Chapter 42   Circulation and Gas Exchange
Lecture Outline
Overview: Trading with the Environment

• Every organism must exchange materials and energy with its environment, and this exchange ultimately occurs at the cellular level.
• Cells live in aqueous environments.
• The resources that they need, such as nutrients and oxygen, move across the plasma membrane to the cytoplasm.
• Metabolic wastes, such as carbon dioxide, move out of the cell.
• Most animals have organ systems specialized for exchanging materials with the environment, and many have an internal transport system that conveys fluid (blood or interstitial fluid) throughout the body.
• For aquatic organisms, structures such as gills present an expansive surface area to the outside environment.
• Oxygen dissolved in the surrounding water diffuses across the thin epithelium covering the gills and into a network of tiny blood vessels (capillaries).
• At the same time, carbon dioxide diffuses out into the water.

Concept 42.1 Circulatory systems reflect phylogeny

• Diffusion alone is not adequate for transporting substances over long distances in animals—for example, for moving glucose from the digestive tract and oxygen from the lungs to the brain of a mammal.
• Diffusion is insufficient over distances of more than a few millimeters, 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 of glucose to diffuse 100 microns, it will take 100 seconds for it to diffuse 1 mm and almost three hours to diffuse 1 cm.
• The circulatory system solves this problem by ensuring that no substance must diffuse very far to enter or leave a cell.
• The bulk transport of fluids throughout the body functionally connects the aqueous environment of the body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes.
• For example, in the mammalian lung, oxygen from inhaled air diffuses across a thin epithelium and into the blood, while carbon dioxide diffuses out.
• Bulk fluid movement in the circulatory system, powered by the heart, quickly carries the oxygen-rich blood to all parts of the body.
• As the blood streams through the tissues within microscopic vessels called capillaries, chemicals are exchanged between blood and the interstitial fluid that bathes the cells.

 Most invertebrates have a gastrovascular cavity or a circulatory system for internal transport.

• The body plan of a hydra and other cnidarians makes a circulatory system unnecessary.
• A body wall only two cells thick encloses a central gastrovascular cavity that serves for both digestion and for diffusion of substances throughout the body.
• The fluid inside the cavity is continuous with the water outside through a single opening, the mouth.
• Thus, both the inner and outer tissue layers are bathed in fluid.
• In cnidarians such as Aurelia, the mouth leads to an elaborate gastrovascular cavity that has branches radiating to and from the circular canal.
• The products of digestion in the gastrovascular cavity are directly available to the cells of the inner layer, and it is only a short distance to diffuse to the cells of the outer layer.
• Planarians and most other flatworms also have gastrovascular cavities that exchange materials with the environment through a single opening.
• The flat shape of the body and the branching of the gastrovascular cavity throughout the animal ensure that cells are bathed in a suitable medium and that diffusion distances are short.
• For animals with many cell layers, gastrovascular cavities are insufficient for internal distances because the diffusion transports are too great.
• In more complex animals, two types of circulatory systems that overcome the limitations of diffusion have evolved: open circulatory systems and closed circulatory systems.
• Both have a circulatory fluid (blood), a set of tubes (blood vessels), and a muscular pump (the heart).
• The heart powers circulation by using metabolic power to elevate the hydrostatic pressure of the blood (blood pressure), which then flows down a pressure gradient through its circuit back to the heart.
• In insects, other arthropods, and most molluscs, blood bathes organs directly in an open circulatory system.
• There is no distinction between blood and interstitial fluid, collectively called hemolymph.
• One or more hearts pump the hemolymph into interconnected sinuses surrounding the organs, allowing exchange between hemolymph and body cells.
• In insects and other arthropods, the heart is an elongated dorsal tube.
• When the heart contracts, it pumps hemolymph through vessels out into sinuses.
• When the heart relaxes, it draws hemolymph into the circulatory system through pores called ostia.
• Body movements that squeeze the sinuses help circulate the hemolymph.
• In a closed circulatory system, found in earthworms, squid, octopuses, and vertebrates, blood is confined to vessels and is distinct from interstitial fluid.
• One or more hearts pump blood into large vessels that branch into smaller ones coursing through organs.
• Materials are exchanged by diffusion between the blood and the interstitial fluid bathing the cells.
• The fact that open and closed circulatory systems are both widespread in the animal kingdom suggests that both systems offer advantages.
• The lower hydrostatic pressures associated with open circulatory systems make them less costly than closed circulatory systems.
• Because they lack an extensive system of blood vessels, open systems require less energy to build and maintain.
• In molluscs and freshly molted aquatic arthropods, the open circulatory system functions as a hydrostatic skeleton.
• What advantages are associated with closed circulatory systems?
• Closed systems with their higher blood pressure are more effective at transporting circulatory fluids to meet the high metabolic demands of the tissues and cells of larger and more active animals.
• Among the molluscs, only the large and active squid and octopuses have closed circulatory systems.

 Vertebrate phylogeny is reflected in adaptations of the cardiovascular system.

• The closed circulatory system of humans and other vertebrates is often called the cardiovascular system.
• The heart consists of one atrium or two atria, the chambers that receive blood returning to the heart, and one or two ventricles, the chambers that pump blood out of the heart.
• Arteries, veins, and capillaries are the three main kinds of blood vessels.
• Arteries carry blood away from the heart to organs.
• Within organs, arteries branch into arterioles, small vessels that convey blood to capillaries.
• Capillaries with very thin, porous walls form networks called capillary beds, which infiltrate each tissue.
• Chemicals, including dissolved gases, are exchanged across the thin walls of the capillaries between the blood and interstitial fluid.
• At their “downstream” end, capillaries converge into venules, and venules converge into veins, which (usually) return blood to the heart.
• Arteries and veins are distinguished by the direction in which they carry blood, not by the characteristics of the blood they carry.
• All arteries carry blood from the heart toward capillaries.
• Veins return blood to the heart from capillaries.
• A significant exception is the hepatic portal vein that carries blood from capillary beds in the digestive system to capillary beds in the liver.
• Metabolic rate is an important factor in the evolution of cardiovascular systems.
• In general, animals with high metabolic rates have more complex circulatory systems and more powerful hearts than animals with low metabolic rates.
• Similarly, the complexity and number of blood vessels in a particular organ are correlated with that organ’s metabolic requirements.
• Perhaps the most fundamental differences in cardiovascular adaptations are associated with gill breathing in aquatic vertebrates compared with lung breathing in terrestrial vertebrates.
• A fish heart has two main chambers, one atrium and one ventricle.
• Blood is pumped from the ventricle to the gills (the gill circulation) where it picks up oxygen and disposes of carbon dioxide across the capillary walls.
• The gill capillaries converge into a vessel that carries oxygenated blood to capillary beds in the other organs (the systemic circulation) and back via veins to the atrium of the heart.
• In fish, blood must pass through two capillary beds, the gill capillaries and systemic capillaries.
• When blood flows through a capillary bed, blood pressure—the motive force for circulation—drops substantially.
• Therefore, oxygen-rich blood leaving the gills flows to the systemic circulation quite slowly (although the process is aided by body movements during swimming).
• This constrains the delivery of oxygen to body tissues and, hence, the maximum aerobic metabolic rate of fishes.
• 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 and systemic circulations.
• The pulmocutaneous circulation leads to capillaries in the gas-exchange organs (the lungs and skin of a frog), where the blood picks up O2 and releases CO2 before returning to the heart’s left atrium.
• Most of the returning oxygen-rich blood is pumped into the systemic circulation, which supplies all body organs and then returns oxygen-poor blood to the right atrium via the veins.
• This scheme, called double circulation, provides a vigorous flow of blood to the brain, muscles, and other organs because the blood is pumped a second time after it loses pressure in the capillary beds of the lung or skin.
• In the ventricle of the frog, some oxygen-rich blood from the lungs mixes with oxygen-poor blood that has returned from the rest of the body.
• However, a ridge within the ventricle diverts most of the oxygen-rich blood from the left atrium into the systemic circuit and most of the oxygen-poor blood from the right atrium into the pulmocutaneous circuit.
• Nonbird reptiles also have double circulation with a pulmonary circuit (lungs) and a systemic circuit.
• Turtles, snakes, and lizards have a three-chambered heart, although the ventricle is partially blocked by a septum, which results in even less mixing of oxygen-rich and oxygen-poor blood than in amphibians.
• All reptiles except birds have two arteries leading from the heart to the systemic circuit, and arterial valves allow them to divert most of their blood from the pulmonary circuit to the systemic circuit.
• In crocodilians, birds, and mammals, the ventricle is completely divided into separate right and left chambers.
• In this arrangement, the left side of the heart receives and pumps only oxygen-rich blood, while the right side handles only oxygen-poor blood.
• Double circulation restores pressure to the systemic circuit after blood has passed through the lung capillaries and prevents mixing of oxygen-rich and oxygen-poor blood.
• The evolution of a powerful four-chambered heart was an essential adaptation to support the endothermic way of life characteristic of birds and mammals.
• Endotherms use about ten times as much energy as ectotherms of the same size.
• Therefore, the endotherm circulatory system needs to deliver about ten times as much fuel and O2 to their tissues and remove ten times as much wastes and CO2.
• Birds and mammals evolved from different reptilian ancestors, and their powerful four-chambered hearts evolved independently—an example of convergent evolution.

