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Circulation and Gas Exchange Overview: Trading with the Environment Every organism must exchange materials with its environment And this exchange ultimately occurs at the cellular level In unicellular organisms These exchanges occur directly with the environment For most of the cells making up multicellular organisms Direct exchange with the environment is not possible The feathery gills projecting from a salmon Are an example of a specialized exchange system found in animals Figure 42.1 Concept 42.1: Circulatory systems reflect phylogeny Transport systems Functionally connect the organs of exchange with the body cells Most complex animals have internal transport systems That circulate fluid, providing a lifeline between the aqueous environment of living cells and the exchange organs, such as lungs, that exchange chemicals with the outside environment Invertebrate Circulation The wide range of invertebrate body size and form Is paralleled by a great diversity in circulatory systems Gastrovascular Cavities Simple animals, such as cnidarians Have a body wall only two cells thick that encloses a gastrovascular cavity The gastrovascular cavity Functions in both digestion and distribution of substances throughout the body Some cnidarians, such as jellies Have elaborate gastrovascular cavities Circular canal Mouth Radial canal 5 cm Figure 42.2 Open and Closed Circulatory Systems More complex animals Have one of two types of circulatory systems: open or closed Both of these types of systems have three basic components A circulatory fluid (blood) A set of tubes (blood vessels) A muscular pump (the heart) In insects, other arthropods, and most molluscs Blood bathes the organs directly in an open circulatory system Heart Hemolymph in sinuses surrounding ograns Anterior vessel Figure 42.3a Lateral vessels Ostia Tubular heart (a) An open circulatory system In a closed circulatory system Blood is confined to vessels and is distinct from the interstitial fluid Heart Interstitial fluid Small branch vessels in each organ Dorsal vessel (main heart) Auxiliary hearts Figure 42.3b Ventral vessels (b) A closed circulatory system Closed systems Are more efficient at transporting circulatory fluids to tissues and cells Survey of Vertebrate Circulation Humans and other vertebrates have a closed circulatory system Often called the cardiovascular system Blood flows in a closed cardiovascular system Consisting of blood vessels and a two- to four- chambered heart Arteries carry blood to capillaries The sites of chemical exchange between the blood and interstitial fluid Veins Return blood from capillaries to the heart Fishes A fish heart has two main chambers One ventricle and one atrium Blood pumped from the ventricle Travels to the gills, where it picks up O2 and disposes of CO2 Amphibians Frogs and other amphibians Have a three-chambered heart, with two atria and one ventricle The ventricle pumps blood into a forked artery That splits the ventricle’s output into the pulmocutaneous circuit and the systemic circuit Reptiles (Except Birds) Reptiles have double circulation With a pulmonary circuit (lungs) and a systemic circuit Turtles, snakes, and lizards Have a three-chambered heart Mammals and Birds In all mammals and birds The ventricle is completely divided into separate right and left chambers The left side of the heart pumps and receives only oxygen-rich blood While the right side receives and pumps only oxygen-poor blood A powerful four-chambered heart Was an essential adaptation of the endothermic way of life characteristic of mammals and birds Vertebrate circulatory systems AMPHIBIANS REPTILES (EXCEPT BIRDS) MAMMALS AND BIRDS Lung and skin capillaries Lung capillaries Lung capillaries FISHES Gill capillaries Artery Right systemic aorta Pulmocutaneous circuit Gill circulation Heart: ventricle (V) A Atrium (A) Systemic Vein circulation Systemic capillaries Pulmonary circuit A A V Right V Left Right Systemic circuit Systemic capillaries Figure 42.4 Pulmonary circuit Left Systemic V aorta Left A Systemic