissues after it passes through

the capillary beds of the lungs or skin. Indeed, blood pressureis often much higher in the systemic circuit than in the gas ex·change circuit. This contrasts sharply with single circulation, inwhich, as you read earlier, blood flows directly from the respiratory organs to other organs, under reduced pressure.

Adaptations of Double Circulatory Systems

Having considered the general properties of double circula·tion, let's examine the adaptations found in the hearts of dif·ferent vertebrate groups that have this type of circulation. Asyou read, refer to the illustrations in Figure 42.5.

poor blood from the right atrium into the pulmocutaneouscircuit and most of the oxygen-rich blood from the leftatrium into the systemic circuit. When underwater, a frogadjusts its circulation, for the most part shutting off blood

flow to its temporarily ineffective lungs. Blood flow continues to the skin, which acts as the sole site of gas exchangewhile the frog is submerged.

Reptiles (Except Birds) Turtles, snakes, and lizards have athree-chambered heart, with a septum partially dividing theventricle into separate right and left chambers. In alligators,caimans, and other crocodilians, the septum is complete, butthe pulmonary and systemic circuits are connected where thearteries exit the heart. When a crocodilian is underwater, arterial valves divert most of the blood flow from the pulmonary

circuit to the systemic circuit through this connection.

Amphibians Frogs and other amphibians have a heartwith three chambers: two atria and one ventricle. A ridgewithin the ventricle diverts most (about 90%) of the oxygen-

Mammals and Birds In all mammals and birds, the ventricle is completely divided, such that there are two atria and twoventricles. The left side of the heart receives and pumps only


For suggested answers, see Appendix A.

Mammalian Circulation

r~:::~~~a:~·~c1es of heartcontraction drive doublecirculation in mammals

o

Pulmonaryvein

Left atrium

Left ventricle

Capillaries ofabdominal organsand hind limbs

Capillaries ofhead andforelimbs

f---Aorta

blood to the lungs via e the pulmonary arteries. As the bloodflows through 0 capillary beds in the left and right lungs, itloads O 2 and unloads CO2, Oxygen-rich blood returns fromthe lungs via the pulmonary veins to 0 the left atrium of the

heart. Next, the oxygen-rich blood flows into 0 the left ventricle, which pumps the oxygen-rich blood out to body tissuesthrough the systemic circuit. Blood leaves the left ventricle via() the aorta, which conveys blood to arteries leading through

out the body. The first branches from the aorta are the coronary arteries (not shown), which supply blood to the heartmuscle itself. Then branches lead to e capillary beds in thehead and arms (forelimbs). The aorta then descends into the

abdomen, supplying oxygen-rich blood to arteries leading too capillary beds in the abdominal organs and legs (hindlimbs). Within the capillaries, there is a net diffusion of 0 1from the blood to the tissues and of CO2 produced by cellular respiration into the blood. Capillaries rejoin, formingvenules, which convey blood to veins. Oxygen-poor bloodfrom the head, neck, and forelimbs is channeled into a largevein, 0 the superior vena cava. Another large vein, ~ theinferior vena cava, drains blood from the trunk and hind

limbs. The two venae cavae empty their blood into ~ theright atrium, from which the oxygen-poor blood flows intothe right ventricle.

Pulmonaryartery

Right ventricle

inferiorvena cava

Superiorvena cava

Pulmonaryvein

Right atrium

42.1CONCEPT CHECK

oxygen-rich blood, while the right side receives and pumpsonly oxygen-poor blood.

A powerful fouNhambered heart is a key adaptation thatsupports the endothermic way oflife characteristic ofmammals

and birds. Endotherms use about ten times as much energy as

equal-sized ectotherms; therefore, their circulatory systemsneed to deliver about ten times as much fuel and~ to their tissues (and remove ten times as much CO2 and other wastes).

This large traffic of substances is made possible by separate andindependently powered systemic and pulmonary circuits andby large hearts that pump the necessary volume ofblood. As wediscussed in Chapter 34, mammals and birds descended fromdifferent tetrapod ancestors, and their four-chambered heartsevolved independently-an example of convergent evolution.

The timely delivery of O 2 to the body's organs is critical:Brain cells, for example, die within just a few minutes if theirO 2 supply is interrupted. How does the mammalian cardiovascular system meet the body's continuous but variable de

mand for 01? To answer this question, we need to considerhow the parts of the system are arranged and how each partfunctions.

I. How is the flow of hemolymph through an open circulatory system similar to the flow of water throughan outdoor fountain?

2. Three-chambered hearts with incomplete septa wereonce viewed as being less adapted to circulatory function than mammalian hearts. What advantage of suchhearts did this viewpoint overlook?

3. _i,'!:f."l. The heart ofa human fetus has a hole be

tv.'een the left and right ventricles. In some cases, thishole does not close completely before birth. If the holeweren't surgically corrected, how would it affect the O2

content ofthe blood entering the systemic circuit fromthe heart?

Let's first examine the overall organization of the mammaliancardiovascular system, beginning with the pulmonary circuit.(The circled numbers refer to corresponding locations inFigure 42.6). 0 Contraction of the right ventricle pumps

... Figure 42.6 The mammalian cardiovascular system: anoverview. Note that the dual circuits operate simultaneously, not inthe serial fashion that the numbering in the diagram suggests, The twoventricles pump almost in unison; while some blood is traveling in thepulmonary circuit, the rest of the blood is flowing in the systemic circuit.


.... Figure 42.7 The mammalian heart: a closer look. Noticethe locations althe valves. which prevent backflow of blood within theheart, Also notice how the atria and left and right ventricles differ inthe thickness of their muscular walls.

Pclmoo,o"t"'~

Right .....atrium ~

i...Semilunar~al~e

Atrioventricularvalve

Rightventricle

Leftventride

Aorta

Semilunarvalve

Atrioventricularvalve

flaps ofconnective tissue, the valves open when pushed from oneside and close when pushed from the other. An atrioventricular(AV) valve lies between each atrium and ventricle. The AV valvesare anchored by strong fibers that prevent them from turning in·

side out. Pressure generated by the powerful contraction of theventricles closes the AV valves, keeping blood from flowing backinto the atria. Scmilunarvalves are located at the two exitsoftheheart: where the aorta leaves the left ventricle and where the pulmonary artery leaves the right ventricle. These valves are pushedopen by the pressure generated during contraction of the ventricles. When the ventricles relax, pressure built up in the aorta closesthe semilunar valves and prevents Significant backflow. You canfollow these events either with a stethoscope or by pressing yourear tightly against the chest of a friend (or a friendly dog). Thesound pattern is '1ub-dup, lub·dup, lub-dup:' The first heart sound('1ub'') is created by the recoil of blood against the closed AVvalves. The second sound ("dup") is produced by the recoil ofbloocl against the closed semilunar valves.

Ifblood squirts backward through a defective valve, it mayproduce an abnormal sound called a heart murmur. Some

The Mammalian Hearl: A Closer Look

Using the human heart as an example, let's now take a closerlook at how the mammalian heart works (figure 42.7). Locatedbehind the sternum (breastbone), the human heart is about thesize ofa clenched fist and consists mostly ofcardiac muscle (seeFigure40.5). The two atria have relatively thin walls and serve ascollection chambers for blood returning to the heart. Much ofthe blood entering the atria flows into the ventricles while allheart chambers are relaxed. Contraction of the atria transfersthe remainder before the ventricles begin to contract. The ventricles have thicker walls and contract much more forcefullythan the atria-especially the left ventricle, which pumps bloodto all body organs through the systemic circuit. Although theleft ventricle contracts with greater force than the right ventricle, it pumps the same volume of blood as the right ventricleduring each contraction.

The heart contracts and relaxes in a rhythmic cycle. \Vhen itcontracts, it pumps blood; when it relaxes, its chambers fill withblood. One complete sequence of pumping and ftlling is referredto as the cardiaccycle. The contraction phaseofthe cycle is calledsystole, and the relaxation phase is called diastole (Figure 42.8).

The volume of blood each ventricle pumps per minute is thecardiac output Two factors determine cardiac output: the rate ofcontraction, or heart rate (number ofbeats per minute), and thestroke volume, the amount of blood pumped by a ventricle in asingle contraction. The average stroke volume in humans is about70 mL. Multiplying this stroke volume by a resting heart rate of72beats per minute yields a cardiac output of5 Umin-about equalto the total volume ofblood in the human body. During heavy exercise, cardiac output increases as much as fivefold.

Fourvalves in the heart prevent backflowand keep blood moving in the correct direction (see FIgures 42.7 and 42.8). Made of


Semilunarvalvesclosed

oAtrial andventriculardiastole

8 Ventricular systole;atrial diastole

... Figure 42.8 The cardiac cycle. For an adult human at rest witha heart rate of about 72 beats per minute, one complete cardiac cycletakes about 0.8 second. 0 During a relaxation phase (atria andventricles in diastole). blood returning from the large veins flows intothe atria and ventricles through the AV valves, f) A brief period ofatrial systole then forces all blood remaining in the atria into theventricles. 0 During the remainder of the cycle, ventricular systolepumps blood into the large arteries through the semilunar valves. Notethat during all but 0 1 second of the cardiac cycle. the atria are relaxedand are filling with blood returning via the veins.

fit Signals spreadthroughoutventricles.

Heart

42.2

() Signals passto heart apex,

Bundlebranches

~

CONCEPT CHECK

1. Explain why blood in the pulmonary veins has ahigher O2 concentration than blood in the venaecavae, which are also veins.

2. Why is it important that the AV node delay the electrical impulse moving from the SA node and the atriato the ventricles?

3. _','11° 114 After exercising regularly for severalmonths, you find that your resting heart rate has decreased. Given that your body now requires fewercardiac cycles in a given time, what other change inthe function of your heart at rest would you expect tofind? Explain.


6 Signals are delayedat AV node.

function like the spurs and reins used in riding a horse: One setspeeds up the pacemaker, and the other set slows it down. Forexample, when you stand up and start walking, the sympatheticnerves increase your heart rate, an adaptation that enables yourcirculatory system to provide the additional O2 needed by themuscles that are powering your activity.lfyou then sit down andrelax, the parasympathetic nerves decrease your heart rate, anadaptation that conserves energy. Hormones secreted into theblood also influence the pacemaker. For instance, epinephrine,the "fight-or-flight" hormone secreted by the adrenal glands,causes the heart rate to increase. A third type of input that affects the pacemaker is body temperature. An increase of onlyI'e raises the heart rate by about 10 beats per minute. This isthe reason your heart beats faster when you have a fever.

Having examined the operation of the circulatory pump,we turn in the next section to the forces and structures that influence blood flow in the vessels ofeach circuit.

o Pacemakergenerates wave ofsignals to contract.

ECG~... Figure 42.9 The control of heart rhythm. The sequence of electrICal events in the heartis shown at the top: the corresponding components of an electrocardiogram (ECG) are highlightedbelow in gold, In step 4, the portion of the ECG to the right of the gold "spike" representselectrical activity that reprimes the ventricles for the next round of contraction.

Maintaining the Heart'sRhythmic Beat

In vertebrates, the heartbeat originates inthe heart itself. Some cardiac muscle cellsare autorhythmic, meaning they contractand relax repeatedly without any signalfrom the nervous system. You can evensee these rhythmic contractions in tissuethat has been removed from the heartand placed in a dish in the laboratory! Because each ofthese cellshas its own intrinsic contraction rhythm, how are their contractions coordinated in the intact heart? The answer lies in agroup of autorhythmic cells located in the wall of the rightatrium, near where the superior vena cava enters the heart. Thiscluster ofcells is called the sinoatrial (SA) nodc, orpacemaker,and it sets the rate and timing at which all cardiac muscle cellscontract. (In contrast to vertebrates, some arthropods havepacemakers located in the nervous system, outside the heart.)

The SA node generates electrical impulses much like thoseproduced by nerve cells. Because cardiac muscle cells areelectrically coupled through gap junctions (see Figure 6.32),impulses from the SA node spread rapidly within heart tissue.In addition, these impulses generate currents that are conducted to the skin via body fluids. The medical test called anelectrocardiogram (ECG or, sometimes, EKG) uses electrodes placed on the skin to detect and record these currents.The resulting graph has a characteristic shape that representsthe stages in the cardiac cycle (Figure 42.9).

Impulses from the SA node first spread rapidly through thewalls of the atria, causing both atria to contract in unison. During atrial contraction, the impulses originating at the SA nodereach other autorhythmic cells that are located in the wall between the left and right atria. These cells form a relay pointcalled the atrioventricular (AV) node. Here the impulses aredelayed for about 0.1 second before spreading to the walls oftheventricles. This delay allows the atria to empty completely before the ventricles contract. Then, the signals from the AV nodeare conducted throughout the ventricular walls by specializedmuscle fibers called bundle branches and Purkinje fibers.