Concept 42.2 Double circulation in mammals depends on the anatomy and pumping cycle of the heart

• In the mammalian cardiovascular system, the pulmonary and system circuits operate simultaneously.
• The two ventricles pump almost in unison.
• While some blood is traveling in the pulmonary circuit, the rest of the blood is flowing in the systemic circuit.
• To trace the double circulation pattern of the mammalian cardiovascular system, we’ll start with the pulmonary (lung) circuit.
• The pulmonary circuit carries blood from the heart to the lungs and back again.
• The right ventricle pumps blood to the lungs via the pulmonary arteries.
• As blood flows through capillary beds in the right and left lungs, it loads O2 and unloads CO2.
• Oxygen-rich blood returns from the lungs via the pulmonary veins to the left atrium of the heart.
• Next, the oxygen-rich blood flows to the left ventricle, as the ventricle opens and the atrium contracts.
• The left ventricle pumps oxygen-rich blood out to the body tissues through the systemic circuit.
• Blood leaves the left ventricle via the aorta, which conveys blood to arteries leading throughout the body.
• The first branches from the aorta are the coronary arteries, which supply blood to the heart muscle.
• The next branches lead to capillary beds in the head and arms.
• The aorta continues in a posterior direction, supplying oxygen-rich blood to arteries leading to arterioles and capillary beds in the abdominal organs and legs.
• Within the capillaries, blood gives up much of its O2 and picks up CO2 produced by cellular respiration.
• Venous return to the right side of the heart begins as capillaries rejoin to form venules and then veins.
• Oxygen-poor blood from the head, neck, and forelimbs is channeled into a large vein called the anterior (or superior) vena cava.
• Another large vein called the posterior (or inferior) vena cava drains blood from the trunk and hind limbs.
• The two venae cavae empty their blood into the right atrium, from which the oxygen-poor blood flows into the right ventricle.
• The mammalian heart is located beneath the breastbone (sternum) and consists mostly of cardiac muscle.
• The two atria have relatively thin walls and function as collection chambers for blood returning to the heart.
• The ventricles have thicker walls and contract much more strongly than the atria.
• A cardiac cycle is one complete sequence of pumping, as the heart contracts, and filling, as it relaxes and its chambers fill with blood.
• The contraction phase is called systole, and the relaxation phase is called diastole.
• For a human at rest with a pulse of about 75 beats per minute, one complete cardiac cycle takes about 0.8 sec.
• During the relaxation phase (atria and ventricles in diastole) lasting about 0.4 sec, blood returning from the large veins flows into atria and ventricles.
• A brief period (about 0.1 sec) of atrial systole forces all the remaining blood out of the atria and into the ventricles.
• During the remaining 0.3 sec of the cycle, ventricular systole pumps blood into the large arteries.
• Cardiac output is the volume of blood pumped per minute, and it depends on two factors: the rate of contraction or heart rate (number of beats per second) and stroke volume, the amount of blood pumped by the left ventricle in each contraction.
• The average stroke volume for a human is about 75 mL.
• The typical resting cardiac output, about 5.25 L/min, is equivalent to the total volume of blood in the human body.
• Cardiac output can increase about fivefold during heavy exercise.
• Heart rate can be measured indirectly by measuring your pulse—the rhythmic stretching of arteries caused by the pressure of blood pumped by the ventricles.
• Four valves in the heart, each consisting of flaps of connective tissue, prevent backflow and keep blood moving in the correct direction.
• Between each atrium and ventricle is an atrioventricular (AV) valve, which keeps blood from flowing back into the atria when the ventricles contract.
• The AV valves are anchored by strong fibers that prevent them from turning inside out.
• Two sets of semilunar valves, one between the left ventricle and the aorta and the other between the right ventricle and the pulmonary artery, prevent backflow from these vessels into the ventricles while they are relaxing.
• The heart sounds we can hear with a stethoscope are caused by the closing of the valves.
• The sound pattern is “lub-dup, lub-dup, lub-dup.”
• The first heart sound (“lub”) is created by the recoil of blood against the closed AV valves.
• The second sound (“dup”) is the recoil of blood against the semilunar valves.
• A defect in one or more of the valves causes a heart murmur, which may be detectable as a hissing sound when a stream of blood squirts backward through a valve.
• Some people are born with heart murmurs.
• Other murmurs are due to damage to the valves by infection.
• Most heart murmurs do not reduce the efficiency of blood flow enough to warrant surgery.
• Because the timely delivery of oxygen to the body’s organs is critical for survival, several mechanisms have evolved to assure continuity and control of the heartbeat.
• Certain cells of vertebrate cardiac muscle are self-excitable, meaning they contract without any signal from the nervous system.
• Each cell has its own intrinsic contraction rhythm.
• However, these cells are synchronized by the sinoatrial (SA) node, or pacemaker, which sets the rate and timing at which all cardiac muscle cells contract.
• The SA node is located in the wall of the right atrium.
• Because the vertebrate heart has a pacemaker made up of specialized muscle tissues located within the heart itself, it is referred to as a myogenic heart.
• In contrast, the pacemakers of most arthropod hearts originate in motor nerves arising from the outside, an arrangement called a neurogenic heart.
• The cardiac cycle is regulated by electrical impulses that radiate throughout the heart.
• Cardiac muscle cells are electrically coupled by intercalated disks between adjacent cells.
• The SA node generates electrical impulses, much like those produced by nerves that spread rapidly through the wall of the atria, making them contract in unison.
• The impulse from the SA node is delayed by about 0.1 sec at the atrioventricular (AV) node, the relay point to the ventricle, allowing the atria to empty completely before the ventricles contract.
• Specialized muscle fibers called bundle branches and Purkinje fibers conduct the signals to the apex of the heart and throughout the ventricular walls.
• This stimulates the ventricles to contract from the apex toward the atria, driving blood into the large arteries.
• The impulses generated during the heart cycle produce electrical currents that are conducted through body fluids to the skin.
• Here, the currents can be detected by electrodes and recorded as an electrocardiogram (ECG or EKG).
• While the SA node sets the tempo for the entire heart, it is influenced by a variety of physiological cues.
• Two sets of nerves affect heart rate, with one set speeding up the pacemaker and the other set slowing it down.
• Heart rate is a compromise regulated by the opposing actions of these two sets of nerves.
• The pacemaker is also influenced by hormones.
• For example, epinephrine from the adrenal glands increases heart rate.
• The rate of impulse generation by the pacemaker increases in response to increases in body temperature and with exercise.