capillaries A V Right A V Left Systemic circuit Systemic capillaries Concept 42.2: Double circulation in mammals depends on the anatomy and pumping cycle of the heart The structure and function of the human circulatory system Can serve as a model for exploring mammalian circulation in general Mammalian Circulation: The Pathway Heart valves Dictate a one-way flow of blood through the heart Blood begins its flow With the right ventricle pumping blood to the lungs In the lungs The blood loads O2 and unloads CO2 Oxygen-rich blood from the lungs Enters the heart at the left atrium and is pumped to the body tissues by the left ventricle Blood returns to the heart Through the right atrium The mammalian cardiovascular system 7 Capillaries of head and forelimbs Anterior vena cava Pulmonary artery Aorta Pulmonary artery 9 6 Capillaries of right lung Capillaries of left lung 2 4 3 Pulmonary vein 5 1 Right atrium 3 11 Left atrium Pulmonary vein 10 Left ventricle Right ventricle Aorta Posterior vena cava 8 Figure 42.5 Capillaries of abdominal organs and hind limbs The Mammalian Heart: how double circulation works Pulmonary artery Aorta Pulmonary artery Anterior vena cava Left atrium Right atrium Pulmonary veins Pulmonary veins Semilunar valve Semilunar valve Atrioventricular valve Atrioventricular valve Posterior vena cava Figure 42.6 Right ventricle Left ventricle The heart contracts and relaxes In a rhythmic cycle called the cardiac cycle The contraction, or pumping, phase of the cycle Is called systole The relaxation, or filling, phase of the cycle Is called diastole The cardiac cycle 2 Atrial systole; ventricular diastole Semilunar valves closed 0.1 sec Semilunar valves open 0.3 sec 0.4 sec AV valves open 1 Atrial and ventricular diastole Figure 42.7 AV valves closed 3 Ventricular systole; atrial diastole The heart rate, also called the pulse Is the number of beats per minute The cardiac output Is the volume of blood pumped into the systemic circulation per minute Maintaining the Heart’s Rhythmic Beat Some cardiac muscle cells are self-excitable Meaning they contract without any signal from the nervous system A region of the heart called the sinoatrial (SA) node, or pacemaker Sets the rate and timing at which all cardiac muscle cells contract Impulses from the SA node Travel to the atrioventricular (AV) node At the AV node, the impulses are delayed And then travel to the Purkinje fibers that make the ventricles contract The impulses that travel during the cardiac cycle Can be recorded as an electrocardiogram (ECG or EKG) The control of heart rhythm 1 Pacemaker generates wave of signals to contract. SA node (pacemaker) 2 Signals are delayed 3 Signals pass at AV node. AV node to heart apex. 4 Signals spread Throughout ventricles. Bundle branches Heart apex ECG Figure 42.8 Purkinje fibers The pacemaker is influenced by Nerves, hormones, body temperature, and exercise Concept 42.3: Physical principles govern blood circulation The same physical principles that govern the movement of water in plumbing systems Also influence the functioning of animal circulatory systems Blood Vessel Structure and Function The “infrastructure” of the circulatory system Is its network of blood vessels All blood vessels Are built of similar tissues Have three similar layers Artery Vein Basement membrane Endothelium 100 µm Valve Endothelium Smooth muscle Connective tissue Endothelium Capillary Smooth muscle Connective tissue Artery Vein Venule Figure 42.9 Arteriole Structural differences in arteries, veins, and capillaries Correlate with their different functions Arteries have thicker walls To accommodate the high pressure of blood pumped from the heart In the thinner-walled veins Blood flows back to the heart mainly as a result of muscle action Direction of blood flow in vein (toward heart) Valve (open) Skeletal muscle Valve (closed) Figure 42.10 Blood Flow Velocity Physical laws governing the movement of fluids through pipes Influence blood flow and blood pressure The velocity of blood flow varies in the circulatory system And is slowest in the capillary beds