Chapter%2022%20Respiratory - Surviving in Thin Air THE HIGH...

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Unformatted text preview: Surviving in Thin Air THE HIGH MOUNTAINS of the Himalayas have claimed the lives of even the world’s top mountain climbers; the journey into thin air can weaken their muscles, cloud their minds, and sometimes fill their lungs with fluid. The air at the height of the world's high- est peak, 9,700mm Mount Everest, is so low in oxygen (02) that most people would pass out instantly if exposed to it. But if you were ever to make it to the top of Mount Everest, you might see birds flying by. Twice a year, flocks of geese migrate over the Himalayas, traveling between winter quarters in India and summer breeding grounds in Russia. These geese, along with other species of migratory birds, can travel easily at heights that would leave most people drowsy, lethargic, or dead. How do geese and ducks manage to fly at such heights? One factor is the efficiency of their lungs, which can draw far more oxygen from the air than our own lungs can. These birds also have blood containing hemoglobin With a very high It is the continuou , affinity for oxygen, picking it b- d 0 up in the lungs and carrying supply of axygen to it to tissues throughout the cells that makes the body. Their circulatory system difference between has large number of capillar- ies (tiny blood vessels} that and death in the thin carry oxygen-rich blood to of the Himalayas; their flight muscles, and the muscles themselves pack a protein that stores a ready supply 0f oxygen. All these adaptations allow the high-flying birds YOU see in these photos to travel even where the air is very thin. Humans can try to adapt to higher elevations, but success less certain. Most people function well only below 3,300 m and are helpless at higher elevations without an oxygen mask. The: are permanent villages at extremely high elevations in the Hi malayas and the Andes, but the people living there have adapt? in ways that allow them to function with relatively little oxygéfl including large lungs, a large heart, and blood that carries addi your blood to carry more oxygen. After long-term training, some Everest climbers have been able to sur- vive for a short time at the top of the world’s highest peak without oxygen masks. Runners and cyclists may also use this type of training, moving to high altitudes to gain stronger lungs and more oxygen-rich blood and then returning to sea level to blow past competitors who trained at lower elevations. Our study of cellular respiration in Chapter 6 showed why animals require oxygen. Without 02, the metabolic machinery that releases energy from food molecules shuts down. It is the continuous supply of O2 to body cells that makes the difference between life and death in the thin air of the Himalayas. The process of gas exchange, often called respiration, is the interchange of O2 and the waste product C02 be- tween an animal and its environment. In this chap- ter, we will explore the respiratory systems of animals. I I n ' high Himalayas. But many die doing such work, their bodies succumbing to altitude— related illnesses under the burdens of long travel and heavy loads. Altitude-caused disorders include everything from mild headaches, dizziness, and nausea to life-threatening fluid buildup in the lungs and swelling of the brain. Avoiding these disorders requires careful conditioning for high altitudes, and the higher one goes, the longer the adjustment takes. As you ' move from sea level up into the mountains, your body starts ad— justing immediately. Your heart pumps faster, and some blood ' vessels may increase in diameter if you stay in the mountains more than a few days. Within weeks, the rate and depth of your breathing increase to bring more air into your lungs. ‘ At the same 1time, your body m._ may develop more capillar— ‘r _ r ies, and your red blood ‘ _ cell count may go 7 up, allowing Gas exchange makes it possible for animals to put to work the food molecules the digestive system provides. Figure “22§1.ptesents an overview of the three phases of gas ex? change an animal with lungs. 0 Breathing is the first phase of the gas exchange process. When an animal breathes, a large, moist internal surface is exposed to air. 02 diffuses across the cells lining the lungs and into surround- ing blood vessels. At the same time, CO2 diffuses out of the blood and into the lungs. As the animal exhales, C02 is re- moved from the body. 9 A second phase of gas exchange is the transport of gases by the circulatory system. The 02 that has diffused into the blood attaches to hemoglobin in red blood cells and is carried from the lungs to the body’s tissues. C02 is also transported in blood from the tissues back to the lungs. -. 9 In the third phase of gas exchange, body cells take up 02 from the blood and release C02 to the blood. This 02 is required for cells to obtain energy from the food molecules the body has digested and absorbed. As we learned in Mod- ule 6.4, 02 functions in cellular respiration as the final elec- tron acceptor in the stepwise breakdown of fuel molecules. H20 and CO2 are waste products, and ATP is produced to power Cellular work. Thus, our cells require a continuous supply of 02 and must dispose of €02. Gas exchange involves the respiratory system and the circulatory system in servicing the cells of the body. Humans cannot survive for more than a “few minutes without 02.- Why? 