Concept 42.3 Physical principles govern blood circulation

• All blood vessels are built of similar tissues.
• The walls of both arteries and veins have three similar layers.
• On the outside, a layer of connective tissue with elastic fibers allows the vessel to stretch and recoil.
• A middle layer has smooth muscle and more elastic fibers.
• Lining the lumen of all blood vessels, including capillaries, is an endothelium, a single layer of flattened cells that minimizes resistance to blood flow.
• Structural differences correlate with the different functions of arteries, veins, and capillaries.
• Capillaries lack the two outer layers, and their very thin walls consist only of endothelium and its basement membrane, thus enhancing exchange.
• Arteries have thicker middle and outer layers than veins.
• The thicker walls of arteries provide strength to accommodate blood pumped rapidly and at high pressure by the heart.
• Their elasticity (elastic recoil) helps maintain blood pressure even when the heart relaxes.
• The thinner-walled veins convey blood back to the heart at low velocity and pressure.
• Blood flows through the veins mainly because skeletal muscle contractions squeeze blood in veins.
• Within larger veins, flaps of tissues act as one-way valves that allow blood to flow only toward the heart.

 Physical laws governing the movement of fluids through pipes affect blood flow and blood pressure.

• The observation that blood travels more than a thousand times faster in the aorta than in capillaries follows from the law of continuity, describing fluid movement through pipes.
• If a pipe’s diameter changes over its length, a fluid will flow through narrower segments faster than it flows through wider segments because the volume of flow per second must be constant throughout the entire pipe.
• Each artery conveys blood to such an enormous number of capillaries that the total cross-sectional area is much greater in capillary beds than in any other part of the circulatory system.
• The resulting slow flow rate and thin capillary walls enhance the exchange of substances between the blood and interstitial fluid.
• As blood leaves the capillary beds and passes to venules and veins, it speeds up again as a result of the reduction in total cross-sectional area.
• Fluids exert a force called hydrostatic pressure against surfaces they contact, and it is that pressure that drives fluids through pipes.
• Fluids always flow from areas of high pressure to areas of lower pressure.
• Blood pressure, the hydrostatic force that blood exerts against vessel walls, is much greater in arteries than in veins and is highest in arteries when the heart contracts during ventricular systole, creating the systolic pressure.
• When you take your pulse by placing your fingers on your wrist, you can feel an artery bulge with each heartbeat.
• The surge of pressure is partly due to the narrow openings of arterioles impeding the exit of blood from the arteries, the peripheral resistance.
• Thus, when the heart contracts, blood enters the arteries faster than it can leave, and the vessels stretch from the pressure.
• The elastic walls of the arteries snap back during diastole, but the heart contracts again before enough blood has flowed into the arterioles to completely relieve pressure in the arteries.
• As a consequence of the elastic arteries working against peripheral resistance, there is substantial diastolic pressure even during diastole.
• Blood flows into arterioles and capillaries continuously.
• The arterial blood pressure of a healthy human oscillates between about 120 mm Hg at systole and less than 80 mm Hg at diastole.
• Blood pressure is determined partly by cardiac output and partly by peripheral resistance.
• Contraction of smooth muscles in walls of arterioles constricts these vessels, increasing peripheral resistance and increasing blood pressure upstream in the arteries.
• When the smooth muscles relax, the arterioles dilate, blood flow through arterioles increases, and pressure in the arteries falls.
• Nerve impulses, hormones, and other signals control the arteriole wall muscles.
• Stress, both physical and emotional, can raise blood pressure by triggering nervous and hormonal responses that will constrict blood vessels.
• Cardiac output is adjusted in concert with changes in peripheral resistance.
• This coordination maintains adequate blood flow as the demands on the circulatory system change.
• For example, during heavy exercise, arterioles in the working muscles dilate, admitting a greater flow of oxygen-rich blood to the muscles and decreasing peripheral resistance.
• At the same time, cardiac output increases, maintaining blood pressure and supporting the necessary increase in blood flow.
• In large land animals, blood pressure is also affected by gravity.
• In addition to the peripheral resistance, additional pressure is necessary to push blood to the level of the heart.
• In a standing human, it takes an extra 27 mm of Hg pressure to move blood from the heart to the brain.
• In an organism like a giraffe, this extra force is about 190 mm Hg (for a total of 250 mm Hg).
• Special check valves and sinuses, as well as feedback mechanisms that reduce cardiac output, prevent this high pressure from damaging the giraffe’s brain when it puts its head down.
• By the time blood reaches the veins, its pressure is not affected much by the action of the heart.
• The resistance of tiny arterioles and capillaries has dissipated the pressure generated by the pumping heart.
• Rhythmic contractions of smooth muscles in the walls of veins and venules account for some movement of blood.
• More important, the activity of skeletal muscles during exercise squeezes blood through the veins.
• Also, inhalation changes pressure in the thoracic (chest) cavity, causing the venae cavae and other large veins near the heart to expand and fill with blood.

 Transfer of substances between the blood and the interstitial fluid occurs across the thin walls of capillaries.

• At any given time, only about 5–10% of the body’s capillaries have blood flowing through them.
• Capillaries in the brain, heart, kidneys, and liver are usually filled to capacity, but in many other sites, the blood supply varies over times as blood is diverted.
• For example, after a meal, blood supply to the digestive tract increases.
• During strenuous exercise, blood is diverted from the digestive tract and supplied to skeletal muscles.
• Two mechanisms, both dependent on smooth muscles controlled by nerve signals and hormones, 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, decreasing blood flow through it to a capillary bed.
• When the muscle layer relaxes, the arteriole dilates, allowing blood to enter the capillaries.
• In the other mechanism, rings of smooth muscles, called precapillary sphincters because they are located at the entrance to capillary beds, control the flow of blood between arterioles and venules.
• Some blood flows directly from arterioles to venules through thoroughfare channels that are always open.
• The exchange of substances between the blood and interstitial fluid that bathes the cells takes place across the thin endothelial walls of the capillaries.
• Some substances are carried across endothelial cells in vesicles that form by endocytosis on one side and then release their contents by exocytosis on the other side.
• Others simply diffuse between the blood and the interstitial fluid across cells or through the clefts between adjoining cells.
• Transport through these clefts occurs mainly by bulk flow due to fluid pressure.
• Blood pressure within the capillary pushes fluid, containing water and small solutes, through the capillary clefts.
• This causes a net loss of fluid at the upstream end of the capillary.
• Blood cells and most proteins in the blood are too large and remain in the capillaries.
• As blood proceeds along the capillary, blood pressure continues to drop and the capillary becomes hyperosmotic compared to the interstitial fluids.
• The resulting osmotic gradient pulls water into the capillary by osmosis near the downstream end.
• About 85% of the fluid that leaves the blood at the arterial end of the capillary bed reenters from the interstitial fluid at the venous end.
• The remaining 15% is eventually returned to the blood by the vessels of the lymphatic system.