Physiological cues alter heart tempo by regulating the SAnode. Two sets of nerves, the sympathetic and parasympatheticnerves, are largely responsible for this regulation. TIlese nerves

people are born with heart murmurs; inothers, the valves may be damaged byinfection (from rheumatic fever, for instance). When a valve defect is severeenough to endanger health, surgeonsmay implant a mechanical replacementvalve. However, not all heart murmursare caused by a defect, and most valvedefects do not reduce the efficiency ofblood flow enough to warrant surgery.


Vein

Smoothmuscle

Endothelium

Connectivetissue

'f Figure 42.10 Thestructure of blood vessels.

Basal lamina

Capillary

100llmI I

Vein

Blood Flow Velocity

tractions. Signals from the nervous system and hormonescirculating in the blood act on the smooth muscles in ar~

teries, controlling blood flow to different parts of the

body. The thinner·walled veins convey blood back to the

heart at a lower velocity and pressure. Valves in the veinsmaintain a unidirectional flow of blood in these vessels

(see Figure 42.10).

To understand how blood vessel diameter influences bloodflow, consider how water flows through a thick hose con

nected to a faucet. When the faucet is turned on, water flowsat the same velocity everywhere in the hose. However, ifa narrow nozzle is attached to the end of the hose, the water willexit the nozzle at a much greater velocity. Because water doesn'tcompress under pressure, the volume of water movingthrough the nozzle in a given time must be the same as the

volume moving through the rest of the hose. The cross·sectional area of the nozzle is smaller than that of the hose, so

the water speeds up in the nozzle.

IArteriole

Artery

Capillary

SEM

Endothelium""

-Smooth 1}~)1:<-""muscle

Red blood cell

Artery

Blood Vessel Structure and Function

The vertebrate circulatory system enables blood to deliveroxygen and nutrients and remove wastes throughout thebody. In doing so, the circulatory system relies on a branching network of vessels much like the plumbing system thatdelivers fresh water to a city and removes its wastes. Furthermore, the same physical principles that govern the op·eration of plumbing systems apply to the functioning ofblood vessels.

Blood vessels contain a central lumen (cavity) lined with an

endothelium, a single layer of flattened epithelial cells. The

smooth surface of the endothelium minimizes resistance tothe flow of blood. Surrounding the endothelium are layers oftissue that differ among capillaries, ar·teries, and veins, reflecting the special-ized functions of these vessels.

Capillaries are the smallest bloodvessels, having a diameter only slightlygreater than that of a red blood cell(Figure 42.10). Capillaries also havevery thin walls, which consist of justthe endothelium and its basal lamina.This structural organization facili~

tates the exchange of substances be~

tween the blood in capillaries and theinterstitial fluid.

The walls of arteries and veins havea more complex organization thanthose of capillaries. Both arteries andveins have two layers of tissue surrounding the endothelium: an outerlayer of connective tissue containingelastic fibers, which allow the vessel tostretch and recoil, and a middle layercontaining smooth muscle and moreelastic fibers. However, arteries andveins differ in important ways. For a

given blood vessel diameter, an arteryhas a wall about three times as thick asthat of a vein (see Figure 42.10). The

thicker walls of arteries are very strong,accommodating blood pumped at highpressure by the heart, and their elasticrecoil helps maintain blood pressurewhen the heart relaxes between con-

r;;~~:~~;o~~i~od pressure andflow reflect the structure andarrangement of blood vessels


Blood pressure fluctuates over two different time scales.The first is the oscillation in arterial blood pressure during

each cardiac cycle (see bottom graph in Figure 42.11). Bloodpressure also fluctuates on a longer time scale in responseto signals that change the state of smooth muscles in arteriole walls. For example, physical or emotional stress cantrigger nervous and hormonal responses that cause smoothmuscles in arteriole walls to contract, a process calledvasoconstriction. When that happens, the arterioles narrow, thereby increasing blood pressure upstream in the arteries. When the smooth muscles relax, the arteriolesundergo vasodilation, an increase in diameter that causesblood pressure in the arteries to fall.

Vasoconstriction and vasodilation are often coupled to

changes in cardiac output that also affect blood pressure. This

Regulation of Blood Pressure

Changes in Blood Pressure During the Cardiac Cycle

Arterial blood pressure is highest when the heart contractsduring ventricular systole. The pressure at this time is calledsystolic pressure (see Figure 42.11). The spikes in bloodpressure caused by the powerful contractions of the ventricles stretch the arteries. By placing your fingers on your wrist,you can feel a pulse-the rhythmic bulging ofthe artery wallswith each heartbeat. The surge of pressure is partly due to thenarrow openings of arterioles impeding the exit of bloodfrom the arteries. Thus, when the heart contracts, blood enters the arteries faster than it can leave, and the vessels stretch

from the rise in pressure. During diastole, the elastic walls ofthe arteries snap back. As a consequence, there is a lower butstill substantial blood pressure when the ventricles are relaxed (diastolic pressure). Before enough blood has flowedinto the arterioles to completely relieve pressure in the arteries, the heart contracts again. Because the arteries remainpressurized throughout the cardiac cycle (see Figure 42.11),blood continuously flows into arterioles and capillaries.

Blood, like all fluids, flows from areas ofhigher pressure to areas of lower pressure. Contraction of a heart ventricle gener

ates blood pressure, which exerts a force in all dire<tions. Theforce dire<ted lengthwise in an artery causes the blood to flowaway from the heart, the site of highest pressure. The force ex·

erted against the elastic wall of an artery stretches the wall,and the recoil of arterial walls plays a critical role in maintaining blood pressure, and hence blood flow, throughout the cardiac cycle. Once the blood enters the millions oftiny arteriolesand capillaries, the narrow diameter ofthese vessels generatessubstantial resistance to flow. This resistance dissipates muchof the pressure generated by the pumping heart by the timethe blood enters the veins.

