Lecture 7 Readings PDF

Transport of Oxygen and Carbon Dioxide in Body Fluids 659 „„The O2 affinities of respiratory pigments are often critical partial pressure regardless of how much CO2 it donates to the for pigment function. When O2 is transferred from one respiratory pig...

Transport of Oxygen and Carbon Dioxide in Body Fluids 659 „„The O2 affinities of respiratory pigments are often critical partial pressure regardless of how much CO2 it donates to the for pigment function. When O2 is transferred from one respiratory pigment to another in an individual animal— solution. From Chapter 22, we know that after the solution comes as when blood hemoglobin donates O2 to myoglobin— to equilibrium with the gas, the concentration of carbon dioxide it is usual for the pigment receiving the O2 to have a in solution in the form of CO2 will be simply proportional to the higher O2 affinity. Comparing related species, those CO2 partial pressure. Thus the amount of CO2 taken up in dissolved with long evolutionary histories in O2-poor environments form by our liter of solution will depend simply on the principles often have evolved blood respiratory pigments with of gas solubility. In contrast, the extent of bicarbonate formation particularly high O2 affinities. is governed, not by the principles of solubility, but by the action „„Respiratory-pigment physiology undergoes acclimation, of compounds that act as buffers of pH. In blood, these are the as by changes in pigment amounts, synthesis of new blood buffers. For our immediate purposes, the function of the molecular forms, or modulation of preexisting forms. blood buffers that deserves emphasis is that, under conditions when the concentration of H+ is being driven upward, they are able to restrain the rise in concentration by removing free H+ Carbon Dioxide Transport ions from solution (we’ll return to a fuller description of buffer Carbon dioxide dissolves in blood as CO2 molecules, but usually only function shortly). a small fraction of the carbon dioxide in blood is present in this chemi- How do blood buffers determine the amount of HCO3 – for- cal form (about 5% in human arterial blood). Thus the first step in mation? A straightforward way to see the answer is to return to understanding carbon dioxide transport is to discuss the other chemi- the analysis of the solution mentioned in the last paragraph and cal forms in which carbon dioxide exists in blood. Because carbon apply the principles of mass action (see page 50) to Equation 24.5. dioxide can be present in multiple chemical forms, not just CO2, we According to the principles of mass action, the following equation must distinguish the material from its exact chemical forms. We do holds true at equilibrium: this by speaking of “carbon dioxide” when we refer to the sum total of _  HCO3   H +  the material in all its chemical forms and by specifying the chemical = K (24.6) form (e.g., CO2) when we refer to a particular form. [CO2 ] When carbon dioxide dissolves in aqueous solutions, it un- where the square brackets signify the concentrations of the various dergoes a series of reactions. The first is hydration to form carbonic chemical entities, and K is a constant. Because [CO2] is a constant acid (H2CO3): at equilibrium in our solution at a given CO2 partial pressure, and because K is also a constant, Equation 24.6 reveals that the CO2 + H2O ~ H2CO3 (24.3) amount of HCO3 – formed per unit volume of solution depends The second is dissociation of the carbonic acid to yield bicarbonate inversely on the H+ concentration. If [H+] is kept relatively low, (HCO3 –) and a proton: [HCO3 –] at equilibrium will be relatively high, meaning that a lot of HCO3 – will be formed as the system approaches equilib- H2CO3 ~ H+ + HCO3− (24.4) rium. However, if [H+] is allowed to rise to high levels, [HCO3 –] Bicarbonate can then dissociate further to yield carbonate (CO32–) at equilibrium will be low, meaning little HCO3 – will be formed. and an additional proton. This final dissociation, however, occurs When carbon dioxide enters our solution from the gas and un- to only a small extent in the body fluids of most animals. Moreover, dergoes the reaction in Equation 24.5, the degree to which the although carbonic acid is an important intermediate compound, H+ made by the reaction is allowed to accumulate, driving [H+] it never accumulates to more than very slight concentrations. For