 The lymphatic system returns fluid to the blood and aids in body defense.

• Fluids and some blood proteins that leak from the capillaries into the interstitial fluid are returned to the blood via the lymphatic system.
• Fluid enters this system by diffusing into tiny lymph capillaries intermingled among capillaries of the cardiovascular system.
• Once inside the lymphatic system, the fluid is called lymph, with a composition similar to the interstitial fluid.
• The lymphatic system drains into the circulatory system near the junction of the venae cavae with the right atrium.
• Lymph vessels, like veins, have valves that prevent the backflow of fluid toward the capillaries.
• Rhythmic contraction of the vessel walls helps draw fluid into lymphatic capillaries.
• Like veins, lymph vessels depend mainly on the movement of skeletal muscle to squeeze fluid toward the heart.
• Along lymph vessels are organs called lymph nodes.
• The lymph nodes filter the lymph and attack viruses and bacteria.
• Inside a lymph node is a honeycomb of connective tissue with spaces filled with white blood cells specialized for defense.
• When the body is fighting an infection, these cells multiply, and the lymph nodes become swollen.
• In addition to defending against infection and maintaining the volume and protein concentration of the blood, the lymphatic system transports fats from the digestive tract to the circulatory system.

Concept 42.4 Blood is a connective tissue with cells suspended in plasma

• In invertebrates with open circulation, blood (hemolymph) is not different from interstitial fluid.
• However, blood in the closed circulatory systems of vertebrates is a specialized connective tissue consisting of several kinds of cells suspended in a liquid matrix called plasma.
• The plasma includes the cellular elements (cells and cell fragments), which occupy about 45% of the blood volume, and transparent, straw-colored plasma.
• Plasma, about 55% of the blood volume, consists of water, ions, various plasma proteins, nutrients, waste products, respiratory gases, and hormones, while the cellular elements include red and white blood cells and platelets.
• Blood plasma is about 90% water.
• Dissolved in the plasma are a variety of ions, sometimes referred to as blood electrolytes.
• These are important in maintaining osmotic balance of the blood and help buffer the blood at a pH of about 7.4.
• Also, proper functioning of muscles and nerves depends on the concentrations of key ions in the interstitial fluid, which reflects concentrations in the plasma.
• Blood’s plasma proteins have many functions.
• Collectively, they act as buffers against pH changes, help maintain osmotic balance, and contribute to the blood’s viscosity.
• Some specific proteins transport otherwise insoluble lipids in the blood.
• Other proteins—the immunoglobulins, or antibodies—help combat viruses and other foreign agents that invade the body.
• Fibrinogen proteins help plug leaks when blood vessels are injured.
• Blood plasma with clotting factors removed is called serum.
• Plasma carries a wide variety of substances in transit from one part of the body to another, including nutrients, metabolic wastes, respiratory gases, and hormones.
• Suspended in blood plasma are two classes of cells: red blood cells, which transport oxygen, and white blood cells, which function in defense.
• A third cellular element, platelets, are pieces of cells that are involved in clotting.
• Red blood cells, or erythrocytes, are by far the most numerous blood cells.
• Each cubic millimeter of blood contains 5 to 6 million red cells, 5,000 to 10,000 white blood cells, and 250,000 to 400,000 platelets.
• There are about 25 trillion red blood cells in the body’s 5 L of blood.
• The main function of red blood cells, oxygen transport, depends on rapid diffusion of oxygen across the red blood cell’s plasma membranes.
• Human erythrocytes are small biconcave disks, presenting a large surface area.
• Mammalian erythrocytes lack nuclei, an unusual characteristic that leaves more space in the tiny cells for hemoglobin, the iron-containing protein that transports oxygen.
• Red blood cells also lack mitochondria and generate ATP exclusively by anaerobic metabolism.
• An erythrocyte contains about 250 million molecules of hemoglobin.
• Each hemoglobin molecule binds up to four molecules of O2, so one erythrocyte can transport a billion O2 molecules.
• As red blood cells pass through the capillary beds of lungs, gills, or other respiratory organs, oxygen diffuses into the erythrocytes and hemoglobin binds O2 and NO.
• In the systemic capillaries, hemoglobin unloads oxygen, which then diffuses into body cells.
• NO relaxes the capillary walls, allowing them to expand and helping deliver O2 to the cells.
• There are five major types of white blood cells, or leukocytes: monocytes, neutrophils, basophils, eosinophils, and lymphocytes.
• Their collective function is to fight infection.
• For example, monocytes and neutrophils are phagocytes, which engulf and digest bacteria and debris from the body’s dead cells.
• Lymphocytes develop into specialized B cells and T cells, which produce the immune response against foreign substances.
• White blood cells spend most of their time outside the circulatory system, patrolling through interstitial fluid and the lymphatic system, fighting pathogens.
• A microliter of human blood normally has about 5,000 to 10,000 leukocytes, but their numbers increase temporarily when the body is fighting infection.
• The third cellular element of blood, platelets, are fragments of cells about 2 to 3 microns in diameter.
• They have no nuclei and originate as pinched-off cytoplasmic fragments of large cells in the bone marrow.
• Platelets function in blood clotting.
• The cellular elements of blood wear out and are replaced constantly throughout a person’s life.
• For example, erythrocytes usually circulate for only about 3 to 4 months and are then destroyed by phagocytic cells in the liver and spleen.
• Enzymes digest the old cell’s macromolecules, and the monomers are recycled.
• Many of the iron atoms derived from hemoglobin in old red blood cells are incorporated into new hemoglobin molecules.
• Erythrocytes, leukocytes, and platelets all develop from a single population of cells, pluripotent stem cells, in the red marrow of bones, particularly the ribs, vertebrae, breastbone, and pelvis.
• “Pluripotent” means that these cells have the potential to differentiate into any type of blood cells or cells that produce platelets.
• This population arises in the early embryo and renews itself while replenishing the blood with cellular elements.
• A negative-feedback mechanism, sensitive to the amount of oxygen reaching the tissues via the blood, controls erythrocyte production.
• If the tissues do not produce enough oxygen, the kidney synthesizes and secretes a hormone called erythropoietin (EPO), which stimulates production of erythrocytes.
• If blood is delivering more oxygen than the tissues can use, the level of erythropoietin is reduced, and erythrocyte production slows.
• Physicians use synthetic EPO to treat people with anemia, a condition of low hemoglobin levels.
• Some athletes abuse EPO by injecting themselves with the drug to increase their erythrocyte levels.
• This practice is known as blood doping. It is banned by the International Olympic Committee and other sports federations.
• Through a recent breakthrough in isolating and culturing pluripotent stem cells, researchers may soon have effective treatments for a number of human diseases, such as leukemia.
• Individuals with leukemia have a cancerous line of stem cells that produce leukocytes.
• These cancerous cells crowd out cells that make red blood cells and produce an unusually high number of leukocytes, many of which are abnormal.
• One strategy now being used experimentally for treating leukemia is to remove pluripotent stem cells from a patient, destroy the patient’s bone marrow, and restock it with noncancerous pluripotent cells.
• As few as 30 of these cells can repopulate the bone marrow.
• Blood contains a self-sealing material that plugs leaks from cuts and scrapes.
• A clot forms when the inactive form of the plasma protein fibrinogen is converted to fibrin, which aggregates into threads that form the framework of the clot.
• The clotting mechanism begins with the release of clotting factors from platelets.
• An inherited defect in any step of the clotting process causes hemophilia, a disease characterized by excessive bleeding from even minor cuts and bruises.
• The clotting process begins when the endothelium of a vessel is damaged and connective tissue in the wall is exposed to blood.
• Platelets adhere to collagen fibers and release a substance that makes nearby platelets sticky.
• The platelets form a plug.
• The seal is reinforced by a clot of fibrin when vessel damage is severe.
• More than a dozen clotting factors have been discovered, and the mechanism is still not fully understood.
• A genetic mutation that affects any step of the clotting process causes hemophilia, a disease characterized by excessive bleeding from even minor cuts.
• Anticlotting factors in the blood normally prevent spontaneous clotting in the absence of injury.
• Sometimes, platelets clump and fibrin coagulates within a blood vessel, forming a clot called a thrombus, and blocking the flow of blood.
• These potentially dangerous clots are more likely to form in individuals with cardiovascular diseases, diseases of the heart and blood vessels.