Blood Pressure

• • • • • •.~ -" ,~ -" 0 •0 , •• >

~ '~ !2 0 > •

" • v< t > •< • •u 0•>

~~~~~~i~-jfSystolicpressure

5,000

1 4,000..'::!. 3,000

~ 2,000<i 1.000

0

u 50•'g 40

'" 30-~ 20vQ 10•> 0

" 120I100E.s 80

• 60, 40

" 20~ 0

•t0<

An analogous situation exists in the circulatory system,but blood slows as it moves from arteries to arterioles to capillaries. Why? The reason is that the number of capillaries isenormous. Each artery conveys blood to so many capillaries

that the total cross-sectional area is much greater in capillary

beds than in the arteries or any other part of the circulatorysystem (Figure 42.11). The result is a dramatic decrease in

velocity from the arteries to the capillaries: Blood travels 500times slower in the capillaries (about 0.1 cmlsec) than in theaorta (about 48 cm/sec).

The reduced velocity of blood flow in capillaries is critical to the function of the circulatory system. Capillaries arethe only vessels with walls thin enough to permit the transfer of substances between the blood and interstitial fluid.The slower flow of blood through these tiny vessels allowstime for exchange to occur. After passing through the capillaries, the blood speeds up as it enters the venules andveins, which have smaller total cross-sectional areas (see

Figure 42.11).

.. Figure 42.11 The interrelationship of cross-sectionalarea of blood vessels. blood flow velocity, and bloodpressure. Owing to an increase in total cross-sectional area. bloodflow velocity deueases markedly in the arterioles and is lowest in thecapillaries, Blood pressure, the main force driving blood from the heartto the capillaries, is highest in the aorta and other arteries.


EXPERIMENT

coordination of regulatory mechanisms maintains adequateblood flow as the body's demands on the circulatory systemchange. During heavy exercise, for example, the arterioles inworking muscles dilate, causing a greater flow of oxygen-richblood to the muscles. By itself, this increased flowto the muscles

... FI 42.12 •

How do endothelial cells controlvasoconstriction?

In 1988, Masashi Yanagisawa set out to identify the endothelial factor that triggers vasoconstriction mmammals. He isolated endothelial cells from blood vessels and grewthem in liquid medium. Then he collected the liquid, which contained substances secreted by the cells, Next, he bathed a smallpiece of an artery in the liquid, The artery tissue contracted, indicating that the cells grown in culture had secreted a factor thatcauses vasoconstriction Using biochemical procedures, Yanaglsawa separated the substances in the fluid on the basis of size,charge, and other properties, He then tested each substance forits ability to cause arterial contraction, After several separationsteps and many tests, he purified the vasoconstriction factor,

would cause a drop in blood pressure (and therefore blood flow)in the body as a whole. However, cardiac output increases at thesame time, maintaining blood pressure and supporting the necessary increase in blood flow.

Recent experiments have identified the mole<ules that serve assignals for vasodilation and vasoconstriction. Three scientists inthe United States-Robert Furchgott, Louis Ignarro, and FeridMurad-demonstrated that the gas nitric oxide (NO) serves as amajor inducer ofvasodilation in the cardiovascular system. Theirresearch was honored with a Nobel Prize in 1998. Independentstudies by Masashi Yanagisawa, then a graduate student at theUniversity ofTsukuba in Japan, identified a peptide, endothelin,as a potent inducer of vasoconstriction (Figure 42.12). As discussed in the interview with Dr. Yanagisawa on pages 850-851,his findings led to fundamental discoveries about signals that regulate not only blood vessel diameter but also the embryonic development ofthe digestive system.

Blood Pressure and Gravity

(ys Trp

Blood pressure is generally measured for an artery in the armat the same height as the heart (Figure 42.13). For a healthy20-year-old human at rest. arterial blood pressure in the systemic circuit is typically about 120 millimeters of mercury(mm Hg) at systole and 70 mm Hg at diastole, a combinationdesignated 120/70. (Arterial blood pressure in the pulmonarycircuit is six to ten times lower.)

Gravity has a significant effect on blood pressure. \Vhen youare standing, for example, your head is roughly 0.35 m higherthan your chest, and the arterial blood pressure in your brainis about 27 mm Hg less than that near your heart. If the bloodpressure in your brain is too low to provide adequate blood flow,you will likely faint. By call5ing your body to collapse to the grolUld,fainting effectively places your head at the level of your heart.quickly increasing blood flow to your brain.

The challenge ofpumping blood against gravity is particularlygreat for animals with very long necks. Agiraffe, for example, requires a systolic pressure of more than 250 mm Hg near theheart. \'(!hen a giraffe lowers its head to drink, one-way valvesand sinuses, along with feedback me<hanisms that reduce cardiac output, prevent this high pressure from damaging its brain.We can calculate that a dinosaur with a neck nearly 10 m longwould have required even greater systolic pressure-nearly760 mm Hg-to pump blood to its brain when its head wasfully raised. However, calculations based on anatomy and inferred metabolic rate suggest that dinosaurs did not have aheart powerful enough to generate such high pressure. Basedon this evidence as well as studies of neck bone structure, somebiologists have concluded that the long-necked dinosaurs fedclose to the ground rather than on high foliage.

Gravity is also a consideration for blood flow in veins, especially those in the legs. Although blood pressure in veins is relatively low, several mechanisms assist the return of venous bloodto the heart. First, rhythmic contractions of smooth muscles in

203

Endothelin

Parent polypeptide

is e I I lie Tr (00-c

Endothelin

GICVaTr

RESULTS

CONCLUSION Endothelial cells produce and translate endo·thelin mRNA in response to signals, such as hormones. that circulate in the blood, The resulting polypeptide is cleaved to formactive endothelin, the substance that triggers vasoconstriction,Yanagisawa and colleagues subsequently demonstrated that endothelial cells also make the enzyme that catalyzes this cleavage.

The vasoconstriction factor, which Yanagisawanamed endothelin, is a peptide that contains 21 amino acids, Twodisulfide bridges between cysteines stabilize the peptide structure,

M. Yanag'\aWa el al .• A novel potent oaSOCOllSlrJCtorpept,de prodllCed by oas<:ular endothelial cells. NafUte 331:411-.415 (19M)

Using the amino acid sequence of the peptide as a guide,Yanagisawa identified the endothelin gene. The polypeptideencoded by the gene is much longer than endothelm, containing203 amino aCids The amino aCids in endothelm extend fromposition S3 ((ys) to position 73 (Trp) in the longer polypeptide:

Yanaglsawa also showed that treating endothelial cells with othersubstances already known to promote vasoconstriction, such asthe hormone epinephrine, led to increased production ofendothelin mRNA,

SOURCE

-mrU1iI Given what you know about epithelial tissue organization (see Figure 40.5) and the function of endothelin, whatwould you predict about the location of endothelin seuetion withregard to endothelial cell surfaces?


.. Figure 42.13 Measurement of blood pressure. Blood pressure is recorded as two numbersseparated by a slash. The first number is the systolic pressure; the second is the diastolic pressure.

the walls ofvenulesand veins aid in the

movement of the blood. Second, and

more important, the contraction of

skeletal muscles during exercise

squeezes blood through the veins to

\\wd the heart (Figure 42.14). This is

why periodically walking up and down

the aisle during a long airplane flight

helps prevent potentially dangerous

blood clots from forming in veins.

Third, the change in pressure within

the thoracic (chest) cavity during in

halation causes the venae cavae and

other large veins near the heart to ex

pand and fill with blood.

In rare instances, runners and

other athletes can suffer heart failure if

they stop vigorous exercise abruptly.

\Vhen the leg muscles suddenly cease

contracting and relaxing, less bloodreturns to the heart, which continues

to beat rapidly. If the heart is weak or

damaged, this inadequate blood flow

may cause the heart to malfunction.

To reduce the risk of stressing the

heart excessively, athletes are encour

aged to follow hard exercise with

moderate activity, such as walking, to

~cool down" until their heart rate ap

proaches its resting level.

Pressure incuff below70 mm Hg

8The cuff is allowed to deflatefurther, just until the blood flowsfreely through the artery and thesounds below the cuff disappear.The pressure at this point is thediastolic pressure.

120

Blood pressure reading: 120/70

Pressure in cuffdrops below120 mm Hg

Pressure in cuffgreater than120 mm Hg

Arteryclosed

Rubbercuffinflatedwith air

eThe cuff is allowed to deflate gradually. Whenthe pressure exerted by the cuff falls just belowthat in the artery, blood pulses into the forearm,generating sounds that can be heard with thestethoscope. The pressure measured at this pointis the systolic pressure.

oAsphygmomanometer, an inflatable cuff attached to a pressuregauge, measures blood pressure in an artery. The cuff is inflated until thepressure closes the artery, so that no blood flows past the cuff. When thisoccurs, the pressure exerted by the cuff exceeds the pressure in the artery.

.. Figure 42.14 Blood flow in veins. Skeletal musclecontraction squeezes and constricts veins. Flaps of tissue within theveins act as one-way valves that keep blood moving only toward theheart. If you sit or stand too long. the lack of muscular activity maycause your feet to swell as blood pools in your veins.

Direction of blood flow----iltLin vein (toward heart) ';--,-----Valve (open)

Skeletal muscle

~~-Valve (closed)

Capillary Function

At any given time, only about 5-10% of the body's capillar

ies have blood flowing through them. However, each tissue

has many capillaries, so every part of the body is supplied

with blood at all times. Capillaries in the brain, heart, kid

neys, and liver are usually filled to capacity, but at many

other sites the blood supply varies over time as blood is di

verted from one destination to another. For example, blood

flow to the skin is regulated to help control body tempera

ture, and blood supply to the digestive tract increases aftera meal. During strenuous exercise, blood is diverted from

the digestive tract and supplied more generously to skeletal

muscles and skin. This is one reason why exercising heavily

immediately after eating a big meal may cause indigestion.

Given that capillaries lack smooth muscles, how is blood

flow in capillary beds altered? There are two mechanisms, both

of which rely on signals that regulate flow into capillaries. One

mechanism involves contraction of the smooth muscle in the

wall ofan arteriole, which reduces the vessel's diameter and de

creases blood flow to the adjoining capillary beds. \Vhen the

smooth muscle relaxes, the arterioles dilate, allowing blood to

enter the capillaries. The other mechanism for altering flow,


7 ..

Net fluidmovement in

INTERSTITIAL FLUID

Net fluid

Direction ofblood flow

Body tissue/

Capillary

Venule

Thoroughfarechannel

Precapillary sphincters

(a) Sphincters relaKed

r'"1....""~.BIOOd pressure

Venous end

Inward flow~

Arterial end of capillary

Fluid Return by the Lymphatic System

Throughout the body, only about 85% of the fluid thatleaves the capillaries because of blood pressure reentersthem as a result of osmotic pressure. Each day, this imbalance results in a loss of about 4 L of fluid from capillariesto the surrounding tissues. There is also some leakage ofblood proteins, even though the capillary wall is not verypermeable to large molecules. The lost fluid and proteinsreturn to the blood via the lymphatic system, which

\Vhile blood pressure tends to drive fluid out of the capillaries, the presence of blood proteins tends to pull fluid back intothe capillaries. Many blood proteins (and all blood cells) are toolarge to pass readily through the endothelium, and they remainin the capillaries. The proteins, especiallyalbumin, create an osmotic pressure difference between the capillary interior and theinterstitial fluid. In places where the blood pressure is greaterthan the osmotic pressure difference, there is a net loss of fluidfrom thecapillaries. In contrast, where the osmotic pressure dif~

ference exceeds the blood pressure, there is a net movement offluid from the tissues into the capillaries (Figure 42.16).

... Figure 42.16 Fluid eKchange between capillaries and tkeinterstitial fluid. This diagram shows a hypothetical capillary in whichosmotIC pressure is constant along its length, At the arterial end. whereblood pressure exceeds osmotic pressure. fluid flows out of the capillaryinto the interstitial fluid. At the venous end. the blood pressure is lessthan osmotic pressure. and fluid flows from the interstitial fluid into thecapillary, In many capillaries. blood pressure may be higher or lower thanosmotIC pressure throughout the entire length of the capillary,

Venule

(b) Sphincters contracted

.. Figure 42.15 Blood flow in capillary beds. Precapillarysphincters regulate the passage of blood into capillary beds. Someblood flows directly from arterioles to ~enules through capillaries calledthoroughfare channels. which are always open.

shown in Figure 42.15, involves the action of precapillarysphincters, rings of smooth muscle located at the entrance tocapillary beds. The signals that regulate blood flow includenerve impulses, hormones traveling throughout the bloodstream, and chemicals produced locally. For example, the chemical histamine released by cells at a wound site causes smoothmuscle relaxation, dilating blood vessels and increasing bloodflow. The dilated vessels also provide disease~fighting whiteblood cells greater access to invading microorganisms.

As you have read, the critical exchange of substances bern'een the blood and interstitial fluid takes place across thethin endothelial walls of the capillaries. Some substances arecarried across the endothelium in vesicles that form on oneside by endocytosis and release their contents on the opposite side by exocytosis. Small molecules, such as O2 and CO2,

simply diffuse across the endothelial cells or through theopenings within and between adjoining cells. These openings also provide the route for transport ofsmall solutes suchas sugars, salts, and urea, as well as for bulk flow of fluid intotissues driven by blood pressure within the capillary.


CONCEPT CHECK

includes a network of tiny vessels intermingled among capillaries of the cardiovascular system. After entering thelymphatic system by diffusion, the fluid is called lymph; itscomposition is about the same as that of interstitial fluid.

The lymphatic system drains into large veins of the circu

latory system at the base of the neck (see Figure 43.7). Asyou read in Chapter 41, this joining of the lymphatic andcirculatory systems functions in the transfer of lipids fromthe small intestine to the blood.

The movement of lymph from peripheral tissues to theheart relies on much the same mechanisms that assist bloodflow in veins. Lymph vessels, like veins, have valves that prevent the backflow of fluid. Rhythmic contractions of the vesselwalls help draw fluid into the small lymphatic vessels. In addition, skeletal muscle contractions playa role in moving lymph.

Disorders that interfere with the lymphatic system highlight its role in maintaining proper fluid distribution in thebody. Disruptions in the movement of lymph often cause

edema, swelling resulting from the excessive accumulation offluid in tissues. Severe blockage of lymph flow, as occurs whencertain parasitic worms lodge in lymph vessels, results in extremely swollen limbs or other body parts, a condition knownas elephantiasis.

Along a lymph vessel are organs called lymph nodes. By filtering the lymph and by housing cells that attack viruses andbacteria, lymph nodes play an important role in the body's defense. Inside each lymph node is a honeycomb of connectivetissue with spaces filled by white blood cells. When the bodyis fighting an infection, these cells multiply rapidly, and thelymph nodes become swollen and tender (which is why yourdoctor may check for swollen lymph nodes in your neck,

armpits, or groin when you feel sick). Because lymph nodeshave filtering and surveillance functions, doctors may examine the lymph nodes of cancer patients to detect the spread ofdiseased cells.

In recent years, evidence has appeared suggesting that thelymphatic system also has a role in harmful immune responses, such as those responsible for asthma. Because ofthese and other findings, the lymphatic system, largely ignored until the 1990s, has become a very active and promisingarea of biomedical research.

42.JI. What is the primary cause of the low velocity ofblood

flow through capillaries?2. What short-term changes in cardiovascular function

might best enable skeletal muscles to help an animalescape from a dangerous situation?

3. _',IMilly If you had additional hearts distributed

throughout your body, what would be one likely advantage and one likely disadvantage?

For suggested answers. see Appendix A.

r:'II~":~':::~~ents function inexchange, transport, and defense

As we discussed earlier, the fluid transported by an open circulatory system is continuous with the fluid that surrounds all ofthe body cells and therefore has the same composition. In contrast, the fluid in a closed circulatory system can be much morehighly specialized, as is the case for the blood of vertebrates.

Blood Composition and Function

Vertebrate blood is a connective tissue consisting of cells suspended in a liquid matrix called plasma. Dissolved in theplasma are ions and proteins that, together with the bloodcells, function in osmotic regulation, transport, and defense.

Separating the components ofblood using a centrifuge revealsthat cellular elements (cells and cell fragments) occupy about45% of the volume of blood (Figure 42.17, on the next page).The remainder is plasma.

Plasma

Among the many solutes in plasma are inorganic salts in theform of dissolved ions, sometimes referred to as bloodelectrolytes (see Figure 42.17). Although plasma is about 90%water, the dissolved salts are an essential component of theblood. Some of these ions buffer the blood, which in humansnormally has a pH of 7.4. Salts are also important in maintaining the osmotic balance of the blood. In addition, the concentration of ions in plasma directly affects the compositionof the interstitial fluid, where many of these ions have a vitalrole in muscle and nerve activity. To serve all of these func

tions, plasma electrolytes must be kept within narrow concentration ranges, a homeostatic function we will explore inChapter 44.

Plasma proteins act as buffers against pH changes, helpmaintain the osmotic balance between blood and interstitialfluid, and contribute to the blood's viscosity (thickness). Particular plasma proteins have additional functions. The immunoglobulins, or antibodies, help combat viruses and otherforeign agents that invade the body (see Chapter 43). Othersare escorts for lipids, which are insoluble in water and can

travel in blood only when bound to proteins. A third group ofplasma proteins are clotting factors that help plug leaks whenblood vessels are injured. (The term serum refers to bloodplasma from which these clotting factors have been removed.)

Plasma also contains a wide variety of other substances intransit from one part of the body to another, including nutrients, metabolic wastes, respiratory gases, and hormones.Plasma has a much higher protein concentration than interstitial fluid, although the two fluids are otherwise similar. (Capillary walls, remember, are not very permeable to proteins.)


Cellular elements 45%

Number functionsper Jll (mml) of blood

Constituent

Water

Ions (blood electrolytes)

SodiumPotassiumCalciumMagnesiumChlorideBICarbonate

Major functions

Solvent fO(

carrying othersubstances

OsmotiC balance.pH buffering. andrequlallon ofmembranepermeability