up, is determined by the buffers in the solution. If the buffers most purposes, therefore, the reaction of CO2 with water can be are ineffective, the H+ produced by the reaction will simply ac- viewed simply as yielding HCO3 – and protons: cumulate as free H+ in the solution; thus [H+] will rise rapidly to a high level, and the entire reaction will quickly reach an end CO2 + H2O ~ HCO3− + H+ (24.5) point with little uptake of carbon dioxide and little formation of Equation 24.5 emphasizes that carbon dioxide acts as an acid in HCO3 –. However, if the buffers are highly effective, so that most aqueous systems because it reacts to produce H+; as mentioned H+ is removed from solution as it is formed, [H+] will stay low, earlier, it has been aptly termed a “gaseous acid.” and a great deal of carbon dioxide will be able to undergo reac- tion, causing a large buildup of HCO3 –. The extent of bicarbonate formation Let’s now speak about buffers in more detail. Buffer reactions depends on blood buffers are represented by the general equation Almost no bicarbonate is generated when CO2 is dissolved in dis- HX ~ H+ + X− (24.7) tilled water or a simple salt (NaCl) solution. However, bicarbonate is typically the dominant form in which carbon dioxide exists in where X– is a chemical group or compound that can combine the bloods of animals. How can we explain these two, seemingly reversibly with H+. When H+ is added to a buffered solution, the contradictory, statements? The answer lies in the factors that affect buffer reaction is shifted to the left, removing some of the H+ from bicarbonate formation, which we now examine. free solution (as already stressed). However, if H+ is extracted from Suppose that we bring a liter of an aqueous solution—initially a buffered solution, the reaction shifts to the right, releasing free devoid of carbon dioxide—into contact with a gas that acts as H+ from compound HX. In brief, a buffer reaction acts to stabilize a source of CO2, and that this gas remains at a constant CO2 [H+]. Together, HX and X– are termed a buffer pair. According to  Circulation 669 BOX The Structure and Function of 25.1 Vertebrate Cardiac Muscle Vertebrate cardiac muscle has distinctive traits that suit it for its specialized role. Some Cardiac 0 Membrane potential (mV) muscle cell of these traits are discussed fully in this chapter. Others are discussed in detail in Chap- Neuron ters 12, 13, and 20. Here we briefly note the properties that receive detailed treatment in the other chapters. Structurally, cardiac mus- cle is a type of striated mus- –100 cle, and in this respect it re- 0 300 sembles skeletal muscle (see 0.5 µm Time (ms) Table 20.3). Cardiac muscle Figure A Cardiac muscle cells with interca- Figure B Action potentials is structurally very different lated disc from skeletal muscle, howev- er, in that adjacent muscle cells (muscle potential and contracts, adjacent cells each heartbeat for the heart to pump fibers) are joined by specialized inter- quickly generate action potentials and blood effectively. calated discs (Figure A; see also page contract almost synchronously. Because The cellular process that initiates 561) that impart important mechanical of the control of contraction in this way, each heartbeat is also a property of the and electrical properties. Where two cells all the cells in the wall of a heart cham- cardiac muscle. The pacemaker, which meet at an intercalated disc, they are ber contract together. initiates each beat, is composed of spe- joined together by strong mechanical Individual action potentials in cardiac cialized muscle cells. Although we say adhesions, so that mechanical forces muscle cells are long, drawn-out events much about this topic in this chapter, developed in one cell are transmitted by comparison with those in neurons the electrical properties of the cells that to the other, helping to achieve mutu- (Figure B). During an action potential in compose the pacemaker are discussed ally reinforcing force generation. The two a cardiac muscle cell, depolarization in Chapter 12. As explained there (see cells that meet at an intercalated disc of the cell membrane—which is the im- pages 329–330), the membrane poten- are also joined to each other there by mediately effective stimulus for contrac- tial across the cell membrane in these gap junctions (see Figures 2.7 and 13.2), tion—lasts about 100–500 milliseconds cells