 Cardiovascular diseases are the leading cause of death in the United States and most other developed nations.

• More than half of the deaths in the United States are caused by cardiovascular diseases, diseases of the heart and blood vessels.
• The tendency to develop cardiovascular disease is inherited to some extent, but lifestyle also plays a important role.
• Nongenetic factors include smoking, lack of exercise, a diet rich in animal fat, and high levels of cholesterol in the blood.
• One measure of an individual’s cardiovascular health or risk of arterial plaques can be gauged by the ratio of low-density lipoproteins (LDLs) to high-density lipoproteins (HDLs) in the blood.
• LDL is associated with depositing of cholesterol in arterial plaques.
• HDL may reduce cholesterol deposition.
• Exercise increases HDL concentration, while smoking increases LDL:HDL ratio.
• Healthy arteries have smooth inner linings that permit unimpeded blood flow.
• Deposition of cholesterol thickens and roughens this smooth lining.
• Growths called plaques develop in the inner wall of the arteries, narrowing their bore and leading to a chronic cardiovascular disease known as atherosclerosis.
• At plaque sites, the smooth muscle layer of an artery thickens abnormally and becomes infiltrated with fibrous connective tissue and lipids such as cholesterol.
• The rough lining of an atherosclerotic artery encourages the adhesion of platelets, triggering the clotting process, and interfering with circulation.
• Hypertension (high blood pressure) promotes atherosclerosis and increases the risk of heart disease and stroke.
• Atherosclerosis raises blood pressure by narrowing the vessels and reducing their elasticity.
• According to one hypothesis, high blood pressure causes chronic damage to the endothelium that lines the arteries, promoting plaque formation.
• Hypertension is simple to diagnose and can usually be controlled by diet, exercise, medication, or a combination of these.
• A diastolic pressure over 90 is cause for concern, and extreme hypertension (200/100) courts disaster.
• As atherosclerosis progresses, arteries become more and more clogged and the threat of heart attack or stroke becomes much greater, but there may be warnings of this impending threat.
• For example, if a coronary artery is partially blocked, a person may feel occasional chest pains, a condition known as angina pectoris.
• This is a signal that part of the heart is not receiving enough blood, especially when the heart is laboring because of physical or emotional stress.
• However, many people with atherosclerosis experience no warning signs and are unaware of their disease until catastrophe strikes.
• The final blow is usually a heart attack or stroke.
• A heart attack is the death of cardiac muscle tissue resulting from prolonged blockage of one or more coronary arteries, the vessels that supply oxygen-rich blood to the heart.
• A stroke is the death of nervous tissue in the brain.
• Heart attacks and strokes frequently result from a thrombus that clogs a coronary artery or an artery in the brain.
• A key process leading to the clogging of an artery is an inflammatory response triggered by the accumulation of LDLs in the inner lining of an artery.
• Such an inflammation can cause plaques to rupture, releasing fragments that form a thrombus.
• The thrombus may originate at the site of blockage or it may develop elsewhere and be transported (now called an embolus) until it becomes lodged in an artery too narrow for it to pass.
• Cardiac or brain tissue downstream of the blockage may die from oxygen deprivation.
• If damage in the heart interrupts the conduction of electrical impulses through cardiac muscle, heart rate may change drastically or the heart may stop beating altogether.
• Still, the victim may survive if a heartbeat is restored by cardiopulmonary resuscitation (CPR) within a few minutes of the attack.
• The effects of a stroke and the individual’s chance of survival depend on the extent and location of the damaged brain tissue.

Concept 42.5 Gas exchange occurs across specialized respiratory surfaces

• Gas exchange is the uptake of molecular oxygen (O2) from the environment and the discharge of carbon dioxide (CO2) to the environment.
• While often called respiration, this process is distinct from, but linked to, the production of ATP in cellular respiration.
• Gas exchange, in concert with the circulatory system, provides the oxygen necessary for aerobic cellular respiration and removes the waste product, carbon dioxide.
• The source of oxygen, the respiratory medium, is air for terrestrial animals and water for aquatic animals.
• The atmosphere is about 21% O2 (by volume).
• Dissolved oxygen levels in lakes, oceans, and other bodies of water vary considerably, but are always much less than an equivalent volume of air.
• The part of an animal where gases are exchanged with the environment is the respiratory surface.
• Movements of CO2 and O2 across the respiratory surface occur entirely by diffusion.
• The rate of diffusion is proportional to the surface area across which diffusion occurs, and inversely proportional to the square of the distance through which molecules must move.
• Therefore, respiratory surfaces tend to be thin and have large areas, maximizing the rate of gas exchange.
• In addition, the respiratory surface of terrestrial and aquatic animals must be moist to maintain the cell membranes.
• As a result, gases must dissolve in water before diffusing across respiratory surfaces.
• Because the respiratory surface must supply O2 and expel CO2 for the entire body, the structure of a respiratory surface depends mainly on the size of the organism, whether it lives in water or on land, and on its metabolic demands.
• An endotherm requires a larger area of respiratory surface than a similar-sized ectotherm.
• Gas exchange occurs over the entire surface area of protists and other unicellular organisms.
• Similarly, for some relatively simple animals, such as sponges, cnidarians, and flatworms, the plasma membrane of every cell in the body is close enough to the outside environment for gases to diffuse in and out.
• However, in most animals, the bulk of the body lacks direct access to the respiratory medium.
• The respiratory surface is a thin, moist epithelium, separating the respiratory medium from the blood or capillaries, which transport gases to and from the rest of the body.
• Some animals, such as earthworms and some amphibians, use the entire outer skin as a respiratory organ.
• Just below the moist skin is a dense net of capillaries.
• However, because the respiratory surface must be moist, the possible habitats of these animals are limited to water or damp places.
• Animals that use their moist skin as their only respiratory organ are usually small and are either long and thin or flat in shape, with a high ratio of surface area to volume.
• For most other animals, the general body surface lacks sufficient area to exchange gases for the entire body.
• The solution is a respiratory organ that is extensively folded or branched, enlarging the surface area for gas exchange.
• Gills, tracheae, and lungs are the three most common respiratory organs.