'----v---'Separatedbloodelements

Cell type

Erythrocytes 5-6 mdbon(red blood cells).e,leukocytes 5,000-10,000(white blood cells)

Transport oxygenand help transportcarbon dioxide

Defense andImmUnity

Substances transported by blood

Nutrients (such as glucose, fatty aods, VitaminS)Waste products of metabolM1Respiratory gases (01 and COl)

"""""""

Monocyte

lymphocyte

Blood donlng

OsmollC balancepH buffering

Clotting

Defense

Flbnnogen

Immunoglobulins(antIbodies)

Plasma proteins

Albumin

• Figure 42,17 The composition of mammalian blood.

Cellular Elements

Suspended in blood plasma are two classes ofcells: red bloodcells, which transport~ and white blood cells, which function indefense (see Rgure 4217). Blood also contLins platelets, frag·ments ofcells that are involved in the clotting process.

Erythrocytes Red blood cells, or erythrocytes, are by far themost numerous blood cells. Each microliter (ilL, or mm3) ofhuman blood contains 5-6 million red cells, and there areabout 25 trillion of these cells in the body's 5 Lof blood. Theirmain function is~ transport, and their structure is closely related to this function. Human erythrocytes are small disks (7-811m in diameter) that are biconcave-thinner in the center thanat the edges. This shape increases surface area, enhancing therate ofdiffusion ofO2 across their plasma membranes. Maturemammalian erythrocytes lack nuclei. This unusual characteristic leaves more space in these tiny cells for hemoglobin, theiron-containing protein that transports O2 (see Figure 5.21).Erythrocytes also lack mitochondria and generate their ATPexclusively by anaerobic metabolism. Oxygen transport wouk!be less efficient if erythrocytes were aerobic and consumedsome of the O2 they carry.

Despite its small size, an erythrocyte contains about 250million molecules of hemoglobin. Because each molecule of

hemoglobin binds up to four moleculesofO:z, one erythrocytecan transport about a billion ~ molecules. As erythrocytespass through the capillary beds of lungs, gills, or other respiratory organs,~ diffuses into the erythrocytes and binds tohemoglobin. In the systemic capillaries, O2 dissociates fromhemoglobin and diffuses into body celts.

Leukocytes The blood contains five major types of whiteblood cells, or leukocytes. Their function is to fight infections. Some are phagocytic, engulfing and digesting microorganisms as well as debris from the body's own dead cells. Aswe '",ill see in Chapter 43, other leukocytes, called lymphocytes, develop into specialized Bcells and T cells that mountimmune responses against foreign substances. Normally, 1ilLof human blood contains about 5,000-1O,lXXl leukocytes;their numbers increase temporarily whenever the body isfighting an infection. Unlike erythrocytes, leukocytes are alsofound outside the circulatory system, patrolling both interstitial fluid and the lymphatic system.

Platelets Platelets are pinched-offcytoplasmic fragments ofspecialized bone marrow cells. They are about 2-3 11m in diameter and have no nuclei. Platelets serve both structural andmolecular functions in blood clolting.


Prothrombin --L Thrombin

Fibrinogen -L Fibrin

Clotting factors from:Platelets

l_~=:::1: Damaged cellsPlasma (factors include calcium, vitamin K)

Red blood cellFibrin clot

{) This seal is remforced by a clot of fibrin when vessel damageis severe. Fibrin is formed via a multistep process: Clottingfactors released from the clumped platelets or damaged cellsmix with clotting factors in the plasma, forming an activationcascade that converts a plasma protem called prothrombm toits active form, thrombin, Thrombin itself is an enzyme that

catalyzes the final step of the clottingprocess. the conversion of fibrinogento fibrin The threads of fibrm becomeinterwoven into a clot (see colorizedSEM below),

Plateletplug

e The plateletsform a plug thatprovides emergencyprotection agamstblood loss.

Platelet releases chemicalsthat make nearby platelets sticky

o The clotting process begins when theendothelium of a vessel is damaged,exposing connective tissue in thevessel wall to blood. Platelets adhereto collagen fibers in the connectivetissue and release a substance thatmakes nearby platelets sticky.

Monocytes

lymphoidstem cells

Basophils

Neutrophils

Myeloidstem cells

Eosinophils

•Stem cells(in bone marrow)•+

Tcells

•+lymphocytes

Erythrocytes

Bcells

,.

Blood Clotting

... Figure 42.18 Blood clotting.

The occasional cut or scrape is not life-threatening becauseblood components seal the broken blood vessels. A break in ablood vessel wall exposes proteins that attract platelets and initiate coagulation, the conversion ofliquid components ofbloodto a solid dot. The coagulant, or sealant, circulates in an inactive form called fibrinogen. Clotting involves the conversion offibrinogen to its active form, fibrin, which aggregates intothreads that form the framework of the dot. The formation offibrin is the last step in a series of reactions triggered by the release ofclotting factors from platelets (Figure 42,18). Ageneticmutation that affects any step of the clotting process causeshemophilia, a disease characterized by excessive bleeding andbruising from even minor cuts and bumps.

Anticlotting factors in the blood normally prevent spontaneous dotting in the absence of injury. Sometimes, however,clots form within a blood vessel, blocking the flow of blood.Such adot is called a thrombus. We will explore howa thrombus forms and the danger that it poses later in this chapter.

Stem Cells and the Replacement of Cellular Elements

Erythrocytes, leukocytes, and platelets all develop from a common source: multipotent stem cells that are dedicated to replenishing the body's blood cell populations (Figure 42.19).

.. Figure 42.19 Differentiation of blood cells. Some of themultipotent stem cells differentiate mto lymphoid stem cells. whIChthen develop mto Bcells and Tcells, two types of lymphocytes thatfunction in the immune response (see Chapter 43). All other bloodcells differentiate from myeloid stem cells.


The stem cells that produce blood cells are located in the redmarrow of bones, particularly the ribs, vertebrae, sternum, andpelvis. Multipotent stem cells are so named because they havethe ability to form multiple types of cells-in this case, themyeloid and lymphoid cell lineages. \Vhen any stem cell divides,one daughter ceU remains a stem cell while the other takes on aspecialized function.

Throughout a person's life, erythrocytes, leukocytes, andplatelets formed from stem cell divisions replace the worn-outcellular elements of blood. Erythrocytes, for example, usuallycirculate for only three to four months before being replaced;the old cells are consumed by phagocytic cells in the liver andspleen. The production of new erythrocytes involves recyclingof materials, such as the use of iron scavenged from old erythrocytes in new hemoglobin molecules.

A negative-feedback mechanism, sensitive to the amount ofO2 reaching the body's tissues via the blood, controls erythrocyte production. Ifthe tissues do not receive enough 02' the kidneys synthesize and secrete a hormone called erythropoietin(EPO) that stimulates erythrocyte production. If the blood isdelivering more O2 than the tissues can use, the level of EPOfalls and erythrocyte production slows. Physicians use syntheticEPO to treat people with health problems such as anemia, acondition of lower-than-normal hemoglobin levels. Some athletes inject themselves with EPO to increase their erythrocytelevels, although this practice, a form of blood doping, has beenbanned by the International Olympic Committee and othersports organizations. In recent years, a number of well-knownrunners and cyclists have tested positive for EPO-related drugsand have forfeited both their records and their right to participate in future competitions.

Cardiovascular Disease

More than half of all human deaths in the United States arecaused by cardiovascular diseases-disorders of the heart andblood vessels. Cardiovascular diseases range from a minor disturbance of vein or heart valve function to a life-threatening

disruption of blood flow to the heart or brain. The tendency todevelop particular cardiovascular diseases is inherited but isalso strongly influenced by lifestyle. Smoking, lack ofexercise,and a diet rich in animal fat each increase the risk ofa numberofcardiovascular diseases.

Atherosclerosis

One reason cardiovascular diseases cause so many deaths isthat they often aren't detected until they disrupt critical bloodflow. An example is atherosclerosis, the hardening of the arteries by accumulation of fatty deposits. Healthy arteries havea smooth inner lining that reduces resistance to blood flow.Damage or infection can roughen the lining and lead to inflammation. Leukocytes are attracted to the damaged liningand begin to take up lipids, including cholesterol. A fatty deposit, called a plaque, grows steadily, incorporating fibrousconnective tissue and additional cholesterol. As the plaquegrows, the walls of the artery become thick and stiff, and theobstruction of the artery increases (Figure 42.20).

Atherosclerosis sometimes produces warning signs. Partialblockage of the coronary arteries, which supply oxygen-richblood to the heart muscle, may cause occasional chest pain, acondition known as angina pectoris. The pain is most likely tobe felt when the heart is laboring hard during physical or emotional stress, and it signals that part of the heart is not receivingenough 02' However, many people with atherosclerosis arecompletely unaware oftheir condition until catastrophe strikes.

Heart Attacks and Stroke

If unrecognized and untreated, the result ofatherosclerosis isoften a heart attack or a stroke. A heart attack, also called amyocardial infarction, is the damage or death ofcardiac muscle tissue resulting from blockage ofone or more coronary arteries. Because the coronary arteries are small in diameter,they are especially vulnerable to obstruction. Such blockagecan destroy cardiac muscle quickly because the constantlybeating heart muscle cannot survive long without 02' If the

>-----;250 11m

Plaque

,Endothelium

" ._ Otf-----<50 ~m (b) Partly clogged artery(a) Normal artery

(onnectlve... Figure 42.20 Atherosclerosis. Theselight micrographs contrast (a) a cross sedionof a normal (healthy) artery with (b) that of anartery partially blocked by an atheroscleroticplaque, Plaques consist mostly of fibrousconnective tissue and smooth muscle cellsIllfiltrated with lipids.


r~::j::~~~~50ccurs acrossspecialized respiratory surfaces

In the remainder of this chapter, we will focus on the processof gas exchange. Although this process is often called respiratory exchange or respiration, it should not be confused withthe energy transformations of cellular respiration. Gas exchange is the uptake of molecular O2 from the environmentand the discharge of CO2 to the environment.

I. Explain why a physician might order a white cellcount for a patient with symptoms of an infection.

2. Clots in arteries can cause heart attacks and strokes.Why, then, does it make sense to treat hemophiliacsby introducing clotting factors into their blood?

3. • ,,'!:tUla Nitroglycerin (the key ingredient in dy

namite) is sometimes prescribed for heart disease patients. Within the body, the nitroglycerin is convertedto nitric oxide. Why would you expect nitroglycerinto relieve chest pain in these patients?


Partial Pressure Gradients in Gas Exchange

To understand the driving forces for gas exchange, we mustcalculate partial pressure, which is simply the pressure exerted by a particular gas in a mixture of gases. To do so, weneed to know the pressure that the mixture exerts and thefraction of the mixture represented by a particular gas. Let'sconsider O2 as an example. At sea level, the atmosphere exertsa downward force equal to that of a column of mercury (Hg)

7fiJ mm high. Therefore, atmospheric pressure at sea level is7fiJ mm Hg. Since the atmosphere is 21% O 2 by volume, thepartial pressure of02 is 0.21 x 7fiJ, or about lfiJ mm Hg. Thisvalue is called the partialpressure of O2 (abbreviated P~) because it is the portion ofatmospheric pressure contributed byO 2, The partial pressure of CO2, Pc~, is much less, only 0.29

mm Hg at sea level.Calculating partial pressure for a gas dissolved in liquid,

such as water, is also straightforward. \'X'hen water is exposedto air, the amount of a gas that dissolves in the water is proportional to its partial pressure in the air and its solubility in water.Equilibrium is reached when gas molecules enter and leave thesolution at the same rate. Atequilibrium, the partial pressure ofthe gas in the solution equals the partial pressure of the gas in

the air. Therefore, the P0:2 in water exposed to air at sea levelmust be lfiJ mm Hg, the same as that in the atmosphere. However, the concentrations of O 2 in the air and water differ sub

stantially because O2 is much less soluble in water than in air.

42.4CONCEPT CHECKheart stops beating, the victim may nevertheless survive if aheartbeat is restored by cardiopulmonary resuscitation (CPR)or some other emergency procedure within a few minutes ofthe attack. A stroke is the death of nervous tissue in the brain

due to a lack of 02' Strokes usually result from rupture orblockage ofarteries in the head. The effects of a stroke and theindividual's chance of survival depend on the extent and loca

tion of the damaged brain tissue.Heart attacks and strokes frequently result from a throm

bus that dogs an artery. A key step in thrombus formation isthe rupture of plaques by an inflammatory response, analogous to the body's response to a cut infected by bacteria (seeFigure 43.8). A fragment released by plaque rupture is sweptalong in the bloodstream, sometimes lodging in an artery. Thethrombus may originate in a coronary artery or an artery inthe brain, or it may develop elsewhere in the circulatory system and reach the heart or brain via the bloodstream.

Treatment and Diagnosis of Cardiovascular Disease

One major contributor to atherosclerosis is cholesterol. Cholesterol travels in the blood plasma mainly in the form of particles consisting of thousands of cholesterol molecules andother lipids bound to a protein. One type of particlelow-density lipoprotein (tOt), often called"badcholesteroris associated with the deposition of cholesterol in arterialplaques. Another type-high-dcnsity lipoprotein (HOL), or"good cholesterol"-appears to reduce the deposition ofcholes

terol. Exercise de<:reases the LDLlHDL ratio. Smoking and consumption ofcertain processed vegetable oils called transfats (seeChapter 5) have the opposite effect. Many individuals at high riskfor cardiovascular disease are treated with drugs called statins,which lower LDL levels and thereby reduce the frequency ofheart attacks.

The recent recognition that inflammation has a central rolein atherosclerosis and thrombus formation is changing the diagnosis and treatment of cardiovascular disease. For example,aspirin, which blocks the inflammatory response, has beenfound to help prevent the recurrence of heart attacks andstroke. Researchers have also focused attention on C-reactiveprotein (CRP), which is produced by the liver and found in the

blood during episodes ofacute inflammation. Like a high levelof LDL cholesterol, the presence of significant amounts ofCRP in blood is a useful predictor of cardiovascular disease.

Hypertension (high blood pressure) is yet another contributor to heart attack and stroke as well as other health problems.According to one hypothesis, chronic high blood pressuredamages the endothelium that lines the arteries, promotingplaque formation. The usual definition of hypertension inadults is a systolic pressure above 140 mm Hg or a diastolicpressure above 90 mm Hg. Fortunately, hypertension is simpleto diagnose and can usually be controlled by dietary changes,exercise, medication, or a combination of these approaches.


Once we have calculated partial pressures, we can readilypredict the net result of diffusion at gas exchange surfaces: Agas always diffuses from a region of higher partial pressure toa region oflower partial pressure.

Respiratory Media

The conditions for gas exchange vary considerably, dependingon whether the respiratory medium-the source of 02-is airor water. Asalready noted, O2is plentiful in air, making upabout21 %of Earth's atmosphere by volume. Compared to water, air ismuch less dense and less viscous, so it is easier to move and to

force through small passageways. As a result, breathingair is relatively easy and need not be particularly efficient. Humans, forexample, extract only about 25% of the O2in the air we inhale.

Gas exchange with water as the respiratory medium is muchmore demanding. The amount of~ dissolved in a given volume ofwater varies but is always less than in an equivalent volume of air: Water in many marine and freshwater habitatscontains only 4-8 mL ofdissolved O2per liter, a concentrationroughly 40 times less than in air. The warmer and saltier the water is, the less dissolved O2itcan hold. Water's lower O2content,greater density, and greater viscosity mean that aquatic animalssuch as fishes and lobsters must expend considerable energy tocarry out gas exchange. In the contextofthese challenges, adaptations have evolved that in general enable aquatic animals to be

very efficient in gas exchange. Many of these adaptations involve the organization of the surfaces dedicated to exchange.

Respiratory Surfaces

Specialization for gas exchange is apparent in the structure ofthe respiratory surface, the part ofan animal's body where gasexchange occurs. Like all living cells, the cells that carry outgas exchange have a plasma membrane that must be in contact with an aqueous solution. Respiratory surfaces are therefore always moist.

The movement of~ and CO2across moist respiratory surfaces takes place entirely by diffusion. The rate of diffusion isproportional to the surface area across which it occurs and inversely proportional to the square ofthe distance through whichmolecules must move. In other words, gas exchange is fastwhenthe area for diffusion is large and the path for diffusion is short.As a result, respiratory surfaces tend to be large and thin.

The structure of a respiratory surface depends mainly onthe size of the animal and whether it lives in water or on land,but it is also influenced by metabolic demands for gas exchange. Thus, an endotherm generally has a larger area of respiratory surface than a similar-sized lXtotherm.

In some relatively simple animals, such as sponges, cnidarians, and flatworms, every cell in the body is close enough to theexternal environment that gases can diffuse quickly between all

Parapodium (functions as gill)

(a) Marine worm. Many polychaetes (marineworms of the phylum Annelida) have a pairof flanened appendages called parapodiaon each body segment. The parapodiaserve as gills and also function in crawlingand swimming,

(b) Crayfish. Crayfish and other crustaceanshave long, feathery gills covered by theexoskeleton, Specialized body appendagesdrive water over the gill surfaces,

(c) Sea star. The gills of a sea star are simpletubular prOjections of the skin, The hollowcore of each gill is an extension of thecoelom (body cavity), Gas exchange occursby diffusion across the gill surfaces. andfluid in the coelom circulates in and out ofthe gills, aiding gas transport, The surfacesof a sea star's tube feet also function ingas exchange,

... Figure 42.21 Diversity in the structure of gills, external body surfaces thatfunction in gas exchange.


cells and the environment. In many animals, however, the bulkof the body's cells lack immediate access to the environment.The respiratory surface in these animals is a thin, moist epithelium that constitutes a respiratory organ.

The skin serves as a respiratory organ in some animals, in

cluding earthworms and some amphibians. Just below theskin, a dense nern'ork of capillaries facilitates the exchange ofgases between the circulatory system and the environment.Because the respiratory surface must remain moist, earthworms and many other skin-breathers can survive for extended periods only in damp places.

The general body surface of most animals lacks sufficientarea to exchange gases for the whole organism. The solution isa respiratory organ that is extensively folded or branched,thereby enlarging the available surface area for gas exchange.Gills, tracheae, and lungs are three such organs.