does not stay at a stable resting meaning that the cytoplasm of each cell (ms), whereas depolarization in neurons value between beats. Instead, after a is continuous with that of the other cell. typically lasts less than 1 ms. The mecha- heartbeat, the membrane potential At the gap junctions, an action potential nism of the long depolarization is dis- spontaneously drifts in the direction of (wave of cell-membrane depolarization) cussed in Chapter 12 (see Figure 12.23). ever-increasing depolarization. Because in one cell is transmitted electrically— Its function is to ensure that contraction of this drift, the membrane poten- Hill Animal Physiology 4E and therefore very rapidly—to the other is prolonged, rather than being just a Associates Sinauer tial eventually becomes depolarized Hill AnimalThus cell (see pages 338–339). when4E Physiology one brief twitch. Cardiac muscle cells Morales must Studioenough to initiate a new action poten- Sinauer Associates Figure Box 25.01B 02-26-16 cardiac muscle cell generates an action contract for about 100–500 ms during tial, which triggers the next heartbeat. Morales Studio Figure Box 25.01A 03-04-16 other mammals and of birds are similar. The left side of the human vessels to the head, arms, abdomen, and all other body regions, heart, which consists of two chambers—a weakly muscular atrium even the myocardium itself. Passive valves, consisting of flaps of and a strongly muscular ventricle —receives freshly oxygenated connective tissue covered with endothelial tissue, are positioned blood from the lungs and pumps it to the systemic tissues of the between the atrium and ventricle (the left atrioventricular valve) body.2 Blood arrives in the left atrium via the pulmonary veins and between the ventricle and aorta (the aortic valve); these valves that drain the lungs.3 It leaves the left ventricle via a single mas- allow blood to flow freely in the correct direction but prevent it sive artery, the systemic aorta, which branches to send arterial from flowing backward. After blood leaves the systemic aorta, it passes through the systemic circuit—the blood vessels that take 2 The systemic tissues are all the tissues other than the tissues of the blood to and from the systemic tissues—and ultimately returns breathing organs. in the great collecting veins (venae cavae; singular vena cava) 3 By definition, veins are vessels that carry blood toward the heart, and to the heart, where it enters the right atrium and then the right arteries are vessels that carry blood away from the heart. ventricle. The function of the right side of the heart is to pump  670 Chapter 25 Figure 25.1 The human heart A section Systemic aorta through the heart, shown in relation to the at- Pulmonary trunk tached blood vessels. Vessels are colored red if Pulmonary valve Pulmonary artery they carry freshly oxygenated blood and blue if they carry partly deoxygenated blood. To lung To lung Left atrium Pulmonary veins Superior vena cava From 1 Blood that has been oxygenated From lung in the lungs travels to the heart lung in the pulmonary veins and enters the left atrium. Right atrium 4 After passing through the 2 Blood flows through the left systemic circuit, the blood—now atrioventricular valve to enter partly deoxygenated—flows into the left ventricle. the venae cavae, then into the right atrium. 3 The strongly muscular left ventricle pumps the oxygenated 5 Blood flows through the right blood through the aortic valve atrioventricular valve to enter into the systemic aorta, from the right ventricle. which it flows to the entire systemic circuit. Left ventricle Inferior vena cava Aortic valve 6 The right ventricle pumps the deoxygenated blood Myocardium through the pulmonary valve into the pulmonary trunk, from which it flows to the lungs in the pulmonary circuit. blood through the pulmonary circuit—the blood vessels that contraction. The contraction of the ventricle on the fixed volume take blood to and from the lungs. The right ventricle propels of blood within causes the blood pressure inside the ventricle to blood into a large vessel, the pulmonary trunk, which divides to rise rapidly. As soon as the ventricular pressure rises high enough form the pulmonary arteries to the lungs. As in the left heart, to exceed the aortic pressure, the aortic valve flips open, and the passive flap valves prevent backward flow in the right heart; these blood in the