 Gills are respiratory adaptations of most aquatic animals.

• Gills are outfoldings of the body surface that are suspended in water.
• In some invertebrates, such as sea stars, gills have a simple shape and are distributed over much of the body.
• Many segmented worms have flap-like gills that extend from each body segment, or long feathery gills clustered at the head or tail.
• The gills of clams, crayfish, and many other animals are restricted to a local body region.
• The total surface area of gills is often much greater than that of the rest of the body.
• Water has both advantages and disadvantages as a respiratory medium.
• There is no problem keeping the cell membranes of the respiratory surface moist, since the gills are surrounded by the aqueous environment.
• However, O2 concentrations in water are low, especially in warmer and saltier environments.
• Thus, gills must be very effective to obtain enough oxygen.
• Ventilation, which increases the flow of the respiratory medium over the respiratory surface, ensures that there is a strong diffusion gradient between the gill surface and the environment.
• Without ventilation, a region of low O2 and high CO2 concentrations can form around the gill as it exchanges gas with the environment.
• Crayfish and lobsters have paddle-like appendages that drive a current of water over their gills.
• Fish gills are ventilated by a current of water that enters the mouth, passes through slits in the pharynx, flows over the gills, and exits the body.
• Because water is dense and contains little oxygen per unit volume, fishes must expend considerable energy in ventilating their gills.
• Gas exchange at the gill surface is enhanced by the opposing flows of water and blood at the gills.
• This flow pattern is countercurrent exchange.
• As blood moves through a gill capillary, it becomes more and more loaded with oxygen, but it simultaneously encounters water with even higher oxygen concentrations because it is just beginning its passage over the gills.
• All along the gill capillary, there is a diffusion gradient favoring the transfer of oxygen from water to blood.
• The countercurrent exchange mechanism is so efficient that the gills can remove more than 80% of the oxygen from water to blood.
• Gills are generally unsuited for an animal living on land.
• An expansive surface of wet membrane exposed to air would lose too much water by evaporation.
• In addition, the gills would collapse as their fine filaments, no longer supported by water, cling together, reducing surface area for exchange.
• Most terrestrial animals have their respiratory surfaces within the body, opening to the atmosphere through narrow tubes.

 Tracheal systems and lungs are respiratory adaptations of terrestrial animals.

• As a respiratory medium, air has many advantages over water.
• Air has a much higher concentration of oxygen.
• Also, since O2 and CO2 diffuse much faster in air than in water, respiratory surfaces exposed to air do not have to be ventilated as thoroughly as gills.
• When a terrestrial animal does ventilate, less energy is needed because air is far lighter and much easier to pump than water and much less volume needs to be breathed to obtain an equal amount of O2.
• Air does have problems as a respiratory medium.
• The respiratory surface, which must be large and moist, continuously loses water to the air by evaporation.
• This problem is greatly reduced by a respiratory surface folded into the body.
• The tracheal system of insects is composed of air tubes that branch throughout the body.
• The largest tubes, called tracheae, open to the outside, and the finest branches extend to the surface of nearly every cell where gas is exchanged by diffusion across the moist epithelium that lines the terminal ends.
• The open circulatory system does not transport oxygen and carbon dioxide.
• For a small insect, diffusion through the trachea brings in enough O2 and removes enough CO2 to support cellular respiration.
• Larger insects with higher energy demands ventilate their tracheal systems with rhythmic body movements that compress and expand the air tubes like bellows.
• An insect in flight has a very high metabolic rate, consuming 10 to 200 times more O2 than it does at rest.
• Alternating contraction and relaxation of flight muscles compresses and expands the body, rapidly pumping air through the tracheal system.
• The flight muscles are packed with mitochondria, and the tracheal tubes supply each with ample oxygen.
• Unlike branching tracheal systems, lungs are restricted to one location.
• Because the respiratory surface of the lung is not in direct contact with all other parts of the body, the circulatory system transports gases between the lungs and the rest of the body.
• Lungs have a dense net of capillaries just under the epithelium that forms the respiratory surface.
• Lungs have evolved in spiders, terrestrial snails, and vertebrates.
• Among the vertebrates, amphibians have relatively small lungs that do not provide a large surface, and many lack lungs altogether.
• They rely heavily on diffusion across other body surfaces, especially their moist skin, for gas exchange.
• In contrast, most reptiles (including all birds) and all mammals rely entirely on lungs for gas exchange.
• Turtles may supplement lung breathing with gas exchange across moist epithelial surfaces in their mouth and anus.
• Lungs and air-breathing have evolved in a few fish species (lungfishes) as adaptations to living in oxygen-poor water or to spending time exposed to air.
• In general, the size and complexity of lungs are correlated with an animal’s metabolic rate (and hence rate of gas exchange).
• For example, the lungs of endotherms have a greater area of exchange surface than the lungs of similar-sized ectotherms.
• Located in the thoracic (chest) cavity, the lungs of mammals have a spongy texture and are honeycombed with a moist epithelium that functions as the respiratory surface.
• A system of branching ducts conveys air to the lungs.
• Air enters through the nostrils and is then filtered by hairs, warmed and humidified, and sampled for odors as it flows through the nasal cavity.
• The nasal cavity leads to the pharynx, an intersection where the paths for air and food cross.
• When food is swallowed, the larynx moves upward and tips the epiglottis over the glottis.
• The rest of the time, the glottis is open, and air enters the upper part of the respiratory tract.
• The wall of the larynx is reinforced by cartilage.
• In most mammals, the larynx is adapted as a voice box in which vibrations of a pair of vocal cords produce sounds.
• These sounds are high-pitched when the vocal cords are stretched tight and vibrate rapidly and low-pitched when the cords are less tense and vibrate slowly.
• From the larynx, air passes into the trachea, or windpipe, whose shape is maintained by rings of cartilage.
• The trachea forks into two bronchi, one leading into each lung.
• Within the lung, each bronchus branches repeatedly into finer and finer tubes, called bronchioles.
• The epithelium lining the major branches of the respiratory tree is covered by cilia and a thin film of mucus.
• The mucus traps dust, pollen, and other particulate contaminants, and the beating cilia move the mucus upward to the pharynx, where it is swallowed.
• At their tips, the tiniest bronchioles dead-end as a cluster of air sacs called alveoli.
• Gas exchange occurs across the thin epithelium of the lung’s millions of alveoli.
• These have a total surface area of about 100 m2 in humans, sufficient to carry out gas exchange for the whole body.
• Oxygen in the air entering the alveoli dissolves in the moist film and rapidly diffuses across the epithelium into a web of capillaries that surrounds each alveolus.
• Carbon dioxide diffuses in the opposite direction.
•  
• Concept 42.6 Breathing ventilates the lungs
• The process of breathing, the alternate inhalation and exhalation of air, ventilates lungs.
• A frog ventilates its lungs by positive pressure breathing.
• During a breathing cycle, muscles lower the floor of the oral cavity, enlarging it and drawing in air through the nostrils.
• With the nostrils and mouth closed, the floor of the oral cavity rises and air is forced down the trachea.
• Elastic recoil of the lungs, together with compression of the muscular body wall, forces air back out of the lungs during exhalation.
• In contrast, mammals ventilate their lungs by negative pressure breathing.
• This works like a suction pump, pulling air instead of pushing it into the lungs.
• Muscle action changes the volume of the rib cage and the chest cavity, and the lungs follow suit.
• The lungs are enclosed by a double-walled sac, with the inner layer of the sac adhering to the outside of the lungs and the outer layer adhering to the wall of the chest cavity.
• A thin space filled with fluid separates the two layers.
• Because of surface tension, the two layers behave like two plates of glass stuck together by the adhesion and cohesion of a film of water.
• The layers can slide smoothly past each other, but they cannot be pulled apart easily.
• Surface tension couples movements of the lungs to movements of the rib cage.
• Lung volume increases as a result of the contraction of the rib muscles and diaphragm, a sheet of skeletal muscle that forms the bottom wall of the chest cavity.
• Contraction of the rib muscles expands the rib cage by pulling the ribs upward and the breastbone outward.
• At the same time, the diaphragm contracts and descends like a piston.
• These changes increase the lung volume, and as a result, air pressure within the alveoli becomes lower than atmospheric pressure.
• Because air flows from higher pressure to lower pressure, air rushes into the respiratory system.
• During exhalation, the rib muscles and diaphragm relax.
• This reduces lung volume and increases air pressure within the alveoli.
• This forces air up the breathing tubes and out through the nostrils.
• Actions of the rib muscles and diaphragm account for changes in lung volume during shallow breathing, when a mammal is at rest.
• During vigorous exercise, other muscles of the neck, back, and chest further increase ventilation volume by raising the rib cage even more.
• In some species, rhythmic movements during running cause visceral organs, including the stomach and liver, to slide forward and backward in the body cavity with each stride.
• This “visceral pump” further increases ventilation volume by adding to the piston-like action of the diaphragm.
• The volume of air an animal inhales and exhales with each breath is called tidal volume.
• It averages about 500 mL in resting humans.
• The maximum tidal volume during forced breathing is the vital capacity, which is about 3.4 L and 4.8 L for college-age females and males, respectively.
• The lungs hold more air than the vital capacity, but some air, the residual volume, remains in the lungs because the alveoli do not completely collapse.
• Since the lungs do not completely empty and refill with each breath cycle, newly inhaled air is mixed with oxygen-depleted residual air.
• Therefore, the maximum oxygen concentration in the alveoli is considerably less than in the atmosphere.
• Although this limits the effectiveness of gas exchange, the carbon dioxide in residual air is critical for regulating the pH of blood and breathing rate in mammals.
• Ventilation is much more complex in birds than in mammals.
• Besides lungs, birds have eight or nine air sacs that do not function directly in gas exchange, but act as bellows that keep air flowing through the lungs.
• The entire system—lungs and air sacs—is ventilated when the bird breathes.
• Air flows through the interconnected system in a circuit that passes through the lungs in one direction only, regardless of whether the bird is inhaling or exhaling.
• Instead of alveoli, which are dead ends, the sites of gas exchange in bird lungs are tiny channels called parabronchi, through which air flows in one direction.
• This system completely exchanges the air in the lungs with every breath.
• Therefore, the maximum lung oxygen concentrations are higher in birds than in mammals.
• Partly because of this efficiency advantage, birds perform much better than mammals at high altitude.
• For example, while human mountaineers experience tremendous difficulty obtaining oxygen when climbing Earth’s highest peaks, several species of birds easily fly over the same mountains during migration at altitudes of 9,000 m or more.