'agp LIJ51U25JO aui pue sues My mouth“ 'dlv 51L]: aanpmd or E0 seignbai UOQBJldSBJ Jelnnaj 'uolpuru or, iapro u! div to ([ddns (pears e aJli'IbaJ snag a The part of an animal where gases are exchanged with the en- vironment is called the respiratory surface. (In this context, the word respiratory refers to the process of breathing, not to cellular respiration.) Respiratory surfaces are made up of liv— ing cells, whose plasma membranes must be wet to_ function properly. Thus, the respiratory surfaces of terrestrial as well as aquatic animals must be moist, and gases must be dissolved in water before they can diffuse across them. The surface area of the respiratory surface must be extensive enough to take up sufficient 02 for every cell in the body and to dispose of all waste COZ. Usually, a single layer of cells covers or lines the entire respiratory surface. Being thin and moist, the layer al- lows O2 to difiuse rapidly into the circulatory system or di— rectly into body tissues and allows CO2 to difiuse out. The four figures on the facing page illustrate, in simplified form, four types of respiratory organs, structures where gas 454 UNIT V Animals: Form and Function Overview: Gas exchange involves breathing, transport of gases, and exchange of gases with tissue cells l Animals exchange 02 and C02 across moist body surfaces Lung Circulatory system 9 Transport ofgases by the circulatory system Mitochondria 9 Exchange of gases with body cells Capillary Figure 22.1 The three phases of gas exchange exchange with the external environment occurs- In eaCh case, the circle represents a cross section of the animal’s body through the respiratory surface. The yellow areas repre- Sent the respiratOry surfaces; the green circles represent bodl’ surfaces with little or no role in respiration. The boxed eIl largements show a portion of the respiratory surface in the process of exchanging 02 and COZ. ' ' ' Some animals use their entire outer skin as a gas EX change organ. The earthworm in Figure 222A is an exam ple. Notice in the cross-sectional diagram that its -Wh01 body surface is yellow; there are no specialized gas exchafllir surfaces. Oxygen diffuses into a dense net of thin—iii'fillle capillaries lying just beneath the skin. Earthworms 311 other "skin-breathers” must live in damp places or in WaleI because their whole body surface has to stay moist. Animals that breathe only through their skin and lack specialized Ea. Cross section of respiratory surface (the skin covering the body) '— Capillaries Body surface Respiratory surface (gill) Capillary exchange organs are generally small, and many are long and thin or flattened. Small size or flatness provides a high ratio of respiratory Surface to body volume, allowing for sufficient ‘ gas exchange for the entire body. In most animals, the skin surface is not extensive enough i to exchange gases for the whole body. Consequently, certain ‘ parts of the body have become adapted as respiratory sur- faces. Gills have evolved in most aquatic animals. Lungs or an internal system of gas exchange tubes called tracheae have evolved in most terrestrial animals. Gills, lungs, and UaCheae all have extensive surfaces for gas exchange, as ; ShOWn in Figures 22.2B—22.2D. Gills are extensions, or outfoldings, of the body surface ' Specialized for gas exchange. Many marine worms have flap- l-lke gills that extend from each body segment. The gills of C1611113 and crayfish are clustered in one body location. A fish figure 22.2B).has a set of feather-like gills on each side of its head. As indicated in the enlargement, O2 djfiuses across the gill surfaces into capillaries, and CO2 diffuses in the op- Posite direction, out of the capillaries and into the external e11ViIOImlerrt. Since the respiratory surfaces of aquatic ani- mals extend into the surrounding water, keeping the surface IHoist is not a problem. In most terrestrial animals, the respiratory surfaces are f0ltfled into the body rather than projecting from it. The in- Body surface Respiratory surface (air tubes) (no capillaries) Figure 22.2C Tracheal system Figure 22.2D Lungs Body surface Respiratory surface (within lung) Capillary folded surfaces open to the air only through narrow tubes, an arrangement that helps retain the moisture that is essen— tial for the cells of the respiratory. surfaces to function. The tracheal system of insects is an extensive system of branching internal tubes (Figure 22.2C). As we will see in Module 2.2.4, the smallest branches exchange gases directly with body cells. Thus, gas exchange in insects requires no as- sistance from the circulatory system. Most terrestrial vertebrates have lungs (Figure 22.2D), which are internal sacs lined with moist epithelium. As the di- agram indicates, the inner surfaces of the lungs branch‘extene sively, forming a large respiratory surface. Gases are carried between the lungs and the body cells by the circulatory system. We examine gills, tracheae, and lungs more closely in the next Several modules. I ' What is the main difference between gills and lungs in terms of- their spatial relationship to the rest of an animal’s body? 'saaeiins KJOZLQJgClSBJ {mm 5325 leuJalul are sfiung asenuoo or 50932:“) suatuuorytua Buipunonns our oiug Kpoq eLn Luci; piemno spuarxa smfi i0 aoeyns {hounds-cu engsuaixe aqi . CHAPTER 22 Gas Exchange 455 Gills are adapted for gas exchange in aquatic environments Oceans, lakes, and other bodies of water contain 02 in the to any mechanism that increases the flow of the surroun - ' form of dissolved gas. The gills of fishes and many inverte— water or air over the respiratory surface (gills, tracheae' OI brate animals, including lobsters and clams, tap this source lungs). Increasing this flow ensures a fresh supply of 02 and: of Oz. The total surface area of the gills is often much greater the removal of C02. The blue arrows in the drawings repre ' than that of the rest of the body. - sent the one-way flow of water into the mouth, across th An advantage of exchanging gases in water is that there is gills, and out the side of the fish’s body. Swimming fish 33m no problem keeping the respiratory surface wet. On the ply open then mouths and let water flow past the gills, Fish I ‘ other—hand, the amount of available oxygen (dissolved 02) also pump water across the gills by the coordinated Openin ' in water is only about 3—5% of What it is in the air, and the and closing of the mouth and operculum, the stiff flap tha warmer and saliie‘r the water, the less dissolved 02 it holds. covers and protects the gills. Because water is dense and con Thus, gills—especially those of large, active animals in Warm rains so little oxygen, most fish must expend considerable oceans-must be very efficient to obtain enough oxygen energyinventilating their from water. _ The arrangement of capillaries in a fish gill greatly en_ The drawings