ventricle accelerates extremely rapidly, gushing out valves are positioned between the atrium and ventricle (the right into the aorta (thus increasing aortic pressure). The opening of the atrioventricular valve) and between the ventricle and pulmonary aortic valve marks the start of the phase of ventricular ejection. trunk (the pulmonary valve). After blood has been oxygenated Toward the end of this phase, the aortic pressure comes to exceed in the lungs, it returns to the left atrium. the ventricular pressure slightly, but ejection of blood into the aorta continues for a while—at a rapidly falling rate—because of blood The heart as a pump: The action of a heart can momentum. Ultimately, the ventricle starts to relax. The ventricular be analyzed in terms of the physics of pumping pressure then falls rapidly away from the aortic pressure, and the During the beating cycle of any type of heart, the period of con- aortic valve shuts. A period of isovolumetric relaxation follows, traction is called systole (pronounced with a long e: sis-tuh-lee), as ventricular pressure falls with both the inflow and outflow and the period of relaxation is termed diastole (dy-as-tuh-lee). The valves shut. When the ventricular pressure drops below the atrial heart is a pump, and we Hillcan understand its workings Animal Physiology 4E as a pump by pressure, the atrioventricular valve opens inward to the ventricle, Sinauer Associates analyzing pressure, flow, and volume during these periods. Here, and ventricular filling begins. Most filling of the ventricle occurs Morales Studio as an example, we analyze the workings Figure 25.01 of the human left heart 03-08-16 before atrial systole—that is, before the atrial muscle contracts; the (left atrium and ventricle) shown in Figure 25.2. motive force for this filling is the pressure built up by accumulation At the time marked by the arrow at the bottom of Figure 25.2, of pulmonary venous blood in the atrium. When atrial systole oc- ventricular systole begins. Whereas the pressure inside the ventricle curs, it forces some additional blood into the ventricle just before was lower than that inside the atrium during the time just before the next ventricular systole. the arrow, as soon as the ventricle starts to contract (marked by the In thinking of any heart as a pump, its most important attribute arrow), the ventricular pressure rises abruptly to exceed the atrial is the volume of blood it pumps per unit of time, known as the pressure, causing the atrioventricular valve between the chambers cardiac output. (In the case of the mammalian or avian heart, to flip shut. For a brief interval of time (about 0.05 s), however, the the term cardiac output refers specifically to the output of the left ventricular pressure remains below the pressure in the systemic ventricle into the systemic aorta unless stated otherwise.) The cardiac aorta, meaning that the aortic valve is not forced open. During this output is the product of the heart rate and the stroke volume, the interval, therefore, both the inflow and outflow valves of the ventricle volume of blood pumped per heart cycle: are shut. The volume of blood in the ventricle during this time is Cardiac output = heart rate × stroke volume thus constant, and the interval is called the phase of isovolumetric (25.1) (mL/minute) (beats/minute) (mL/beat) contraction (“contraction with unchanging volume”) or isometric 728 Chapter 27 Salinity increases from below 5 g/kg at the top of the Chesapeake Bay… Baltimore The salinity distribution 5 shifts a little as the tide rises and falls each day. 10 Washington When river flow into the Bay is especially great— r as during spring ve rainfall— Ri the Bay becomesk more an A typical estuarine shoreline (Chesapeake Bay) pt dilute oeverywhere. C h 15 Figure 27.4 Salinity trends in an estuary Salinity varies greatly along the length of the Chesapeake Bay, one of the world’s most thoroughly studied estuaries. The map shows a typical pattern of salinity in the water near the surface. Numbers are salinities in grams Pa per kilogram. Each line connects all the surface waters of a particu- tu xe nt lar salinity; therefore, for example, the line for 20 g/kg connects all Rive the places where the salinity of the surface water is 20 g/kg. (Salinity r Po distribution after McHugh 1967.) tom ac Riv er Many of the places where brackish waters occur are classified as estuaries. An