 Control centers in the brain regulate the rate and depth of breathing.

• While we can voluntarily hold our breath or breathe faster and deeper, most of the time autonomic mechanisms regulate our breathing.
• This ensures that the work of the respiratory system is coordinated with that of the cardiovascular system, and with the body’s metabolic demands for gas exchange.
• Our breathing control centers are located in two brain regions, the medulla oblongata and the pons.
• Aided by the control center in the pons, the medulla’s center sets basic breathing rhythm, triggering contraction of the diaphragm and rib muscles.
• A negative-feedback mechanism via stretch receptors prevents our lungs from overexpanding by inhibiting the breathing center in the medulla.
• The medulla’s control center monitors the CO2 level of the blood and regulates breathing activity appropriately.
• Its main cues about CO2 concentration come from slight changes in the pH of the blood and cerebrospinal fluid bathing the brain.
• Carbon dioxide reacts with water to form carbonic acid, which lowers the pH.
• When the control center registers a slight drop in pH, it increases the depth and rate of breathing, and the excess CO2 is eliminated in exhaled air.
• Oxygen concentrations in the blood usually have little effect of the breathing control centers.
• However, when the O2 level is severely depressed—at high altitudes, for example—O2 sensors in the aorta and carotid arteries in the neck send alarm signals to the breathing control centers, which respond by increasing breathing rate.
• Normally, a rise in CO2 concentration is a good indicator of a fall in O2 concentrations because these are linked by the same process, cellular respiration.
• However, deep, rapid breathing (hyperventilation) purges the blood of so much CO2 that the breathing center temporarily ceases sending impulses to the rib muscles and diaphragm.
• The breathing center responds to a variety of nervous and chemical signals and adjusts the rate and depth of breathing to meet the changing demands of the body.
• However, breathing control is only effective if it is coordinated with control of the circulatory system, so that there is a good match between lung ventilation and the amount of blood flowing through alveolar capillaries.
• For example, during exercise, cardiac output is matched to the increased breathing rate, which enhances O2 uptake and CO2 removal as blood flows through the lungs.
•  
• Concept 42.7 Respiratory pigments bind and transport gases

 Gases diffuse down pressure gradients in the lungs and other organs.