in Figure 22.3 show the architecture of fish hances gas exchange. Blood flows in the direction Opposit'e gills, which are among the most efficient gas exchange or- to the movement of water past the gills. This makes it possi. gans in the aquatic world. There are four supporting gill ble to transfer oxygen to the blood by a very efficient process arches on each side of the body. Two rows of filaments called countercurrent exchange. Countercurrent exchange project from each gill arch. Each filament bears many plate- is the transfer of something from a fluid moving in one (ii- like structures called lamellae (singular, 1amella),rwhich are rection to another fluid moving in the opposite direction, the actual respiratory surfaces. A lamella is full of‘tiny capil- The name comes from the fact that the two fluids are mov- laries that are separated from the outside by only one or a ing counter to each other. Their opposite flow maintains a dif. few layers of cells. Capillaries are so narrow that red blood fusion gradient that enhances transfer of the substance. Let’s cells must pass through them in single file. As a result, every see how this principle works in a fish gill. ' red blood cell comes in close contact with oxygen dissolved In the circular enlargement on the right of Figure 22.3, in the surrounding water. notice that the direction of water flow over the surface of a What you can’t see in the drawings are the movements lamella (blue arrow) is opposite that of the blood flow that ventilate the gills, We use the term ventilation to refer the lamella (arrow turning from blue to red). The changingin Gill arch Oxygen-poor blood \ Direction of water 7 ' flow ' Oxygen-rich blood \ ‘ Gill arch % 02 in water flowing over lamellae LM 250x % 02 in blood flowing through capillaries in lamellae Gill filaments Countercurrent eXchange Figure 22.3 The structure of fish gills 4:56 UNIT V Animals: Form and Function tensities of these arrows and the numbers on them indicate the changing amount of Oz dissolved in each fluid: the darker the color, the more 02. Notice that as blood flows through a lamella and picks up more and more 02, the blood ‘ comes in contact with water that has even more 02 available because it is just beginning its passage over the gills. As a re- sult, a diffusion gradient is maintained that favors the trans- fer of 02 from the water to the blood along the entire length of the Capillary. r This countercurrent exchange mechanism is so efficient that fish gills can remove more than 80% of the oxygen dis- solved in the water flowing through them. The basic mecha— nism of countercurrent exchange is also important in temperature regulation, as you will see in Chapter 25. between the air and body cells There are two big advantages to exchanging gases by breath— ing air: Air contains a much higher concentration of Oz, and air is much lighter and easier to move than water. Thus, a terrestrial animal expends much less energy than an aquatic animal ventilating its respiratory surface. The main problem facing any aierreathing animal is the loss of water to the air by evaporation. With respiratory surfaces occurring as tiny tubes deep in the body of an insect, evaporation is reduced, and the respiratory system loses very little water. The trachea] system of insects is made up of air tubes that branch throughout the body (Figure 22.4.4). The largest Tracheae Opening for ait The tracheal system of an insect The tracheal/system of insects provides direct exchange Gills are unsuitable for'an animal living on land. An expan- sive surface of wet membrane extending out from the body and exposed to air would lose too much water to evaporation. Most terrestrial animals house their respiratory surfaces within the body, opening to the atmosphere through narrow tubes, as we see next. (This is a tough one!) What would be the maximum per- ' Centage of the water’s oxygen a gill could extract if its blood flowed in the same direction as the water instead of counter to it? poem 0:, REM mug. asngip lafiuol ou nine: 20 pure ’qioq u! paniossgp 20 lo iunome auras our SEM slain mun daais 5:31 pm! ssal awoaaq p]n0M quaipeifi uonenuaauoa aqi 'uonoaiip auras an), u! pamog (an; 52 poogq an; only JalEM our LU!)le pasngip 2(j w 134,05 I tubes, called tracheae, open to the outside and are reinforced by rings of chitin, as shown in the blowup on the bottom right of the figure. Enlarged portions of tracheae form air sacs near organs that require a large supply of 02. The micrograph in Figure 22.4A shows how these tubes branch repeatedly. The smallest branches, called tr‘acheoles, extend to nearly every cell in the insects body. The tiny tips of the tracheoles are closed and contain fluid (dark blue in the figure). Gas is exchanged with body cells by diffusion across the moist epithelium that lines these tips. Thus, the circulatory system of insects is not involved in transporting oxygen. For a small insect, diffusion through the tracheae brings in enough 02 and removes enough (302 to support cellular respiration. larger insects may ventilate their tracheal sys- tems with rhythmic body movements that compress and ex pand the air tubes like bellows. An inS‘ect in flight (Figure 22.413) has a very high metabolic rate and consumes 10 to 200 times more 02 than it does at rest. In many insects, al— ternating contraction and relaxation of the flight muscles rapidly pumps air through the tracheal system. in what basic way does the process of gas exchange in insects differ from that in both fish and humans? 