Gills in Aquatic AnimalsGills are outfoldings of the body surface that are suspended inthe water. As illustrated in Figure 42.21, on the facing page,the distribution of gills over the body can vary considerably.Regardless of their distribution, gills often have a total surfacearea much greater than that of the rest of the body.

Movement of the respiratory medium over the respiratorysurface, a process called ventilation, maintains the partial pressure gradients of O2 and CO2 across the gill that are necessaryfor gas exchange. To promote ventilation, most gill-bearing an-

imals either move their gills through the water or move waterover their gills. For example, crayfish and lobsters have paddle·like appendages that drive a current of water over the gills,whereas mussels and clams move water with cilia. Octopuses

and squids ventilate their gills by taking in and ejecting water,with the side benefit of locomotion by jet propulsion. Fishes usethe motion of swimming or coordinated movements of themouth and gill covers to ventilate their gills. In both cases, a current ofwater enters the mouth, passes through slits in the pharynx, flows over the gills, and then exits the body (Figure 42.22).

The arrangement of capillaries in a fish gill allows forcountercurrent exchange, the exchange of a substance orheat between two fluids flOWing in opposite directions. In afish gill, this process maximizes gas exchange efficiency. Because blood flows in the direction opposite to that of waterpassing over the gills, ateach point in its travel blood is less saturated with O2 than the water it meets (see Figure 42.22). Asblood enters a gill capillary, it encounters water that is com·

pleting its passage through the gill. Depleted of much ofits dissolved O2, this water nevertheless has a higher Paz than theincoming blood, and O2 transfer takes place. As the bloodcontinues its passage, its Paz steadily increases, but so doesthat of the water it encounters, since each successive positionin the blood's travel corresponds to an earlier position in thewater's passage over the gills. Thus, a partial pressure gradientfavoring the diffusion of O2 from water to blood exists alongthe entire length of the capillary.

Fluid flowthrough

gill filament

r-__,POl (mm Hg) icc,W.'_te_'__-",-

Net diffusion of O2Irom waterto blood

Oxygen-poor blood "

Oxygen-rich blood

"Gill filamentorganization

Gill filaments

Operculum

1I'!r:::=O~Blood~essels

... Figure 42.22 The structure andfunction of fish gills. A fish continuouslypumps water through its mouth and o~er gillarches, using coordinated mo~ements of the jawsand operculum (gill cover) lor this ~entilation. (ASWimming fish can simply open its mouth and letwater flow pa)\ its gills,) Each gill arch has tworows of gill filaments, composed of flattenedplates called lamellae. Blood flowing throughcapillaries within the lamellae picks up O2 fromthe water. Notice that the countercurrent flow ofwater and blood maintains a partial pressuregradient down which O2 diffuses from the waterinto the blood over the entire length 01 acapillary.

Gillarch

Anatomy of gills


Muscle fiber

~Tracheoles

2.5 ~m

(c) The micrograph above shows cross sections oftracheoles in a tiny piece of insect flight muscle(TEM). Each of the numerous mitochondria in themuscle cells lies within about 5 11m of a tracheole.

Body wall

Lungs

changed by diffusion across the moist epithelium that lines thetips of the tracheal branches (Figure 42.23b). Because the tra

cheal system brings air within a very short distance of virtuallyall body cells in an insect, it can transport O2 and CO2 withoutthe participation of the animal's open circulatory system.

For small insects, diffusion through the tracheae brings inenough O2 and removes enough CO2 to support cellular respiration. Larger insects meet their higher energy demands by ventilating their tracheal systems with rhythmic body movements thatcompress and expand the air tubes like bellows. For example, aninsect in flight has a very high metabolic rate, consuming 10 to200 times more~ than it does at rest. In many flying insects, alternating contraction and relaxation ofthe flight muscles pumpsair rapidly through the tracheal system. The flight muscle cells arepacked with mitochondria that support the high metabolic rate,and the tracheal tubes supply each of these ATP-generating or·ganelles",~th ample~ (Figure 42.23c). Thus,adaptationsoftracheal systems are directly related to bioenergetics.

Unlike tracheal systems, which branch throughout the insectbody, lungs are localized respiratory organs. Representing aninfolding of the body surface, they are typically subdividedinto numerous pockets. Because the respiratory surface of alung is not in direct contact with all other parts of the body,the gap must be bridged by the circulatory system, whichtransports gases between the lungs and the rest of the body.Lungs have evolved in organisms with open circulatory systems, such as spiders and land snails, as well as in vertebrates.

Among vertebrates that lack gills, the use of lungs for gasexchange varies. Amphibian lungs, when present, are relativelysmall and lack an extensive surface for exchange. Amphibians

Body

cell -"''--'''0'-Tracheole ------'~

(b) Air enters the tracheae through openings on theinsect's body surface and passes into smaller tubescalled tracheoles. The tracheoles are closed. and theirterminal ends contain fluid (blue'gray), When theanimal is active and using more 01, most of the fluid iswithdrawn into the body. This increases the surfacearea of air-filled tracheoles in contact with cells.

(a) The respiratory systemof an insect conSists ofbranched internal tubesthat deliver air directlyto body cells. Rings ofchitin reinforce thelargest tubes. calledtracheae, keeping themfrom collapsing.Enlarged portions oftracheae form air sacsnear organs that reqUirea large supplyof oxygen.

'f Figure 42.23 Tracheal systems.

Although the most familiar respiratory structure among terrestrial animals is the lung, the most common is actually thetracheal system of insects. Made up of air tubes that branchthroughout the body, this system is one variation on the themeof an internal respiratory surface. The largest tubes, called tracheae, open to the outside (Figure 42.23a). The finest branchesextend close to the surface of nearly every cell, where gas is ex-

Tracheal Systems in Insects

Countercurrent exchange mechanisms are remarkably efficient. In the fish gill, more than 8O'i'6 of the O2 dissolved in thewater is removed as it passesover the respiratory surface. Countercurrent exchange also contributes to temperature regulation(see Chapter 40) and to the functioning of the mammalian kidney, as we will see in Chapter 44.

Gills are generally unsuitable for an animal living on land Anexpansive surface of wet membrane exposed directly to air currents in the environment would lose too much water by evaporation. Furthermore, the gills would collapse as their fine filaments,no longer supported by water, would cling together. In most terrestrial animals, respiratory surfaces are enclosed within thebody, exposed to the atmosphere through narrow tubes.


... Figure 42.24 The mammalian respiratory system. From thenasal cavity and pharynx, inhaled air passes through the larynx, trachea,and bronchi to the bronchioles. which end in microscopIC alveoli lined bya thin. moist epithelium. Branches of the pulmonary arteries conveyoxygen-poor blood to the alveoli; branchesof the pulmonary veins transportoxygen-rich blood from the alveoliback to the heart. The leftmicrograph shows the densecapillary bed that envelops thealveoli. The right micrographis a cutaway view of alveoli.

.,"'-"f--Nasal1'-::;;;' cavity

Pharynx ------

Larynx----

(Esophagus)Trachea------j

Right lung ---------,r

Bronchus-,-----+-i""L~

Bronchiole---'-----j,r

Diaphragm-----+-~,.

Branchofpulmonaryvein(oxygen-richblood)

Terminalbronchiole

S'M

Branchofpulmonaryartery(oxygen-poorblood)

150 IJ.m I Colorjzed SEM

instead rely heavily on diffusion across other body surfaces,such as the skin, to carry out gas exchange. In contrast, mostreptiles (including all birds) and all mammals depend entirelyon lungs for gas exchange. Turtles are an exception; they supplement lung breathing with gas exchange across moist epithelial surfaces continuous with their mouth or anus. Lungsand air breathing have evolved in a few aquatic vertebrates (including lungfishes) as adaptations to living in oxygen-poorwater or to spending part of their time exposed to air (for instance, when the water level of a pond recedes).

In general, the size and complexity of lungs are correlatedwith an animal's metabolic rate (and hence its rate of gas exchange). For example, the lungs of endotherms have a greaterarea ofexchange surface than those ofsimilar-sized ectotherms.

Mammalian Respiratory Systems: A Closer Look

In mammals, a system of branching ducts conveys air to thelungs, which are located in the thoracic cavity (Figure 42,24),Air enters through the nostrils and is then filtered by hairs,warmed, humidified, and sampled for odors as it flowsthrough a maze of spaces in the nasal cavity. The nasal cavityleads to the pharynx, an intersection where the paths for airand food cross. When food is swallowed, the larynx (the upper part of the respiratory tract) moves upward and tips the

epiglottis over the glottis (the opening of the trachea, orwindpipe), This allows food to go down the esophagus to thestomach (see Figure 41.11). The rest of the time, the glottis isopen, enabling breathing.

From the larynx, air passes into the trachea. Cartilage reinforcing the walls of both the larynx and the trachea keeps thispart of the airway open. In most mammals, the larynx alsofunctions as a voice box. Exhaled air rushes by the vocalcords, a pair of elastic bands of muscle in the larynx. Soundsare produced when muscles in the voice box are tensed,stretching the cords so they vibrate. High-pitched sounds reosuit from tightly stretched cords vibrating rapidly; low·pitchedsounds come from less tense cords vibrating slowly.

From the trachea fork two bronchi (singular, bronchus), oneleading to each lung, \Vithin the lung, the bronchi branch repeatedly into finer and finer tubes called bronchioles. The entire system of air ducts has the appearance of an inverted tree,the trunk being the trachea. The epithelium lining the majorbranches ofthis respiratory tree is covered by cilia and a thin filmof mucus. The mucus traps dust, pollen, and other particulatecontaminants, and the beating cilia move the mucus upward tothe pharynx, where itcan beswallowed into the esophagus. Thisprocess, sometimes referred to as the "mucus escalator,' plays acritical role in cleansing the respiratory system.


Like fishes, terrestrial vertebrates rely on ventilation to maintainhigh O2 and low CO2 concentrations at the gas exchange surface. The process that ventilates lungs is breathing, the alter-

1. Why is the position oflung tissues within the body anadvantage for terrestrial animals?

2. After a heavy rain, earthworms come to the surface.How would you explain this behavior in terms ofanearthworm's requirements for gas exchange?

3, _ImP.)Il. The walls of alveoli contain elastic fibersthat allow the alveoli to expand and contract witheach breath. If alveoli lost their elasticity, how mightgas exchange be affected? Explain.

For suggested answers. see Appendix A.

Gas exchange occurs in alveoli (singular, alveolus; seeFigure 42.24), air sacs clustered at the tips of the tiniest bronchioles. Human lungs contain millions of alveoli, which togetherhave a surface area of about 100 m2

, fifty times that of the skin.Oxygen in the air entering the alveoli dissolves in the moist filmlining their inner surfaces and rapidly diffuses across the epithelium into a web of capillaries that surrounds each alveolus. Car

bon dioxide diffuses in the oppositedirection, from the capillariesacross the epitheHum of the alveolus and into the air space.

Alveoli are so small that specialized secretions are requiredto relieve the surface tension in the fluid that coats their surface. These secretions, called surfactants, contain a mixtureof phospholipids and proteins. In their absence, the alveoli collapse, blocking the entry of air. A lack of lung surfactants is amajor problem for human babies born very prematurely. Sur·factants typically appear in the lungs after 33 weeks of embryoonic development. Among infants born before week 28, halfsuffer serious respiratory distress. Artificial surfactants arenow used routinely to treat such preterm infants.

Lacking cilia or significant air currents to remove particlesfrom their surface, alveoli are highly susceptible to contamination. White blood cells patrol alveoli, engulfing foreign particles.However, if too much particulate matter reaches the alveoli, thedefenses can break down, leading to diseases that reduce the efficiency of gas exchange. Coal miners and other workers exposed to large amounts of dust from rock are susceptible tosilicosis, a disabling, irreversible, and sometimes fatal lung disease. Cigarette smoke also brings damaging particulates intothe alveoli.

Having surveyed the route that air follows when we breathe,we will turn next to the process of breathing itself.

How an Amphibian Breathes

An amphibian such as a frog ventilates its lungs by positivepressure breathing, inflating the lungs with forced airflow. During the first stage ofinhalation, muscles lower the floor of an amphibian'soral cavity, drawing in air through its nostrils. Next, withthe nostrils and mouth closed, the floor of the oral cavity rises,forcing air down the trachea. Duringexhalation, air is forced backout by the elastic recoil of the lungs and by compression of themuscular body wall. \Vhen male frogs puff themselves up in aggressive or courtship displays, they disrupt this breathing cycle,taking in air several times without allowing any release.

How a Mammal Breathes

nating inhalation and exhalation ofair. A variety of mechanismsfor moving air in and out oflungs have evolved, as we will see byconsidering breathing in amphibians, mammals, and birds.

Unlike amphibians, mammals employ negative pressurebreathing-puUing, rather than pushing, air into their lungs(Figure 42.25). Using muscle contraction to actively expand thethoracic cavity, mammals lower air pressure in their lungs belowthat ofthe air outside their body. Because gas flows from a regionof higher pressure to a region of lower pressure, air rushesthrough the nostrils and mouth and down the breathing tubes tothe alveoli. During exhalation, the muscles controlling the thoracic cavity relax, and the volume ofthe cavity is reduced. The increased air pressure in the alveoli forces airup the breathing tubesand outofthe body. Thus, inhalation is always active and requireswork, whereas exhalation is usually passive.

Expanding the thoracic cavity during inhalation involves theanimal's rib muscles and the diaphragm, a sheet ofskeletal muscle that forms the bottom ....wl of the cavity. Contracting the ribmuscles expands the rib cage, the front waU ofthe thoracic cavity,by puUing the ribs upward and the sternum outward. At the sametime, the diaphragm contracts, expanding the thoracic cavitydownward. The effect of the descending diaphragm is similar tothat ofa pltmger being drawn out ofa syringe.

Within the thoracic cavity, a double membrane surroundsthe lungs. The inner layer of this membrane adheres to theoutside of the lungs, and the outer layer adheres to the wall ofthe thoracic cavity. A thin space filled with fluid separates thet....'o layers. Surface tension in the fluid causes the two layers tostick together like two plates of glass separated by a film ofwater: The layers can slide smoothly past each other, but theycannot be pulled apart easily. Consequently, the volume ofthethoracic cavity and the volume of the lungs change in unison.

Depending on activity level, additional muscles may be recruited to aid breathing. The rib muscles and diaphragm aresufficient to change lung volume when a mammal is at rest.During exercise, other muscles ofthe neck, back, and chest increase the volume of the thoracic cavity by raising the rib cage.In kangaroos and some other species, locomotion causes a

42.5CONCEPT CHECI(


How a Bird Breathes

Because the lungs in mammals donot completely empty with each breath,and because inhalation occurs throughthe same airways as exhalation, each inhalation mixes fresh air with oxygendepleted residual air. As a result, themaximum P0:2 in alveoli is always considerably less than in the atmosphere.

AireKhaled

Air

Ventilation is both more efficient andmore complex in birds than in mammals. When birds breathe, they passair over the gas exchange surface inonly one direction. Furthermore, incoming, fresh air does not mix with airthat has already carried out gas ex-change. To bring fresh air to theirlungs, birds use eight or nine air sacssituated on either side of the lungs(figure 42.26). The air sacs do notfunction directly in gas exchange butact as bellows that keep air (lowingthrough the lungs. Instead of alveoli,which are dead ends, the sites of gasexchange in bird lungs are tiny channels called parabronchi. Passage of airthrough the entire system-lungs andair sacs-requires two cycles of inhalation and exhalation. In some passageways, the direction in which air movesalternates (see Figure 42.26). Withinthe parabronchl, however, air always(lows in the same direction.

Because the air in a bird's lungs is renewed with every exhalation, the maximum p0:2 in the lungs is higher in birdsthan in mammals. This is one reasonbirds function better than mammals athigh altitude. For example, humans

have great difficulty obtaining enough O2 when climbingEarth's highest peaks, such as Mount Everest (8,850 m), in theHimalayas. But bar-headed geese and several other birdspecies easily fly over the Himalayas during migration.

Control of Breathing in Humans

Although you can voluntarily hold your breath or breathefaster and deeper, most of the time your breathing is regulatedby involuntary mechanisms. These control mechanisms ensure that gas exchange is coordinated with blood circulationand with metabolic demand.

Lung

Diaphragm

Airinhaled

INHALATIONDiaphragm contract~

(moves down)

Anteriorair sac~

.. Figure 42.25 Negative pressure breathing. Amammal breathes by changing the airpressure within Its lungs relative to the pressure of the outside atmosphere

.. Figure 42.26 The avian respiratory system. Inflation and deflation of the air sacs (redarrows) ventilates the lungs. forcing air in one direction through tiny parallel tubes in the lungscalled parabronchi (inset, SEM), During inhalation, both sets of air sacs inflate. The posterior sacsfill with fresh air (blue) from the outside, while the anterior sacs fill with stale air (gray) from thelungs. During eKhalation, both sets of air sacs deflate, forcing air from the posterior sacs into thelungs, and air from the anterior sacs out of the system via the trachea. Gas eKchange occurs acrossthe walls of the parabronchi, Two cycles of inhalation and eKhalation are required for the air topass all the way through the system and out of the bird.

rhythmic movement of organs in the abdomen, including thestomach and liver. The result is a piston-like pumping motionthat pushes and pulls on the diaphragm, further increasing thevolume ofair moved in and out of the lungs.

The volume of air inhaled and exhaled with each breath iscalled tidal volume. It averages about 500 mL in resting humans. The tidal volume during maximal inhalation and exhalation is the vital capacity, which is about 3.4 Land 4.8 L forcollege-age women and men, respectively. The air that remains after a forced exhalation is called the residual volume.As we age, our lungs lose their resilience, and residual volumeincreases at the eKpense of vital capacity.


o Other sensors in the aorta andcarotid arteries signal the medulla toincrease the breathing rate whenO~ levels in the blood become very low.

Aorta

o Sensors in the medulla detect changesin the pH (reflecting CO~ concentration)

=:;~~---lof the blood and cerebrospinal fluidbathing the surface of the brain.

1-----J~~Carotidarteries

o Sensors in major blood vessels