as a result of the Systolic pressure Veins Venules Arterioles Capillaries Diastolic pressure Arteries 120 100 80 60 40 20 0 Aorta Area (cm2) 50 40 30 20 10 0 Venae cavae Figure 42.11 5,000 4,000 3,000 2,000 1,000 0 Velocity (cm/sec) high resistance and large total cross-sectional area Pressure (mm Hg) Blood Pressure Blood pressure Is the hydrostatic pressure that blood exerts against the wall of a vessel Systolic pressure Is the pressure in the arteries during ventricular systole Is the highest pressure in the arteries Diastolic pressure Is the pressure in the arteries during diastole Is lower than systolic pressure Blood pressure Can be easily measured in humans 1 A typical blood pressure reading for a 20-year-old is 120/70. The units for these numbers are mm of mercury (Hg); a blood pressure of 120 is a force that can support a column of mercury 120 mm high. 4 The cuff is loosened further until the blood flows freely through the artery and the sounds below the cuff disappear. The pressure at this point is the diastolic pressure remaining in the artery when the heart is relaxed. Blood pressure reading: 120/70 Pressure in cuff above 120 Rubber cuff inflated with air 120 Pressure in cuff below 120 Pressure in cuff below 70 120 70 Sounds audible in stethoscope Artery Artery closed 2 A sphygmomanometer, an inflatable cuff attached to a pressure gauge, measures blood pressure in an artery. The cuff is wrapped around the upper arm and inflated until the pressure closes the artery, so that no blood flows past the cuff. When this occurs, the pressure exerted by the cuff exceeds the pressure in the artery. Figure 42.12 3 A stethoscope is used to listen for sounds of blood flow below the cuff. If the artery is closed, there is no pulse below the cuff. The cuff is gradually deflated until blood begins to flow into the forearm, and sounds from blood pulsing into the artery below the cuff can be heard with the stethoscope. This occurs when the blood pressure is greater than the pressure exerted by the cuff. The pressure at this point is the systolic pressure. Sounds stop Blood pressure is determined partly by cardiac output And partly by peripheral resistance due to variable constriction of the arterioles Capillary Function Capillaries in major organs are usually filled to capacity But in many other sites, the blood supply varies Two mechanisms Regulate the distribution of blood in capillary beds In one mechanism Contraction of the smooth muscle layer in the wall of an arteriole constricts the vessel In a second mechanism Precapillary sphincters control the flow of blood between arterioles and venules Precapillary sphincters Thoroughfare channel (a) Sphincters relaxed Arteriole Venule Capillaries Arteriole Venule (b) Sphincters contracted (c) Capillaries and larger vessels (SEM) Figure 42.13 a–c 20 m The critical exchange of substances between the blood and interstitial fluid Takes place across the thin endothelial walls of the capillaries The difference between blood pressure and osmotic pressure Drives fluids out of capillaries at the arteriole end and into capillaries at the venule end Tissue cell Capillary Capillary Red blood cell INTERSTITIAL FLUID Net fluid movement out Net fluid movement in 15 m At the arterial end of a capillary, blood pressure is greater than osmotic pressure, and fluid flows out of the capillary into the interstitial fluid. At the venule end of a capillary, blood pressure is less than osmotic pressure, and fluid flows from the interstitial fluid into the capillary. Direction of blood flow Pressure Blood pressure Osmotic pressure Inward flow Outward flow Figure 42.14 Arterial end of capillary Venule end Fluid Return by the Lymphatic System The lymphatic system Returns fluid to the body from the capillary beds Aids in body defense Fluid reenters the circulation Directly at the venous end of the capillary bed and indirectly through the lymphatic system Concept 42.4: Blood is a connective tissue with cells suspended in plasma Blood in the circulatory systems of vertebrates Is a specialized connective tissue Blood