'snaa Kpoq our mos; pue or sariin Buinodsuen u! pa/qmui iou 5; spam! lo ulaisA's Morelnzup eql a Figure 2.43 A grasshopper in flight 457' CHAPTER 22 Gas Exchange Reptiles (including birds), mammals, and most amphibians exchange gases in lungs. In contrast to the tracheae of in- sects, lungs are restricted to one location in the body. There- fore, the circulatory system must transport gases between the lungs and the rest of the body. Amphibians have small lungs (some salamanders lack lungs altogether) and rely heavily on the diffusion of gases across body surfaces. The skin of frogs, for example, Supple- ments gas exchange in the lungs. Most reptiles (including all birds) and mammals rely entirely on lungs for gas exchange. In general, the size and complexity of lungs are correlated with an animal's metabolic rate (and thus oxygen need). For example, the lungs of endotherms (birds and mammals) have a greater area of exchange surface than the lungs of sim- ilar-sized ectotherms (amphibians and nonbird reptiles). The total-respiratory surface of human lungs is about 100 m2, equal to the surface area of a racquetball court. Figure 22.5A shows the human respiratory system (along with the eSOphag-us and heart, for orientation). Our lungs are in the chest cavity, which is bounded at the bot- tom by a sheet of muscle called the diaphragm. Air passes to our lungs via a system of branching narrow tubes. Air usually enters our respiratory system through‘the nos— trils. It is filtered byhairs and warmed, humidified, and sam- pled for odors as it flows through a maze of spaces in the Pha rynx (Esophagus) Larynx Trachea Right lung Bronchus Bronchiole Diaphragm Figure 22.5A The human respiratory system 458 {MT V Animals: Form and Function Terrestrial vertebrates have lungs Nasal -r cavity left lung nasal cavity. We can also draw in air through the mouth, but ea: mouth breathing does not allow the air to be processed by} for the nasal cavity. From the nasal cavity or mouth, air passeg _ at to the pharynx, where the paths for air and food cross. A5 . th' we saw in Module 21.6, the air passage in the pharynx is ro‘ . {Tc open for breathing except when we swallow. _ From the pharynx, air is inhaled into the larynx (voice box). When we exhale, the outgoing air rushes by a pair 0 vocal cords in the larynx, and we can produce sounds by voluntarily tensing muscles in the voice box, stretching the cords and making them vibrate. We produce high-pitche sounds when our vocal cords are tense and therefore vibrat. ing very fast. When the cords are less tense, they vibrat slowly and produce low—pitched sounds. From the larynx, inhaled air passes toward the lungs~ through the trachea, or windpipe. Rings of cartilage mam tain the shape of the trachea, much as metal rings keep the hose of a vacuum cleaner from collapsing. The trachea fork into two bronchi (singular, bronchus), one leading to each lung. Within ,the lung, the bronchus branches repeatedl' intofiner and finer tubes called hronchioles. Bronchitis is " condition in which these small tubes become inflamed and ~ constricted, making breathing difficult. As Figure 22.513 shows, the bronchioles dead-end in grep like clusters of air sacs called alveoli (singular, alveolus). Each Figure 22.53 The structure of alveoli Figure 22.5C Air spaces in alveoli Colorizecl SEM 200x of our lungs contains millions of these tiny sacs. Figure 22.5C, produced by a scanning electron microscope, is a cutaway view of alveoli showing the air spaces. The inner surface of each alveolus is lined with a thin layer of epithelial cells that form the respiratory surface. The O2 in inhaled air dissolves in a film of moisture on the epithelial cells. it then diffuses across the epithelium and into a web of blood capillaries that sur- rounds each alveolus. The CO2 diffuses the opposite way— fiom the capillaries, across the epithelium of the alveolus, into the air space of the alveolus, and finally out in the exhaled air. The trachea and major branches of the respiratory system are lined by a moist epithelium covered by cilia and a thin ‘ Virtually everywhere today, the air we breathe exposes the cells in our respiratory system to chemicals that they are not adapted to tolerate. Air pollutants such as sulfur dioxide, car- bon monoxide, and ozone can all cause respiratory prob- lems. One of the worst sources of air pollutants is tobacco smoke. The visible smoke from a cigarette, cigar, or pipe is mainly microscopic particles of carban Sticking to the car— bon particles are many toxic chemicals. A single drag on a cigarette exposes a person to over 4,000 chemicals, more than 50 of which are carcinogens (cancer-causing agents). Tobacco smoke irritates the cells lining the bronchi, inhibit- ing or destroying their cilia. Frequent coughing—common in heavy smokers—becomes the respiratory system’s attempt to clear the mucus no longer moved by the cilia. Smoke’s noxious particles also kill macrophages, defensive cells that reside in the respiratory tract and engulf fine particles and microorgan- isms. Thus, smoking disables the normal cleansing and protec— tive mechanisms of the respiratory system, allowing even more toxin-laden particles to reach the lung’s delicate alveoli. Some of the toxins in tobacco smoke cause lung cancer. The photographs in Figure 22.6 show a Cutaway view of a pair of healthy human lungs (left) and the lungs of a smoker With cancer. The lungs on the right are black from the long- Lung Heart l:iQIJre 22.6 Healthy lungs (left) and cancerous lungs (right) Smoking is a deadly assault on our respiratory system film of mucus. The cilia and mucus are the system’s cleaning elements. The mucus traps dust, pollen, and other contami- nants, and the beating cilia move the mucus upward to the pharynx, where it is usually swallowed. In the next module, ‘ we explore one of the most serious threats to this delicate ep— ithelium and to our lungs. ' Web/CD Activity 22A The Human Respiratory System List the parts of the respiratory system in the order that an inhaled breath of air would encounter them. sn[oa.