~~~~~=:::~t:::="" ...Jdetect changes in blood pH and send

nerve impulses to the medulla. Inresponse, the medulla's control centeralters the rate and depth of breathing,increasing both if CO~ levels rise ordecreasing both if CO< levels fall.

Breathing

control ~'::':"-1ff=----centers "" Medulla

oblongata

f) Nerves from the medulla'scontrol center send impulses

to the diaphragm and fib

muscles, stimulating them to r~"":::::==----Acontract and causing

inhalation.

o A breathing control centerin the medulla sets the basicrhythm, and a control center

in the pons moderates it,smoothing out the

transitions betweeninhalations and eXhalations.I~""":::::::::"'!i!=~

~~

Q In a person at rest, thesenerve impulses result in

about 10 to 14 inhalationsper minute. Between

inhalations, the musclesrelax and the person exhales.

.... Figure 42.27 Automatic control of breathing.

I. How does an increase in the CO2 concentration in theblood affect the pH of cerebrospinal fluid?

2. A slight decrease in blood pH causes the heart's pacemaker to speed up. What is the function of this control mechanism?

3. N'mu". Suppose that you broke a rib in a fall. Ifthe broken end of the rib tore a small hole in themembranes surrounding your lungs, what effect onlung function would you expect?


The O2 concentration in the blood usually has little effecton the breathing control centers. However, when the O2 leveldrops very low (at high altitudes, for instance), O2 sensors in

the aorta and the carotid arteries in the neck send signals tothe breathing control centers, which respond by increasing thebreathing rate.

Breathing control is effective only if it is coordinated withcontrol of the cardiovascular system so that ventilation ismatched to blood flow through alveolar capillaries. During exercise, for instance, an increased breathing rate, which enhances O2 uptake and CO2 removal, is coupled with anincrease in cardiac output.

Networks of neurons that regulate breathing, calledbreathing control centers, are located in two brain regions, themedulla oblongata and the pons (Figure 42.27). Control circuits

in the medulla establish the breathing rhythm, while neurons inthe pons regulate its tempo. (The number and location ofthe circuits in the medulla is a subject of active research.) When youbreathe deeply, a negative-feedback mechanism prevents thelungs from overexpanding: During inhalation, sensors that detect stretching ofthe lung tissue send nerve impulses to the control circuits in the medulla, inhibiting further inhalation.

In regulating breathing, the medulla uses the pH of the sur·rounding tissue fluid asan indicator ofblood CO2 concentration.The reason pH can be used in this way is that blood CO2 is themain determinant ofthe pH ofarebrospina/fluid, the fluid sur·

rounding the brain and spinal cord. Carbon dioxide diffuses fromthe blood to the cerebrospinal fluid, where it reacts with waterand forms carbonic acid (H2C03). The H2C03 can then dissociate into a bicarbonate ion (HC0:J-) and a hydrogen ion (H+):

CO2 + H20~ H2C03~ HC03 - + H+

Increased metabolic activity, such as occurs during exercise,lowers pH by increasing the concentration ofCOl in the blood..In response, the medulla's control circuits increase the depthand rate ofbreathing. Both remain high until the excess CO2 iseliminated in exhaled air and pH returns to a normal value.

CONCEPT CHECI( 42.6


rZ~i:~~::o~~f~r gas exchangeinclude pigments that bindand transport gases

The high metabolic demands of many animals necessitate theexchange of large quantities ofO2 and CO2, Here we'll examine how blood molecules called respiratory pigments facilitatethis exchange through their interaction with O2 and CO2, Wewill also investigate physiological adaptations that enable animals to be active under conditions of high metabolic load orvery limiting P~. As a basis for exploring these topics, let'ssummarize the basic gas exchange circuit in humans.