Composition and Function Blood consists of several kinds of cells Suspended in a liquid matrix called plasma The cellular elements Occupy about 45% of the volume of blood Plasma Blood plasma is about 90% water Among its many solutes are Inorganic salts in the form of dissolved ions, sometimes referred to as electrolytes The composition of mammalian plasma Plasma 55% Constituent Major functions Water Solvent for carrying other substances Icons (blood electrolytes Sodium Potassium Calcium Magnesium Chloride Bicarbonate Osmotic balance pH buffering, and regulation of membrane permeability Plasma proteins Albumin Osmotic balance, pH buffering Fibringen Clotting Immunoglobulins (antibodies) Defense Substances transported by blood Nutrients (such as glucose, fatty acids, vitamins) Waste products of metabolism Respiratory gases (O2 and CO2) Hormones Separated blood elements Another important class of solutes is the plasma proteins Which influence blood pH, osmotic pressure, and viscosity Various types of plasma proteins Function in lipid transport, immunity, and blood clotting Cellular Elements Suspended in blood plasma are two classes of cells Red blood cells, which transport oxygen White blood cells, which function in defense A third cellular element, platelets Are fragments of cells that are involved in clotting The cellular elements of mammalian blood Cellular elements 45% Cell type Separated blood elements Number per L (mm3) of blood Functions Erythrocytes (red blood cells) 5–6 million Transport oxygen and help transport carbon dioxide Leukocytes (white blood cells) 5,000–10,000 Defense and immunity Lymphocyte Basophil Eosinophil Neutrophil Monocyte Platelets Figure 42.15 250,000 400,000 Blood clotting Erythrocytes Red blood cells, or erythrocytes Are by far the most numerous blood cells Transport oxygen throughout the body Leukocytes The blood contains five major types of white blood cells, or leukocytes Monocytes, neutrophils, basophils, eosinophils, and lymphocytes, which function in defense by phagocytizing bacteria and debris or by producing antibodies Platelets Platelets function in blood clotting Stem Cells and the Replacement of Cellular Elements The cellular elements of blood wear out And are replaced constantly throughout a person’s life Erythrocytes, leukocytes, and platelets all develop from a common source A single population of cells called pluripotent stem cells in the red marrow of bones Pluripotent stem cells (in bone marrow) Lymphoid stem cells Myeloid stem cells Basophils B cells T cells Lymphocytes Eosinophils Neutrophils Erythrocytes Figure 42.16 Platelets Monocytes Blood Clotting When the endothelium of a blood vessel is damaged The clotting mechanism begins A cascade of complex reactions Converts fibrinogen to fibrin, forming a clot 1 The clotting process begins when the endothelium of a vessel is damaged, exposing connective tissue in the vessel wall to blood. Platelets adhere to collagen fibers in the connective tissue and release a substance that makes nearby platelets sticky. 2 The platelets form a plug that provides emergency protection against blood loss. 3 This seal is reinforced by a clot of fibrin when vessel damage is severe. Fibrin is formed via a multistep process: Clotting factors released from the clumped platelets or damaged cells mix with clotting factors in the plasma, forming an activation cascade that converts a plasma protein called prothrombin to its active form, thrombin. Thrombin itself is an enzyme that catalyzes the final step of the clotting process, the conversion of fibrinogen to fibrin. The threads of fibrin become interwoven into a patch (see colorized SEM). Collagen fibers Platelet plug Platelet releases chemicals that make nearby platelets sticky Fibrin clot Clotting factors from: Platelets Damaged cells Plasma (factors include calcium, vitamin K) Prothrombin Figure 42.17 Thrombin Fibrinogen Fibrin 5 µm Red blood cell Cardiovascular Disease Cardiovascular diseases Are disorders of the heart and the blood vessels Account for more than half the deaths in the United States One type of