«[e <— aloguaumq <— sanLIOJq <—~ eauaane— quJel (— XUKJGLICl <— lhyiea |EEPN a term buildup of smoke particles, except where pale cancer- ous tumors appear. Smokers account for 90% of all lung can- cer cases. Most victims die within One year of diagnosis. Smokers also have a markedly greater risk than nonsmokers of developing cancers of the bladder, pancreas, mouth, throat, and several other organs. ' Smoking can also cause emphysema, a disease in which the walls of the alveoli lose their elasticity and deteriorate, reducing the lungs' capacity for gas exchange. Breathless- ness and constant fatigue result, as the body is furced to spend more and more energy just breathing. The second highest number of smoking-related deaths come from cardiovascular disease. Smokers have a higher rate of heart attacks and stroke. Smoking raises blood pres— Sure and increases harmful cholesterol levels in the blood. Every year in the United States, sm'oking kills about 440,000 people, more than all the deaths caused by traffic accidents, alcohol and drug abuse, HIV, and murders com- bined. On average, adults who smoke cigarettes die 13 to 14 years earlier than nonsmokers. Moredver, studies show that nonsmokers exposed to secondary cigarette smoke are also at risk. Young children are particularly susceptible, with in- creased risk of asthma, bronchitis, and pneumonia. Clearly, efforts to reduce smoking and sec- ondary exposure to smoke are important to public and personal health. No lifestyle choice has a more positive impact on long-term health than the decision not to smoke. This is true even if you already smoke. Quitting smoking has immediate health benEfits, andfi after about 15 years, the risk of lung cancer and heart disease is similar to that of people who have never smoked. What causes smoker's cough? 'fiuiqfinm [q alesuadwoa 0:] say; Kpoq aui 'JDEJZ]. IGOJEchlSBJ sq; in inn sap!qu padden pue snanur daams 01 £1”in limp, Euniqguu! 'Qlfp safieiuep axows oaaaqol n CHAPTER 22 Gas Exchange 459. Posterior air sacs Breathing ventilates the lungs Breathing is the alternation of inhalation and exhalation. This ventilation of our lungs maintains high 02 and low CO2 concentrations at the respiratory surface. Like all mam- mals, we breathe by pulling air into the lungs and then pushing it back out. ' Figure 22.7A shows the changes that occur in our rib r cage, chest cavity, and lungs during breathing. During inha- lation (left diagram), both the rib cage and chest cavity ex— pand, and the lungs follow suit. The ribs move upward and the rib cage expands as muscles between the ribs contract. At the same time, the diaphragm contracts, moving downward and expanding the chest cavity as it goes. The increase in the volume of the lungs during inhala— tion lowers the air pressure in the alveoli to less than atmos- pheric pressure. Flowing from a region of higher pressure to one of lower pressure, air rushes through the nostrils and down the breathing tubes to the alveoli. This type of venti- lation is called negative pressure breathing. 7 The diagram on the right in Figure 22.7A shows exhala- tion. The rib muscles and diaphragm both relax, decreasing the volume of the rib cage and chest cavity and forcing air out of the lungs. Notice that the diaphragm curves upward into the chest cavity when relaxed. Each year, a human adult may take between 4 million and 10 million breaths. The volume of air in each breath is about 500 mL when we breathe quietly. The maximum volume of air that we can inhale and exhale during forced breathing is called vital capacity. It averages about 3.4 L and 4.8 L for college-age females and males, respectively. (Women tend'to have smaller rib cages and lungs.) The lungs actually hold more air than the vital capacity. Because the alveoli' do not completely collapse, a residual Volume of “dead” air remains in the lungs even after we blow out as much air as we can. As lungs lose resilience (springiness) with age or as the result of disease, such as emphysema, our residual volume increases at the expense of vital capacity. I I Alr Anterior air sacs Trachea Lungs Exhalationi' - r ' ; Inhalation: _ Air sacs empty; lungs fill Air sacs fill Figure 22.73 How a blrcl breathes 460 UNIT V Animals: Form and Function ‘ 1 air flowing through the lungs. As the simplified diagrams in - halation. The posterior sacs_fill"with fresh air (red) from the NT air passes one—way/t'hrough them (red arrows). Be ’5’ Rib cage Rib cage gets expands as K— smailer as rib muscles ' . rib muscles contract $ A” relax inhaled Lung Diaphragm Diaphragm relaXes {moves up) ' Diaphragm contracts {moves down) Figure 22.7A How a human breathes As we mentioned in the introduction, the gas exchange system of birds is different from ours. Let’s now take a look at some of the adaptations that make a bird’s respiratory sys- tem so efficient. Unlike the in—and-out flow of air in the human alveoli, birds have a one-way flow of air through the lungs. Birds have several large air sacs in addition to their lungs. These do not mnchn directly in gas exchange, but act as bellows that keep Figure 22.7B indicate, both sets of air sacs expand during in— outside, while the anterior sacs stale air (blue) from the lungs. During exhalation, both sets of air sacs deflate, forcing air from the posterior sacs into the lungs, and air from the anterior sacs out of the system via the trachea. Instead of alveoli, bird lungs contain tiny parallel tube (shown in the electron micrograph in the circular inset) Gas exchange occurs across the walls of these tubes as ‘ cause of the one—way flow of air, there is no dead I air (residual volume) the bird lung, so lung ; oxygen concentrations are higher in birth: than in ma. als.‘ Birds can extract about 5% more f'oxygen'from a volume of in- haled air than we can. Compare the pathway of air flow in the lungs of mammals and birds. ' A . ' "shun; our Llfinoiui, Kneuolnallplun 5M0]; J|_E ’splgq lJ] an? fenplsai paraldap-uafiKXO '43?