Coordination of Circulation and Gas ExchangeThe partial pressures ofO2 and CO2 in the blood vary at different points in the circulatory system, as shown in Figure 42.28.Blood arriving at the lungs via the pulmonary arteries has alower P~anda higher Pc~ than theairin the alveoli. As bloodenters the alveolar capillaries, CO2 diffuses from the blood tothe air in the alveoli. Meanwhile, O2 in the air dissolves in thefluid that coats the alveolar epithelium and diffuses into theblood. By the time the blood leaves the lungs in the pulmonaryveins, itsp~ has been raised and its Pc~ has been lowered. After returning to the heart, this blood is pumped through the systemic circuit.

In the tissue capillaries, gradients of partial pressure favorthe diffusion of O2 out of the blood and CO2 into the blood.These gradients exist because cellular respiration in the mitochondria of cells near each capillary removes O2 from andadds CO2 to the surrounding interstitial fluid. After the bloodunloads O2 and loads CO:u it is returned to the heart andpumped to the lungs again.

Although this description faithfully characterizes the driving forces for gas exchange in different tissues, it omits the critical role of the specialized carrier proteins we will discuss next.

Respiratory Pigments

The low solubility of O2 in water (and thus in blood) poses aproblem for animals that rely on the circulatorysystem to deliverOz. For example, a person requires almost 2 LofO2 per minuteduring intense exercise, and all of it must be carried in the bloodfrom the lungs to the active tissues. At normal body temperatureand air pressure, however, only 4.5 mL of Ch can dissolve into aliter of blood in the lungs. Even if 8096 of the dissolved O2 weredelivered to the tissues (an unrealistically high percentage), theheart would still need to pump 555 Lof blood per minute!

In fact, animals transport most of their O2 bound to certainproteins called respiratory pigments. Respiratory pigmentscirculate with the blood or hemolymph and are often contained within specialized cells. The pigments greatly increasethe amount of O2 that can be carried in the circulatory fluid(to about 200 mL ofO2 per liter in mammalian blood). In our

Alveolus

POJ '" 100 mm Hg

-

Alveolus

Pco1

",40mmHg

-... Figure 42.28 loading and unloadingof respiratory gases._','M'II. If you consciously forced moreair out ofyour lungs each time you exhaled,how would that affect the values shown inthese diagrams)

••

•

(a) Oxygen

• ••Body tissue•

• •Pcol <::46mmHg ••

• B~dy tissue

(b) carbon dioxide


... Figure 42.29 Dissociation wrves for hemoglobin at 37·C.

Carbon Dioxide Transport

100

100

1Lungs

---.....

80

80

0, unloadedto tissues

during exercise

60

60

Hemoglobinretains less02 at lower pH{higher (02concentration)

40

ITissuesat rest

Po,(mm Hg)

20

-------------------:.---~---or-O2unloadedto tissuesat rest

Tissues durmgexercise

100

"c:0 800Q;0E

60•~

'0c,2 40,0• 20""0

(a) Po, and hemoglobin dissociation at pH 7.4. The curveshows the relative amounts of O2 bound to hemoglobinexposed to solutions with different Po:· At aPo: of 100 mm Hg.typical in the lungs. hemoglobin is about 98% saturated with02. At aPo, of 40 mm Hg, common in the vicinity of tissuesat rest. hemoglobin is about 70% saturated. Hemoglobin canrelease additional 0, to metabolically very active tissues. suchas muscle tissue during exercise.

Po:(mm Hg)

(b) pH and hemoglobin dissociation. Because hydrogen ionsaffect the shape of hemoglobin, adrop in pH shifts the 0,dissociation curve toward the right (the Bohr shih), At agivenPOl' say 40 mm Hg, hemoglobin gives up more 02 at pH 7.2than at pH 7,4, the normal pH of human blood. The pHdecreases in very active tissues because the (0, produced bycellular respiration reacts with water, forming carbonic acid.Hemoglobin then releases more 02, which supports theincreased cellular respiration in the active tissues,

In addition to its role in O2 transport, hemoglobin helps transport CO2 and assists in buffering the blood-that is, preventing harmful changes in pH. Only about 7% ofthe CO2 released

Hemoglobin

Vertebrate hemoglobin con~ II Chains

sists of four subunits (poly~

peptide chains), each with acofactor called a heme groupthat has an iron atom at itscenter. Each iron atom bindsone molecule ofOb hence, asingle hemoglobin moleculecan carry four molecules ofO2, Like all respiratory pig-ments, hemoglobin binds {l Chams

O2 reversibly, loading O2 in Hemoglobin

the lungs or gills and unloading it in other parts of the body.This process depends on cooperativity between the hemoglo~

bin subunits (see Chapter 8). \Vhen O2 binds to one subunit,the others change shape slightly, increasing their affinity for

~. \Vhen four O2 molecules are bound and one subunit un·loads its 02' the other three subunits more readily unload, asan associated shape change lowers their affinity for 02'

Cooperativity in O2 binding and release is evident in the dissociation curve for hemoglobin (Figure 42.29a). Over the

range of p~ where the dissociation curve has a steep slope,even a slight change in p~ causes hemoglobin to load or unload a substantial amount of O2, Notice that the steep part ofthe curve corresponds to the range of P~ found in body tissues. \'V'hen cells in a particular location begin workingharder-during exercise, for instance-P~ dips in their vicin~

ity as the O2 is consumed in cellular respiration. Because oftheeffect ofsubunit cooperativity, a slight drop in P~ causes a rei·

atively large increase in the amount of O2 the blood unloads.The production of CO2 during cellular respiration pro·

motes the unloading of O2 by hemoglobin in active tissues. Aswe have seen, CO2 reacts with water, forming carbonic acid,which lowers the pH of its surroundings. Low pH, in turn, decreases the affinity of hemoglobin for O2, an effect called theBohr shift (Figure 42.29b). Thus, where CO2 production isgreater, hemoglobin releases more 02.0 which can then be usedto support more cellular respiration.

example of an exercising human with an O2 delivery rate of80%, the presence of respiratory pigments reduces the cardiacoutput necessary for O2 transport to a manageable 12.5 L ofblood per minute.

A variety of respiratory pigments have evolved among theanimal taxa. \Vith a few exceptions, these molecules have a dis·tinctive color (hence the term pigment) and consist of a proteinbound to a metal. One example is the blue pigment hemocyanin,which has copper as its oxygen-binding component and isfound in arthropods and many molluscs. The respiratory pigment ofalmost all vertebrates and many invertebrates is hemoglobin. In vertebrates, it is contained in the erythrocytes.


o Carbonic acid dissociates into abicarbonate ion (HC03-) and ahydrogen ion (W).

o Hemoglobin binds most of theWfrom H2C03. preventing theW from acidifying the bloodand thus preventing the Bohrshift.

8 Most of the HC03- diffusesinto the plasma, where it iscarried in the bloodstream tothe lungs,

f) Over 90% of the CO2diffusesinto red blood cells. leavingonly 7% in the plasma asdissolved CO2,

o Some CO2 is picked up andtransported by hemoglobin.

o However, most CO2 reacts withwater in red blood cells,forming carbonic acid (H2C03),

a reaction catal'(2ed bycarbonic anhydrase containedwithin red blood cells.

o Carbon dioxide produced bybody tissues diffuses into theinterstitial fluid and the plasma,

CO2transportfrom tissues

ICapillarywall

CO2 transportto lungs

Hemoglobin {Hb}picks up

CO2 and W

co,

CO2 produced

ICO,

10

DT.'"""

H{O] Hbarbonic add

o

Body tissue

Interstitialfluid

Redbloodcell

Plasmawithin c:apillary

.-......-

Elite Animal Athletes

by respiring cells is transported in solu

tion in blood plasma. Another 23%

binds to the amino ends of the hemo

globin polypeptide chains, and about

70% is transported in the blood in the

form of bicarbonate ions (HC03-).

As shown in Figure 42.30, carbon

dioxide from respiring cells diffuses

into the blood plasma and then into

erythrocytes. There the CO2 reacts

with water (assisted by the enzyme car

bonic anhydrase) and forms H2C03,

which dissociates into H+ and HC03-.

Most of the H+ binds to hemoglobin

and other proteins, minimizing the

change in blood pH. The HC03- dif

fuses into the plasma.

When blood flows through the lungs,

the relative partial pressures of CO2 fa

vor the diffusion of CO2 out of theblood. As CO2 diffuses into alveoli, the

amount of CO2 in the blood decreases.

This decrease shifts the chemical equi

librium in favor of the conversion ofHC03- to C02J enabling further net dif

fusion of CO2 into alveoli.

For some animals, such as long-distance

runners and migratory birds and mam

mals, the O2 demands of daily activities

would overwhelm the capacity of a typ

ical respiratory system. Other animals,

such as diving mammals, are capable of

being active underwater for extended

periods without breathing. \'(fhat evolu

tionary adaptations enable these ani

mals to perform such feats?

The Ultimate Endurance Runner

Hemoglobinreleases

CO2 and W

() In the lungs, HC03- diffusesfrom the plasma into red bloodcells, combining with H+released from hemoglobin andforming H2C03.

o Carbonic acid is converted backto CO2 and water, CO2 is alsounloaded from hemoglobin,

iil CO2 diffuses into the plasmaand the interstitial fluid.

... Figure 42.30 Carbon dioxide transport in the blood.Din what three forms Is CO2 transported in the bloodstream?

-The elite animal marathon runner may

be the pronghorn, an antelope-like mam

mal native to the grasslands of North

America. Second only to the cheetah in

top speed for a land vertebrate, prong

horns are capable of running as fast as

100 km/hr and can sustain an average

speed of65 km/hr over long distances.

Stan Lindstedt and his colleagues at

the University of Wyoming and the Uni

versity of Bern were curious about how

pronghorns achieve their combination

co,

1CO (ID,

ct ~,Alveolar space in lung

mCO2 diffuses into the alveolarspace, from which it is expelledduring exhalation. The reduction of CO2 concentration inthe plasma drives the breakdown of H2C03 into CO2 andwater in the red blood cells(see step 9), a reversal of thereaction that occurs near bodytissues (see step 4)

CHAPTER FORTY-TWO Circulation and Gas Exchange 925

RESULTS

• Goat

• Pronghorn

Muscle Mitochonmass drial volume

Cardiacoutput

•

Lungcapacity

100

90

80

~ 70-• 600

"> SO•>~ 40•~

30

20

10

0

EXPERIMENT Stan Lindstedt and colleagues had demonstrated that the pronghorn's maximal rate of Ol consumption(VOl max) is five times that of a domestic goat, a similar-sizedmammal adapted to c1imbmg rather than running, To discover thephysiological basis for this difference, they measured the following parameters in both animals: lung capacity (a measure of 0luptake). cardiac output (a measure of O2 delivery), muscle mass,and muscle mitochondrial volume, (The last two parameters aremeasures of the muscles' potential O2 use,)

Mi,ij:f.jlijM Suppose you measured Vo max among a large groupof humans, To what extent would you Jxpect those with thehighest values to be the fastest runners)

What is the basis for the pronghorn's unusuallyhigh rate of O2 consumption?

CONCLUSION The dramatic difference in V max between0,the pronghorn and the goat reflects comparable differences ateach stage of O2 metabolism: uptake, delivery, and use,

SOURCE S. L. Lindstedt et ~I , Running energetICs in the pronghorn~ntelope, N~ture 3S3:748-750 (1991).

protein called myoglobin in their muscles. The Weddell sealcan store about 25% of its O2 in muscle, compared with only13% in humans.

Diving mammals not only have a relatively large Oz stockpile but also have adaptations that conserve Oz. They swimwith little muscular effort and glide passively upward or

downward by changing their buoyancy, Their heart rate andOz consumption rate decrease duringa dive. At the same time,regulatory mechanisms route most blood to the brain, spinal

Animals vary greatly in their ability to temporarily inhabit environments in which there is no access to their normal respiratory medium-for example, when an air-breather swimsunderwater. Whereas most humans, even well-trained divers,cannot hold their breath longer than 2 or 3 minutes or swim

deeper than 20 m, the Weddell seal of Antarctica routinelyplunges to 200-500 m and remains there for about 20 minutes(sometimes for more than an hour). (Humans can remain

submerged for comparable periods, but only with the aid ofspecialized gear and compressed air tanks.) Some sea turtles,whales, and other species of seals make even more impressivedives. Elephant seals can reach depths of 1,500 m-almost amile-and stay submerged for as long as 2 hours! One elephant seal carrying a recording device spent 40 days at sea,diving almost continuously with no surface period longerthan 6 minutes.