cardiovascular disease, atherosclerosis Is caused by the buildup of cholesterol within arteries Connective tissue Smooth muscle Plaque Endothelium (a) Normal artery 50 µm (b) Partly clogged artery Figure 42.18a, b 250 µm Hypertension, or high blood pressure Promotes atherosclerosis and increases the risk of heart attack and stroke A heart attack Is the death of cardiac muscle tissue resulting from blockage of one or more coronary arteries A stroke Is the death of nervous tissue in the brain, usually resulting from rupture or blockage of arteries in the head Concept 42.5: Gas exchange occurs across specialized respiratory surfaces Gas exchange Supplies oxygen for cellular respiration and disposes of carbon dioxide Respiratory medium (air of water) O2 CO2 Respiratory surface Organismal level Circulatory system Cellular level Energy-rich molecules from food Figure 42.19 Cellular respiration ATP Animals require large, moist respiratory surfaces for the adequate diffusion of respiratory gases Between their cells and the respiratory medium, either air or water Gills in Aquatic Animals Gills are outfoldings of the body surface Specialized for gas exchange In some invertebrates The gills have a simple shape and are distributed over much of the body (a) Sea star. The gills of a sea star are simple tubular projections of the skin. The hollow core of each gill is an extension of the coelom (body cavity). Gas exchange occurs by diffusion across the gill surfaces, and fluid in the coelom circulates in and out of the gills, aiding gas transport. The surfaces of a sea star’s tube feet also function in gas exchange. Gills Coelom Figure 42.20a Tube foot Many segmented worms have flaplike gills That extend from each segment of their body (b) Marine worm. Many polychaetes (marine worms of the phylum Annelida) have a pair of flattened appendages called parapodia on each body segment. The parapodia serve as gills and also function in crawling and swimming. Parapodia Figure 42.20b Gill The gills of clams, crayfish, and many other animals Are restricted to a local body region (c) Scallop. The gills of a scallop are long, flattened plates that project from the main body mass inside the hard shell. Cilia on the gills circulate water around the gill surfaces. (d) Crayfish. Crayfish and other crustaceans have long, feathery gills covered by the exoskeleton. Specialized body appendages drive water over the gill surfaces. Gills Gills Figure 42.20c, d The effectiveness of gas exchange in some gills, including those of fishes Is increased by ventilation and countercurrent flow of blood and water Oxygen-poor blood Gill arch Gill arch Water flow Blood vessel Oxygen-rich blood Lamella Operculum O2 Figure 42.21 Water flow over lamellae showing % O2 Gill filaments Blood flow through capillaries in lamellae showing % O2 Countercurrent exchange Tracheal Systems in Insects Consists of tiny branching tubes that penetrate the body Air sacs Tracheae Spiracle (a) The respiratory system of an insect consists of branched internal tubes that deliver air directly to body cells. Rings of chitin reinforce the largest tubes, called tracheae, keeping them from collapsing. Enlarged portions of tracheae form air sacs near organs that require a large supply of oxygen. Air enters the tracheae through openings called spiracles on the insect’s body surface and passes into smaller tubes called tracheoles. The tracheoles are closed and contain fluid (blue-gray). When the animal is active and is using more O2, most of the fluid is withdrawn into the body. This increases the surface area of air in contact with cells. Figure 42.22a The tracheal tubes Supply O2 directly to body cells Body cell Air sac Tracheole Trachea Air Tracheoles Mitochondria Body wall Myofibrils (b) This micrograph shows cross sections of tracheoles in a tiny piece of insect flight muscle (TEM). Each of the numerous mitochondria in the muscle cells lies within about 5 µm of a tracheole. Figure 42.22b 2.5 µm Lungs Spiders, land snails, and most terrestrial vertebrates Have internal lungs Mammalian Respiratory Systems: A