“ saxgiu .112 paleuur Klmau pua ‘Kemuied ewes 3L9 liq shun] aui saneal pue Siaiua J12 ‘sieiuuieuJ Ul I breath for a short while or breathe faster and deeper. Most of the time, however, automatic control centers in our brain u sent-lei, for it ensures coordination between the respiratory and circulatory systems and the body’s metabolic needs for ' gas. exchange. Our breathing control centers (represented by the gold drdes in Figure 22.8) are located in parts 'of the brain called e pens and medulla oblongata (medulla, for short). Nerves from the medulla’s control center signal the diaphragm and rib muscles to contract, making us‘inhale. These nerves send out signals that result in about 10 to 14 inhalations per minute when we are at rest. Between inhalations, the muscles relax, and We exhale. The Control center in the pons smooths out the basic rhythm of breathing set by the. medulla. ' How does the medulla’s control center adjust our breath- ing rate in response to the body‘s varying needs? The con- trol center monitors the: (302 level of the blood and regulates breathing‘rate in response. Its main cues about C92- conCentration come from slight changes in the pH of Brain Cerebrospinal fluid Pons stimulated by: Meduila in blood Nerve signals trigger contraction of muscles Diaphragm Rib muscles Figure 22.8 Control centers that regulate breathing Breathing is automatically hontr'olled Breathing control centers C02 increase/ pH decrease Nerve signals indicating C02 and 02 levels creases in the blood. When we exercise vigorously, for in- stance, our metabolism speeds up and our body cells gener- ate more CO2 as a waste product. The C02 goes into the blood, where it reacts with water to form Carbonic acid. The acid lowers the pH of the blood and cerebrospinal fluid slightly. When the medulla senses this pH drop, its breath- ing control center increases the breathing rate and depth. As a result, more CO2 is eliminated in the exhaled air, and the pH returns to normal. When you were a kid, did you ever make yourself dizzy by hyperventilating, excessively taking rapid, deep breaths? Hyperventilating demonstrates the action of your breathing control center, but it’s hard on your body. Deep, rapid breathing purges the blood of so much CO2 that the control center temporarily ceases to send signals to the rib muscles and diaphragm. Breathing stops until the C02 level in; creases enough to switch the breathing center back on. Our breathing control center responds directly to CO2 levels, bur it usually does not resPond directly to oxygen lev— els. Since the same process that consumes Oz, cellular respi- ration, also produces COz, a rise in CO2 (drop in pl-I) is ‘ generally a good indication of a drop in blood ortygen. Thus, by responding to lowered pH, the breathing control center also Controls blood oxygen level. SeCondary control over breathing is ex- erted by sensors in the aorta and carotid ar-' teries that monitor concentrations of 02' as Well as COZ. When the 02 level in the blood is severely depressed, these sensors signal the control center via nerves to increase the rate and depth of breathing. This response may occur, for example, at high altitudes, where the air is so thin that we cannot get enough 02 by breathing normally. The breathing control center responds to a variety of nervous and chemical signals to keep the breathing rate and depth in tune with the "changing demands of the body. Breathing rate must also be coordinated with the activity of the circulatory system. During exercise, the rate at which our heart beats and the amount of blood it pumps with each beat must be matched with the increased breathing rate. We examme the role of the circulatory system in gas exchange more closely in the next module. ‘ C02 and 02 sensors in aorta Explain how hyperventilation disrupts the control of breathing. ’ButLfiBBJq Spuadsns Alpeiodruai uogielgiuaxuedxq ’Jaiuaa lonuoa Buggieaiq aqi uo unripe 511 eyk uonrngui satelnuips Kliaaiipui uniuM ’(ppe aguoqieo eauau pue) 30:) io poogq anti Buifiind Kg 3 CHAPTER 22 Gas Exchange 461 r How does 02 get from our lungs to all the other tissues in our body, and how does CO2 travel from the, tissues to the alveoli? To answer these questions, we must jump ahead a bit to the subject of Chapter 23 and look at the basic organi- -zation_ of our circulatory system. Figure 22.9 is a schematic diagram showing the main components of the human circulatory system and their role in gas exchange. Let’s start with the heart, in the middle of the diagram. One side of the heart handles oxygen-poor blood (colored blue). The other side handles oxygerr-rich blood (red). As indicated in the lower portion of the diagram, oxygen-poor blood returns to the heart from capillaries in body tissues. The heart pumps this blood to the alveolar CBP‘ illaries in the lungs. At the top of the diagram, gases are ex- changed between air in the alveolar spaces and blood in the alveolar capillaries. Blood leaves the alveolar capillaries, hav- ing lost CO2 and gained 02. This oxygen-rich blood returns to the heart and is pumped out to body tissues. . The exchange of gases between capillaries and the cells around them occurs by the diffusion of gases down gradients of pressure. A miicture of gases, such as air, exerts pressure. (You see evidence of gas pressure whenever you open a can of soda, releasing the pressure of the C0211: contains.) Each kind of gas in a mixture accounts for a portion, called the pressure, of the mixture’s total pressure. Molecules of each kind of gas will diffuse down a gradient of its own partial pres- sure independent of the other gases. At the bottom of the g- ure, for instance, 02 moves from oxygen—rich blood, through the interstitial fluid, and into tissue cells because it diffuses from a region of higher partial pressure to a region of lower" partial pressure. The tissue cells maintain this gradient as they consume 02 in cellular respiration. The (302 producedas a waste product of cellular respiration diffuses dewn its own partial-pressure gradient out of the cells and into the capillar- ies. Diffusion also accounts for gas exchange in the alveoli. What is the physical process underlying gas exchange? ruagperli ornssard-legrred sir umop $26 Lpea lo uorsngrcj 3 Oxygen is not very soluble in water, and most of the O2 in ." blood is carried by hemoglobin in the red blood cells. A he- _ A ' sues. There, hemoglobin unloads some or all of its cargo, d _ moglobin molecule consists of four polypeptide chains of two different types, distinguished by the two shades of pur- ple in Figure 22.10 on the next page. Attached to each a polypeptide is a chemical group called a heme (green), at the center of which is an iron atom (black). Each iron atom can carry one 02 molecule. Thus, every hemoglobin molecule 462 UNIT V Animals: Form and Function Blood transports respiratory gases Hemoglobin carries O2 and helpsfltransport CO2 and buffer the blood 7 up Oz in the lungs and transports it to the body’s ti Alveolar epithelial cells Alveolar capillaries , of lung OZ-rich, CO2-poor blood Heart Tissue capillaries Interstitial x02 fluid Figure 22.9 Gas transport Tissue cells and exchange throughout body the body can carry up to four oxygen molecules- Hemoglobin loads-i pending on the 02 needs of the cells. (The partial pressure of. O2 in the tissue reflects how much 0.2 the cells are using.) 