• For a gas, whether present in air or dissolved in water, diffusion depends on differences in a quantity called partial pressure, the contribution of a particular gas to the overall total.
• At sea level, the atmosphere exerts a total pressure of 760 mm Hg.
• Since the atmosphere is 21% oxygen (by volume), the partial pressure of oxygen is 0.21 × 760, or about 160 mm Hg.
• The partial pressure of CO2 is only 0.23 mm Hg.
• When water is exposed to air, the amount of a gas that dissolves in water is proportional to its partial pressure in the air and its solubility in water.
• An equilibrium is eventually reached when gas molecules enter and leave the solution at the same rate.
• At this point, the gas is said to have the same partial pressure in the solution as it does in the air.
• Thus, in a glass of water exposed to air at sea-level air pressure, the partial pressure of O2 is 160 mm Hg and the partial pressure of CO2 is 0.23 mm Hg.
• A gas will always diffuse from a region of higher partial pressure to a region of lower partial pressure.
• Blood arriving at the lungs via the pulmonary arteries has a lower partial pressure of O2 and a higher partial pressure of CO2 than the air in the alveoli.
• As blood enters the alveolar capillaries, CO2 diffuses from blood to the air within the alveoli, and oxygen in the alveolar air dissolves in the fluid that coats the epithelium and diffuses across the surface into the blood.
• By the time blood leaves the lungs in the pulmonary veins, its partial pressure of O2 has been raised and its partial pressure of CO2 has been lowered.
• In the tissue capillaries, gradients of partial pressure favor the diffusion of oxygen out of the blood and carbon dioxide into the blood.
• Cellular respiration removes oxygen from and adds carbon dioxide to the interstitial fluid by diffusion.
• After the blood unloads oxygen and loads carbon dioxide, it is returned to the heart and pumped to the lungs again, where it exchanges gases with air in the alveoli.
• The low solubility of oxygen in water is a fundamental problem for animals that rely on the circulatory systems for oxygen delivery.
• For example, a person exercising consumes almost 2 L of O2 per minute, but at normal body temperature and air pressure, only 4.5 mL of O2 can dissolve in a liter of blood in the lungs.
• If 80% of the dissolved O2 were delivered to the tissues (an unrealistically high percentage), the heart would need to pump 500 L of blood per minute—a ton every 2 minutes.
• In fact, most animals transport most of the O2 bound to special proteins called respiratory pigments instead of dissolved in solution.
• Respiratory pigments, often contained within specialized cells, circulate with the blood.
• The presence of respiratory pigments increases the amount of oxygen that can be carried in the blood to about 200 mL of O2 per liter of blood.
• For our exercising individual, the cardiac output would need to be a manageable 12.5 L of blood per minute to meet the oxygen demands of the systemic system.
• A diversity of respiratory pigments has evolved in various animal taxa to support their normal energy metabolism.
• One example, hemocyanin, found in the hemolymph of arthropods and many molluscs, has copper as its oxygen-binding component, coloring the blood bluish.
• The respiratory pigment of almost all vertebrates is the protein hemoglobin, contained within red blood cells.
• Hemoglobin consists of four subunits, each with a cofactor called a heme group that has an iron atom at its center.
• Because iron actually binds the O2, each hemoglobin molecule can carry four molecules of O2.
• Like all respiratory pigments, hemoglobin must bind oxygen reversibly, loading oxygen at the lungs or gills and unloading it in other parts of the body.
• Loading and unloading depend on cooperation among the subunits of the hemoglobin molecule.
• The binding of O2 to one subunit induces the remaining subunits to change their shape slightly such that their affinity for oxygen increases.
• When one subunit releases O2, the other three quickly follow suit as a conformational change lowers their affinity for oxygen.
• Cooperative oxygen binding and release is evident in the dissociation curve for hemoglobin.
• Where the dissociation curve has a steep slope, even a slight change in PO2 causes hemoglobin to load or unload a substantial amount of O2.
• This steep part corresponds to the range of partial pressures found in body tissues.
• Because of the effect of subunit cooperativity, a slight drop in PO2 causes a relatively large increase in the amount of oxygen the blood unloads.
• As in all proteins, hemoglobin’s conformation is sensitive to a variety of factors.
• For example, a drop in pH lowers the affinity of hemoglobin for O2, an effect called the Bohr shift.
• Because CO2 reacts with water to form carbonic acid, an active tissue will lower the pH of its surroundings and induce hemoglobin to release more oxygen.
• In addition to oxygen transport, hemoglobin also helps transport carbon dioxide and assists in buffering blood pH.
• About 7% of the CO2 released by respiring cells is transported in solution.
• Another 23% binds to amino groups of hemoglobin.
• About 70% is transported as bicarbonate ions.
• Carbon dioxide from respiring cells diffuses into the blood plasma and then into red blood cells.
• The CO2 first reacts with water, assisted by the enzyme carbonic anhydrase, to form H2CO3, which then dissociates into a hydrogen ion H+ and a bicarbonate ion (HCO3−)
• Most of the H+ attaches to hemoglobin and other proteins, minimizing the change in blood pH.
• The HCO3− diffuses into the plasma.
• As blood flows through the lungs, the process is rapidly reversed as diffusion of CO2 out of the blood shifts the chemical equilibrium in favor of the conversion of HCO3− to CO2.

 Elite animal athletes have adaptations that allow them to meet extreme oxygen demands.

• The elite animal marathon runner may be the antelope-like pronghorn that has roamed the grasslands of North America for 4 million years.
• Pronghorns can run as fast as 100 km/hr, a speed second only to the cheetah.
• Pronghorns can sustain high speeds over long distances, unlike the cheetah.
• Stan Lindstedt and colleagues at the University of Wyoming and University of Bern explored how pronghorns sustain their combination of great speed and great endurance: through enhancements that supply increased oxygen to muscles, or through greater energetic efficiency?
• Pronghorns consume O2 at a rate three times the rate expected for an animal of their size.
• The rate of O2 consumption per gram of tissue by a pronghorn is the same as a mouse.
• The research team compared various physiological characteristics of pronghorns with similar-sized domestic goats, which are adapted to climbing rather than running.
• The maximum rate of O2 consumption by pronghorns is five times that of goats.
• Why? Pronghorns have a larger surface area for diffusion in the lungs, nearly five times the cardiac output, much higher muscle mass, and a higher volume and density of mitochondria than goats.
• In addition, pronghorns maintain higher muscle temperatures.
• The pronghorn’s extreme O2 consumption rate, which underlies their ability to run at high speeds over long distances, results from enhancements of the normal physiological mechanisms present in other animals.
• These enhancements are the result of natural selection, perhaps exerted by the predators that chased pronghorns on the open plains of North America for millions of years.
• When an air-breathing animal swims underwater, it lacks access to its normal respiratory medium.
• Most humans can hold their breath for only 2 to 3 minutes and swim to depths of 20 m or so.
• However, a variety of seals, sea turtles, and whales can stay submerged for much longer times and reach much greater depths.
• The Weddell seal of Antarctica can plunge to depths of 200–500 m and remain there from 20 minutes to more than an hour.
• Elephant seals can dive to 1,500 m and stay submerged for up to 2 hours.
• One adaptation of these deep-divers, such as the Weddell seal, is an ability to store large amounts of O2 in the tissues.
• Compared to a human, a seal can store about twice as much O2 per kilogram of body weight, mostly in the blood and muscles.
• About 36% of our total O2 is in our lungs, and 51% is in our blood.
• In contrast, the Weddell seal holds only about 5% of its O2 in its small lungs and stockpiles 70% in the blood.
• Several adaptations create these physiological differences between the seal and other deep-divers in comparison to humans.
• First, the seal has about twice the volume of blood per kilogram of body weight as a human.
• Second, the seal can store a large quantity of oxygenated blood in its huge spleen, releasing this blood after the dive begins.
• The spleen can store about 24 L of blood.
• Third, diving mammals have a high concentration of an oxygen-storing protein called myoglobin in their muscles.
• This enables a Weddell seal to store about 25% of its O2 in muscle, compared to only 13% in humans.
• Diving vertebrates not only start a dive with a relatively large O2 stockpile, but they also have adaptations that conserve O2.
• They swim with little muscular effort and often use buoyancy changes to glide passively upward or downward.
• Their heart rate and O2 consumption rate decrease during the dive, and most blood is routed to the brain, spinal cord, eyes, adrenal glands, and placenta (in pregnant seals).
• Blood supply is restricted or even shut off to the muscles, and the muscles can continue to derive ATP from fermentation after their internal O2 stores are depleted.
• During dives of more than 20 minutes, a Weddell seal’s muscles deplete the O2 stored in myoglobin and then derive ATP from fermentation instead of respiration.