system of branching ducts Conveys air to the lungs Branch from the pulmonary artery (oxygen-poor blood) Branch from the pulmonary vein (oxygen-rich blood) Terminal bronchiole Nasal cavity Pharynx Left lung Alveoli 50 µm Larynx Esophagus Trachea 50 µm Right lung Bronchus Bronchiole Diaphragm Heart SEM Figure 42.23 Colorized SEM In mammals, air inhaled through the nostrils Passes through the pharynx into the trachea, bronchi, bronchioles, and dead-end alveoli, where gas exchange occurs Concept 42.6: Breathing ventilates the lungs The process that ventilates the lungs is breathing The alternate inhalation and exhalation of air How an Amphibian Breathes An amphibian such as a frog Ventilates its lungs by positive pressure breathing, which forces air down the trachea How a Mammal Breathes Mammals ventilate their lungs By negative pressure breathing, which pulls air into the lungs Rib cage expands as rib muscles contract Air inhaled Rib cage gets smaller as rib muscles relax Air exhaled Lung Diaphragm INHALATION Diaphragm contracts (moves down) Figure 42.24 EXHALATION Diaphragm relaxes (moves up) Lung volume increases As the rib muscles and diaphragm contract How a Bird Breathes Besides lungs, bird have eight or nine air sacs That function as bellows that keep air flowing through the lungs Air Air Anterior air sacs Trachea Posterior air sacs Lungs Lungs Air tubes (parabronchi) in lung INHALATION Air sacs fill EXHALATION Air sacs empty; lungs fill Figure 42.25 1 mm Air passes through the lungs In one direction only Every exhalation Completely renews the air in the lungs Control of Breathing in Humans The main breathing control centers Are located in two regions of the brain, the medulla oblongata and the pons Cerebrospinal fluid 1 The control center in the medulla sets the basic rhythm, and a control center in the pons moderates it, smoothing out the transitions between inhalations and exhalations. Pons 2 Nerve impulses trigger muscle contraction. Nerves from a breathing control center in the medulla oblongata of the brain send impulses to the diaphragm and rib muscles, stimulating them to contract and causing inhalation. Breathing control centers Medulla oblongata 4 The medulla’s control center also helps regulate blood CO2 level. Sensors in the medulla detect changes in the pH (reflecting CO2 concentration) of the blood and cerebrospinal fluid bathing the surface of the brain. 5 Nerve impulses relay changes in CO2 and O2 concentrations. Other sensors in the walls of the aorta and carotid arteries in the neck detect changes in blood pH and send nerve impulses to the medulla. In response, the medulla’s breathing control center alters the rate and depth of breathing, increasing both to dispose of excess CO2 or decreasing both if CO2 levels are depressed. Carotid arteries Aorta Figure 42.26 3 In a person at rest, these nerve impulses result in about 10 to 14 inhalations per minute. Between inhalations, the muscles relax and the person exhales. Diaphragm Rib muscles 6 The sensors in the aorta and carotid arteries also detect changes in O2 levels in the blood and signal the medulla to increase the breathing rate when levels become very low. The centers in the medulla Regulate the rate and depth of breathing in response to pH changes in the cerebrospinal fluid The medulla adjusts breathing rate and depth To match metabolic demands Sensors in the aorta and carotid arteries Monitor O2 and CO2 concentrations in the blood Exert secondary control over breathing Concept 42.7: Respiratory pigments bind and transport gases The metabolic demands of many organisms Require that the blood transport large quantities of O2 and CO2 The Role of Partial Pressure Gradients Gases diffuse down pressure gradients In the lungs and other organs Diffusion of a gas Depends on differences in a quantity called partial pressure A gas always diffuses from a region of higher partial pressure To a region of lower partial pressure In the lungs and in the tissues O2 and CO2 diffuse from where their partial pressures are higher to where they are lower Inhaled air Exhaled air 