3 Hemoglobin is a multipurpose molecule. It also helps the: blood transport CO2 and assists in buffering the blood—12hat is, preventing harmful changes in pH. iron atom 02 loaded in lungs ——~—> «(—- 02 unloaded in tissues Heme group Polypeptide chain Figure 22.10 Hemoglobin loading and unloading of Ci2 When C02 leaves a tissue cell, it diffuses through the in- terstitial fluid, across the wall of a capillary, and into the blood fluid (plasma). Most of the CO2 enters the red blood cells, where some of it combines with hemoglobin. The rest reacts with water molecules, forming carbonic acid (H2C03). Red blood cells contain an enzyme that hastens this reac- tion. H2C03 then breaks apart into a hydrogen ion (PF) and a bicarbonate ion (HCO3‘). Hemoglobin binds most of the H+, minimizing the change in blood pH. The bicarbonate Figure 22.11 shows a human fetus inside the mother's uterus. The fetus literally swims in a protective watery bath, the amniotic fluid. Its lungs are full of fluid and are nonfunc— tional. How does the fetus exchange gases with the outside world? The answer lies in the function of the placenta, a composite organ that includes tissues from 0th the fetus and the mother. A large net of capillaries fans out into the placenta from blood vessels in the umbilical cord of the fetus. These fetal capillaries eXchange gases with the maternal blood that circulates in the placenta, and the maternal circu- latory system carries the, gases to and horn the mother’s lungs. Aiding 02 uptake by the fetus is fetal hemoglobin, a Spectral type that attracts 0‘2 more strongly than does adult hemoglobin. Among the many health n'sks of smoking (see Module 22.6) is a reduction, perhaps by as much as 25%, in the Supply of oxygen reaching the placenta. What happens when a baby is born? Suddenly placental gas exchange ceases, and the baby’s lungs must begin to Work. Carbon dioxide in fetal blood acts as a signal. As soon 38 CO2 stops diffusing from the fetus into the placenta, a C02 rise in fetal blood causes blood pH to fall, stimulating the breathing control centers in the infant’s brain, and the newborn takes its first breath. A human birth and the radical changes in gas exchange mEChanisms that accompany it are extraordinary events. Re- slflfing from millions of years of evolutionary adaptation, these events are on a par with the remarkable flying ability of the geese we discussed in the chapter introduction. For a goose to breathe the thin air and fly great distances high above Earth, or for a human baby to switch almost instantly from liv- ' 111g in water and exchanging gases with maternal blood to The human fetus exchanges gases the mother’s bloodstream . ions diffuse into the plasma, where they are carried to the lungs. This reversible reaction is shown here: C02+HZO <——> r12003 +—> H+ + HCOB' Carbon Water Carbonic Hydrogen Bicarbonate dioxide acid ions As blood flows through capillaries in the lungs, this process is reversed. Carbonic acid forms when bicarbonate combines with F“. The carbonic acid is then converted back to CO2 and water. Finally, the CO2 diffuses from the blood intothe alveoli and out of the body in exhaled air. We have seen how 02 and CO2 are transported between the lungs and body tissue cells via the bloodstream. in the neXt module, we consider a special case of gas exchange be- tween two circulatory systems. Web/ CD Activity 223 Transport of Respiratory Gases O2 in the blood is transported bound to Within cells, while CO2 is mainly transported as . ions within the plasma. aqeuocpeaiq ' ‘- ' p00|q P31 ‘ ‘ ‘ ugqolfiowau a breathing air directly, requires truly remarkable adaptations in the organism’s respiratory system. Also reqriired are adapta- tions of the circulatory system, which, as we have” seen, sup" ports the respiratory system in its gas exchange function. We turn to the circulatory system in Chapter 23. ' How does fetal hemoglobin enhance oxygen transfer from mother to fetus across the placenta? ‘pooiq 1239;01 PODIq l'euiaiew woe Z0 9141 "rind" sdleq 1:21th ‘uiqolfiouieq nan saop IJELil Z0 10; Aguirre £129.16 e seq uiqmfiowaq IBIEH a Placenta, containing maternal blood vessels and fetal capillaries Umbilical cord, containing fetal blood vessels Amniotic fluid Uterus Figure 22.11 A human fetus and placenta in the uterus CHAPTER 22 Gas Exchange 4163 Reviewing the Concepts Mechanisms of {Gas Exchange (introduction-32.8) Gas exchange, the interchange of O2 and CO2 between an organism and its environment, provides 02 for cellular respiration and removes its waste product, C02. Gas ex- change often involves breathing, transport of gases, and exchange of gases with body cells (Introduction—22.1). Respiratory surfaces must be thin and moist for diffu— sion of O2 and CO2 to occur. Some animals use their entire skin as a gas exchange organ. In most animals, specialized body parts—“such as gills, tracheal systems, or lungs—upro- vide large respiratory surfaces for gas exchange (22.2). Gills are extensions of the \ body that absorb O2 dis— solved in water. ln a fish, gas \ - exchange is enhanced by ventilation and the counter— current flow of water and blood (22.3). Tracheal systems in insects transport 02 directly to body Lamella cells through a network of Water flow Blood finely branched tubes (22.4). flow Lungs. Most terrestrial vertebrates have lungs. In inani- mals, air inhaled through the nostrils passes through the pharynx and larynx into the trachea, bronchi, and bron- chioles to the alveoli, where gas exchange occurs. Mucus and cilia in the respiratory passages protect the lungs, but smoking can destroy these protections. Smoking causes lung cancer, heart disease, and emphysema (22.5—22.6). Breathing is the alternation of inhalation and exhalation. The contraction of rib muscles and diaphragm expands the chest cavity and reduces air pressure in the alveoli (negative