One adaptation of diving mammals to prolonged stays underwater is an ability to store large amounts of Oz. Comparedwith humans, the Weddell seal can store about twice as much

Oz per kilogram of body mass. About 36% of our total Oz is inour lungs, and 51% is in our blood. In contrast, the Weddell

seal holds only about 5% of its O2 in its relatively small lungs(and may exhale before diving, which reduces buoyancy),stockpiling 70% in the blood. The seal has about twice the volume of blood per kilogram of body mass as a human. Divingmammals also have a high concentration ofan oxygen-storing

ofexceptional speed and endurance. The researchers exercisedpronghorns on a treadmill to estimate their maximum rate ofO2 consumption (see Figure 40.18). The results were surprising: Pronghorns consume O2 at three times the rate predicted

for an average animal of their size. Normally, as animals in

crease in size, their rate of Oz consumption per gram of bodymass declines. One gram of shrew tissue, for example, consumes as much Oz in a day as a gram of elephant tissue consumes in an entire month. But the rate of Oz consumption pergram of tissue by a pronghorn turned out to be as high as thatof a lO-g mouse!

What adaptations enable the pronghorn to consumeOz at such a high rate? To answer this question, Lindstedtand his colleagues compared various physiological characteristics of pronghorns with those of domestic goats,which lack great speed and endurance (Figure 42.31).

They concluded that the pronghorn's unusually high Ozconsumption rate results from enhancements of normalphysiological mechanisms at each stage of Oz metabo

lism. These enhancements are the result of natural selection, perhaps exerted by the predators that have chasedpronghorns across the open plains of North America formore than 4 million years.

Diving Mammals


I. What determines whether 0 1 and CO2 diffuse into orout of the capillaries in the tissues and near the alve

oli? Explain,2. How does the Bohr shift help deliver O2 to very active

tissues?3, _',IMilla A doctor might use bicarbonate

(HC03-) to treat a patient who is breathing very rap

idly. What assumption is the doctor making about theblood chemistry of the patient?

For suggested answers. see AppendiK A.

cord, eyes, adrenal glands, and, in pregnant seals, the placenta.Blood supply to the muscles is restricted or, during the longestdives, shut off altogether. During dives of more than about 20minutes, a Weddell seal's muscles deplete the O2 stored in

myoglobin and then derive their ATP from fermentation in

stead of respiration (see Chapter 9).The unusual abilities of the Weddell seal and other air

breathing divers to power their bodies during long dives showcase two related themes in our study of organisms-theresponse to environmental challenges over the short term byphysiological adjustments and over the long term as a result ofnatural selection.

CONCEPT CHECK 42.7

-6140"'. Go to the Study Area at www.milsteringbio.(om for BioFliK3-D Animations. MP3 Tutors. Videos. Practice Tests. an eBook. and more,

left atrium and is pumped to the body tissues by the left ventricle. Blood returns to the heart through the right atrium.

Acthity Mammalian Cardiovascular System Structure- 61401.

... The Mammalian Heart: A Closer Look The pulse is ameasure of the number of times the heart beats each minute.The cardiac cycle. one complete sequence of the heart's pumping and filling. consists of a period of contraction. called systole, and a period of relaxation, called diastole. Cardiac outputis the volume of blood pumped by each ventricle per minute.

... Maintaining the Hearl's Rhythmic Beat Impulses originatingat the sinoatrial (SA) node (pacemaker) of the right atrium passto the atrioventricular (AV) node. After a delay, they are conducted along the blUldle branches and Purkinje fibers. The pacemaker is influenced by nerves, hormones, and body temperature.

Pulmonary ~eins

SystemiC arteries

hhaled air

Heartf

AI~eolar repithelial cells L __

CO,

Inhaled air

Systemic veins

Pulmonary arteries

SUMMARY OF KEY CONCEPTS

_ i·iliii'_ 42.1

_i·iliii'_ 42.2Coordinated cycles of heart contraction drive doublecirculation in mammals (pp. 903-905)... Mammalian Circulation Heart valves dictate a one-way

flow of blood through the heart. The right ventricle pumpsblood to the lungs. where it loads O2 and unloads CO2,

Oxygen-rich blood from the lungs enters the heart at the

Circulatory systems link exchange surfaces with cellsthroughout the body Ipp. 898-903)... Gastrovascular Cavities Gastrovascular cavities in small

animals with simple body plans mediate eKchange betweenthe environment and cells that can be reached by short-rangediffusion.

... Open and Closed Circulatory Systems Because diffusionis slow over all but short distances, most complex animalshave internal transport systems. These systems circulate fluidbetween cells and the organs that exchange gases, nutrients,and wastes with the outside environment. In the open circulatory systems of arthropods and most molluscs, the circulatingfluid bathes the organs directly. Closed systems circulate fluidin a closed network of pumps and vessels.

... Organization of Vertebrate Circulatory Systems In vertebrates. blood flows in a closed cardiovascular system consisting of blood vessels and a two- to four-chambered heart.Arteries convey blood to capillaries, the sites of chemical exchange between blood and interstitial fluid. Veins returnblood from capillaries to the heart. Fishes, rays, and sharkshave a single pump in their circulation. Air-breathing vertebrates have two pumps combined in a Single heart. Variationsin ventricle number and separation reflect adaptations to different environments and metabolic needs.


_i.I·'i"- 42.3Patterns of blood pressure and flow reflect the structure

and arrangement of blood vessels (pp. 906-911).. Blood Vessel Structure and function Capillaries have nar

row diameters and thin walls that facilitate exchange. Arteriescontain thick elastic walls that maintain blood pressure. Veinscontain one-way valves that contribute to the return of bloodto the heart.

.. Blood Flow Velocity Physical laws governing the movementof fluids through pipes influence blood flow and blood pressure. The velocity of blood flow varies in the circulatory system, being lowest in the capillary beds as a result of their largetotal cross-sectional area.

.. Blood Pressure Blood pressure is altered by changes in cardiac output and by variable constriction of arterioles .

.. Capillary Function Transfer of substances between theblood and the interstitial fluid occurs across the thin walls ofcapillaries.

.. Fluid Return by the lymphatic System The lymphatic system returns fluid to the blood and parallels the circulatorysystem in its extent and its mechanisms for fluid flow underlow hydrostatic pressure. It also plays a vital role in defenseagainst infection.

Act;\ity (>;)lh of Blood Flow in Mammal~

Acthity Mammalian Cardiovascular System Function

Biology lab, On-lin~ CardioLlb

_i.lili"_ 42.4Blood components function in exchange, transport,

and defense (pp. 911-915).. Blood Composition and Function \X'hole blood consists of

cellular elements (cells and cell fragments called platelets)suspended in a liquid matrix called plasma. Plasma proteinsinfluence blood pH, osmotic pressure, and viscosity and function in lipid transport, immunity (antibodies), and blood clotting (fibrinogen). Red blood cells. or erythrocytes, transportO2, Five types of white blood cells, or leukocytes, function indefense against microbes and foreign substances in the blood.Platelets function in blood dotting, a cascade of reactions thatconverts plasma fibrinogen to fibrin .

.. Cardiovascular Disease The deposition of lipids and tissueson the lining of arteries is a prime contributor to cardiovascular disease that can result in life-threatening damage to theheart or brain.

-.m.It.•Inn'tlgatlon How I, Cardiov.scular Fitnes~ Measured?

-i·II'i"- 42.5Gas exchange occurs across specialized respiratory

surfaces (pp. 915-920).. Partial Pressure Gradients in Gas Exchange At all sites of

gas exchange, gases diffuse from where their partial pressuresare higher to where they are lower.

.. Respiratory Media Air is more conducive to gas exchangebecause of its higher O2 content. lower density. and lowerviscosity.

928 UNlr SEVEN Animal Form and Function

.. Respiratory Surfaces Animals require large, moist respiratory surfaces for the adequate diffusion of O2 and CO2 between their cells and the respiratory medium, either air orwater.

Acti\;ty The Human Respiratory System

_i lilil'_ 42.6Breathing ventilates the lungs (pp, 920-922).. How an Amphibian Breathes An amphibian ventilates its

lungs by positive pressure breathing, which forces air downthe trachea.

.. How a Mammal Breathes Mammals ventilate their lungsby negative pressure breathing, which pulls air into the lungs.Lung volume increases as the rib muscles and diaphragmcontract.

.. How a Bird Breathes Besides lungs, birds have eight or nineair sacs that act as bellows, keeping air flOWing through thelungs in one direction only. Every exhalation completely renews the air in the lungs.

.. Control of Breathing in Humans Control centers in themedulla oblongata and pons of the brain regulate the rate anddepth of breathing. Sensors detect the pH of cerebrospinalfluid {reflecting CO2 concentration in the blood), and themedulla adjusts breathing rate and depth to match metabolicdemands. Secondary control over breathing is exerted by sensors in the aorta and carotid arteries that monitor blood levelsof O2 and CO2 and blood pH.

-i lilil'_ 42.7Adaptations for gas exchange include pigments that

bind and transport gases (pp. 923-927).. Coordination of Circulation and Gas Exchange In the

lungs, gradients of partial pressure favor the diffusion of O2into the blood and CO2 out of the blood. The opposite situation exists in the rest of the body.

.. Respiratory Pigments Respiratory pigments transport 02'greatly increasing the amount of O2 that blood or hemolymphcan carry. Many arthropods and molluscs have coppercontaining hemocyanin; vertebrates and a wide variety ofinvertebrates have hemoglobin. Hemoglobin also helpstransport CO2 and assists in buffering.

.. Elite Animal Athletes The pronghorn's high O 2 consumption rate underlies its ability to run at high speeds over longdistances. Deep-diving air-breathers stockpile O2 and depleteit slowly.

d. heart rate.e. breathing rate.

d. left ventricle.e. right ventricle.

-51401"-l\cthity Transpon of RcspiraloryGases

8ioiollY Labs On-Lint HemoglobinLab

TESTING YOUR KNOWLEDGE

SELF·QUIZ

I. Which of the following respiratory systems is not closely associated with a blood supply?a. the lungs of a vertebrateb. the gills of a fishc. the tracheal system of an insectd. the skin of an earthworme. the parapodia of a polychaete worm

2. Blood returning to the mammalian heart in a pulmonary veindrains first into thea. vena cava.b. left atrium.e. right atrium.

3. Pulse is a direct measure ofa. blood pressure.b. stroke volume.c. cardiac output.

4. The conversion of fibrinogen to fibrina. occurs when fibrinogen is released from broken platelets.b. occurs within red blood cells.e. is linked to hypertension and may damage artery walls.d. is likely to occur too often in an individual with

hemophilia.e. is the final step of a clotting process that involves multiple

clotting factors.

5. In negative pressure breathing, inhalation results froma. forcing air from the throat down into the lungs.b. contracting the diaphragm.c. relaxing the muscles of the rib cage.d. using muscles of the lungs to expand the alveoli.e. contracting the abdominal muscles.

6. \Vhen you hold your breath, which of the following blood gaschanges first leads to the urge to breathe?a. rising O2 d. falling CO2

b. falling O2 e. rising CO2 and falling O2

e. rising CO2

7. Compared with the interstitial fluid that bathes active musclecells, blood reaching these cells in arteries has a

a. higher P02'b. higher Peo:>'c. greater bicarbonate concentration.d.lowerpH.e. lower osmotic pressure.

8. \X'hich of the following reactions prevails in red blood cellstraveling through alveolar capillaries? (Hb = hemoglobin)a. Hb + 4 O2 ---> Hb(02)~

b. Hb(02)~ ---> Hb + 4 O2c. CO2 +H20 ---> H2CO:~

d. H2C03 ---> W + HC03 -

e. Hb + 4 CO2 ---> Hb(C02)~

9. ••p.\i,i", Draw a pair of simple diagrams comparing theessential features of single and double circulation.

For Self-Quiz answers, see Appendix A.

-51401"- ViSit the Study Area at www.masteringbio.com lor aPractice Test

EVOLUTION CONNECTION

10. One of the many mutant opponents that the movie monsterGodzilla contends with is Mothra, a giant mothlike creaturewith a wingspan of several dozen feet. Science fiction creatureslike these can be critiqued on the grounds ofbiomechanicaland physiological principles. \Vhat problems of respiration andgas exchange would Mothra face? The largest insects that haveever lived are Paleozoic dragonflies with half.meter wingspans.Why do you think truly giant insects are improbable?

SCIENTIFIC INQUIRY

11. The hemoglobin ofa human fetus differs from adult hemoglobin. Compare the dissociation curves of the two hemoglobinsin the graph below. Propose a hypothesis for the function ofthis difference between these two versions of hemoglobin.

100

01 80c-,g~ 60'0~g 40'Eo....,~ 20

O+-~~~~a 20 40 60 80 100

POI (mm Hg)

SCIENCE, TECHNOLOGY, AND SOCIETY

12. Hundreds of studies have linked smoking with cardiovascularand lung disease. According to most health authorities. smoking is the leading cause of preventable, premature death in theUnited States. Antismoking and health groups have proposedthat cigarette advertising in all media be banned entirely. \%atare some arguments in favor of a total ban on cigarette advertising? What are arguments in opposition? Do you favor or oppose such a ban? Defend your position.


exchttt~ i - · pdf fileexchange behwen an axolotl or any other animal and its surroundings...

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