160 0.2 O2 CO2 120 27 Alveolar spaces O2 CO2 104 Alveolar epithelial cells 40 O2 CO2 Blood entering alveolar capillaries 40 O2 CO2 2 1 O2 Alveolar capillaries of lung 45 O2 CO2 104 Pulmonary veins Systemic arteries Systemic veins CO2 40 45 Heart Tissue capillaries O2 3 4 O2 CO2 Blood entering tissue capillaries 100 40 O2 CO2 O2 CO2 Tissue cells Figure 42.27 40 O2 CO2 Pulmonary arteries Blood leaving tissue capillaries Blood leaving alveolar capillaries <40 >45 O2 CO2 Respiratory Pigments Respiratory pigments Are proteins that transport oxygen Greatly increase the amount of oxygen that blood can carry Oxygen Transport The respiratory pigment of almost all vertebrates Is the protein hemoglobin, contained in the erythrocytes Like all respiratory pigments Hemoglobin must reversibly bind O2, loading O2 in the lungs and unloading it in other parts of the body Heme group Iron atom O2 loaded in lungs O2 unloaded In tissues Figure 42.28 Polypeptide chain O2 O2 Loading and unloading of O2 Depend on cooperation between the subunits of the hemoglobin molecule The binding of O2 to one subunit induces the other subunits to bind O2 with more affinity Cooperative O2 binding and release Is evident in the dissociation curve for hemoglobin A drop in pH Lowers the affinity of hemoglobin for O2 O2 saturation of hemoglobin (%) (a) PO2 and Hemoglobin Dissociation at 37°C and pH 7.4 O2 unloaded from hemoglobin during normal metabolism 100 80 O2 reserve that can be unloaded from hemoglobin to tissues with high metabolism 60 40 20 0 0 20 40 60 Tissues during Tissues at rest exercise 80 100 Lungs (b) pH and Hemoglobin Dissociation Figure 42.29a, b O2 saturation of hemoglobin (%) PO2 (mm Hg) 100 pH 7.4 80 60 pH 7.2 40 20 0 0 20 40 Bohr shift: Additional O2 released from hemoglobin at lower pH (higher CO2 concentration) 60 PO2 (mm Hg) 80 100 Carbon Dioxide Transport Hemoglobin also helps transport CO2 And assists in buffering Carbon from respiring cells Diffuses into the blood plasma and then into erythrocytes and is ultimately released in the lungs 1 2 Carbon dioxide produced by body tissues diffuses into the interstitial fluid and the plasma. Over 90% of the CO2 diffuses into red blood cells, leaving only 7% in the plasma as dissolved CO2. Tissue cell Some CO2 is picked up and transported by hemoglobin. 1 Blood plasma CO2 within capillary Capillary wall 2 CO2 Carbonic acid dissociates into a biocarbonate ion (HCO3–) and a hydrogen ion (H+). HCO3– 7 Hemoglobin binds most of the H+ from H2CO3 preventing the H+ from acidifying the blood and thus preventing the Bohr shift. Figure 42.30 9 Carbonic acid is converted back into CO2 and water. 10 CO2 formed from H2CO3 is unloaded from hemoglobin and diffuses into the interstitial fluid. To lungs CO2 transport to lungs HCO3– 8 H2CO3 Hb 9 11 CO2 Hemoglobin releases CO2 and H+ H2O CO2 6 In the HCO3– diffuse from the plasma red blood cells, combining with H+ released from hemoglobin and forming H2CO3. 6 HCO3– + H+ 5 8 Red Hemoglobin H2CO3 blood Carbonic acid Hb picks up cell CO2 and H+ 5 + H+ Bicarbonate However, most CO2 reacts with water in red blood cells, forming carbonic acid (H2CO3), a reaction catalyzed by carbonic anhydrase contained. Within red blood cells. Most of the HCO3– diffuse into the plasma where it is carried in the bloodstream to the lungs. 3 4 HCO3– 4 7 Interstitial CO 2 fluid H2O 3 CO2 transport from tissues CO2 produced CO2 CO2 10 CO2 11 Alveolar space in lung diffuses into the alveolar space, from which it is expelled during exhalation. The reduction of CO2 concentration in the plasma drives the breakdown of H2CO3 Into CO2 and water in the red blood cells (see step 9), a reversal of the reaction that occurs in the tissues (see step 4). Elite Animal Athletes Migratory and diving mammals Have evolutionary adaptations that allow them to perform extraordinary feats The Ultimate Endurance Runner The extreme O2 consumption of the antelopelike pronghorn Underlies its ability to run at high speed over long distances Figure 42.31 Diving Mammals Deep-diving air breathers Stockpile O2 and deplete it slowly