pressure breathing). Vital capacity is the maximum volume of air that can be inhaled and exhaled, but the lungs still hold a residual volume. Air flows in one direction through the more efficient lungs of birds (22.7). Breathing control centers in the brain keep breathing in tune with body needs, sensing and responding to the CO2 level in the blood. A drop in blood pH triggers an increase in the rate and depth of breathing (22.8). . ‘ Transport of Gases in the Body (22.9—22.1 '3) Circulation. The heart pumps oxygen-poor blood to the lungs, Where it picks up 02 and drops off (“.02. Then the heart pumps the oxygen-rich blood to body cells, where it drops off 02 and picks up C02. Gases diffuse down partial- pressure gradients in lungs and body tissues (22.9). Hemoglobin in red blood cells transports oxygen, helps buffer the blood, and carries some C02. Most CO2 is trans- ported as bicarbonate ions in the plasma (22.10). A human fetus exchanges gases with maternal blood in the placenta. Fetal hemoglobin enhances oxygen transfer from maternal blood. At birth, rising CO2 in fetal blood stimulates the breathing control centers to initiate breathing (22.11). 464 UNIT V Animals: Form and Function Connecting the Concepts 1. Complete this map to review some of the concepts of gas exchange. often relies on ( requires . moistthin ' red blood cells contain to transport gases between waste product of needed for . mammals nd ventilate by 2. Label the parts of the human reSpiratory system. Mad-92s; c. :1. Testing Your Knowledge Multiple Choice 3. When you hold your breath, which of the following ftISt ' leads to the urge to breathe? a: falling (302 b. falling 02 c. rising CO2 d. rising pH of the blood e. both c and d 4. Countercurrent gas exchange in the gillsof a fish ‘ a. speeds up the flow of water through the gills. ' b. maintains a gradient that enhances diffusion. enables the fish to obtain oxygen without swimming. means that blood and water flow at different rates. allows 02 to diffuse against its partial-pressure gradient. . When you inhale, the diaphragm a. relaxes and moves upward. b. relaxes and moves downward. c. contracts and moves upward. d. contracts and moves downward. e. is not involved in the breathing movements. 5°94!" lln which of the following organisms does oxygen diffuse directly across a respiratory surface to cells, without being carried by fire blood? a. a grasshopper cl. a sparrow b. a whale e. a mouse c. an earthworm . What is the function of the cilia in the trachea and bronchi? to sweep air into and out of the lungs to increase the surface area for gas exchange to vibrate when air is exhaled to produce-sounds to dislodge food that may have slipped past the epiglottis ' to sweep mucus with trapped particles up and out of the respiratory tract . What do the alveoli of mammalian lungs, the gill fila- ments of fish, and the tracheal tubes of insects have in common? ' ‘ a. use of a circulatory system to transport gases b. respiratory surfaces that are invaginations (infoldings) of the body wall c. countercurrent exchange . d. a large, moist surface area for gas exchange e. all of the above . Which of the following is the best explanation for why birds can fly over the Himalayas while most humans require oxygen masks to climb these mountains? 3. Birds are much smaller and require less oxygen. b. Birds use positive pressure breathing, whereas humans use negative pressure breathing. c. With their one-way flow of airancl efficient ventila- tion, the lungs of birds Extract more 02 from the d. The circulatory system of birds is much more efficient at delivering oxygen to tissues than is that of humans. e. Humans are endothenns and thus require more oxy- gen than do birds, which are ectothenns. an 9'.» F” Describing, Comparing, and Explaining 10. What are two advantages of breathing air, compared to obtaining dissolved oxygen from water? What is a com— parative disadvantage of breathing air? 11. Trace the path of an oxygen molecule from the air to a muscle cell in your arm, naming all the structures involved along the way. Applying the Concepts 12. Partial pressure reflects the relative amount of gas in a mixture and is measured in millimeters of mercury (mm Hg), Llamas are native to the Andes Mountains in South America. The partial pressure of O2 (abbreviated P02) in the atmosphere where llamas live is about half of the POz at sea level. As a result, the P02 in the lungs of llamas is about 50 mm Hg, whereas it is about 100 mm Hg in human lungs at sea level. A dissociation curve shows the 0/0 saturation (the amount of O2 bound to hemoglobin) at increasing P02. As you see in this graph, the dissocia— lion curves for llama and human hemoglobin differ. Compare these two curves and explain how the hemo- globin of llamas is an adaptation to living where the air is "thin." .100 80 Ct O g 02 saturation ofhemoglobin (96) O 20 40 60 80 100 P02 (mm Hg) 13. Onelof the many mutant opponents that the movie monster Godzilla contends with is Mothra, a giant moth-like creature with a wingspan of 7 to 8 In. Science fiction creatures like these can be critiqued on the grounds of biomechanical and physiological principles. - Focusing on the principles of gas exchange that you learned about in this chapter, what problems would Mothra face? Why do you think truly giant insects are improbable? 14. Hundreds of studies have linked smoking with cardio- vascular and lung disease. According to most health authorities, smoking is the leading cause of preventable, premature death in the United States. Antismoking and health groups have proposed that cigarette advertising in all media be banned entirely. What are some argu- ments 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. ‘" - .. Answers to all questions can be found in Appendix 3. For study help and Activities, 90 to campbellbiology.com or the student CD—ROM. CHAPTER 22 Gas Exchange 465 ...
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This note was uploaded on 04/10/2008 for the course BIOL 155 taught by Professor Stewart during the Spring '08 term at California State University Los Angeles .

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