estuary is any body of water that is partially surrounded by land and that has inflows of both freshwater and seawater. Estuaries are of great importance in human affairs. They also are among the most interesting aquatic habitats physiologi- Ra cally because of the dramatic variability they often display in their pp ah salinity spatially and temporally—presenting animals with unusual an 20 no challenges because of the great variation in environmental osmotic ck Ri pressure and ion concentrations. Brackish waters are usually defined ve r to have salinities between 0.5 and 30 g/kg—corresponding to Yo r kR osmotic pressures of 15 to 850 mOsm. As illustrated by the Chesa- ive …to over 25 g/kg at peake Bay (Figure 27.4), a single estuary may exhibit almost this r the mouth of the Bay, 270 km (170 miles) entire range of salinities within a distance of 270 km (170 miles). distant. 25 Estuaries are the characteristic habitat of blue crabs, which—when Jam Cape not molting—are effective osmoregulators at brackish salinities. Charles es As a blue crab travels into waters of various salinities in an estuary, Riv 30 its blood osmotic pressure remains almost constant everywhere it er 0 25 miles goes (Figure 27.5). Cape 0 50 km Henry Natural Terrestrial Environments Animals on land are surrounded by a fluid—air—that contains certain ions vary in concentration from one body of freshwater to water only in the gaseous state and, of course, is essentially free of another in ways that are biologically consequential. Calcium (Ca2+) salts. One might guess that terrestrial animals would commonly is particularly noteworthy. Although always dilute, it is distinctly be able to gain water from the air when the humidity of the air is higher in concentration in some bodies of freshwater—termed high. That is not the case, however. For most terrestrial animals “hard”—than in others—termed “soft.” These variations in Ca2+ most of the time, the atmosphere is a sink for water: Animals Hill Animal Physiology 4E concentration can exert substantial effects on the water–salt Sinauer Associates lose water to it by evaporation. Because living in air is inherently physiology of freshwater Figure 27.04 12-18-15 animals by affecting their membrane dehydrating, the study of the water–salt physiology of terrestrial permeabilities and sometimes other functional properties. animals is dominated by the study of water. Where ocean water mixes with freshwater along coastlines, For scientists interested in the water relations of terrestrial waters of intermediate salinity, termed brackish waters, are formed. animals, the world’s deserts are particularly important and in- In these places, both osmotic pressure and ion concentrations in triguing habitats because they present animals with the extremes the water are often highly variable not only from place to place but of terrestrial water stress. Deserts cover substantial areas of also from time to time. Earth’s landmasses (Figure 27.6) and on average are expand-  Water and Salt Physiology 729 ing. Although deserts are often hot, they are not necessarily so, because they may occur at high altitudes or polar latitudes. 1000 Deserts are defined by their dryness, not their temperature. A simple definition of a desert is that it receives less than about 25 Blood osmotic pressure (mOsm) cm (10 inches) of rain or other precipitation per year; we discuss 750 more refined definitions in Chapter 30. To understand the relations of animals to atmospheric water in deserts or other terrestrial habitats, one must study the principles of 500 evaporation, the change of water from a liquid to a gas. Evaporation is a special case of gas diffusion, which (as discussed in Chapter 22) is analyzed using the partial pressures of gases.8 250 Gases always diffuse in net fashion from regions of high partial pressure to regions of low partial pressure. Thus, if a body fluid, or any other aqueous solution, is in contact with the atmosphere, net 0 evaporation occurs if the partial pressure of water in the solution 0 250 500 750 1000 1250 mOsm exceeds that in the atmosphere, and the rate of evaporation increases 0 10 20 30 40 g/kg as the difference in partial pressure increases. Ambient osmotic pressure and salinity What do we mean by the partial pressure of water in a solution and in the atmosphere? Let’s start with the latter. Gaseous water, Figure 27.5 The responses of a resident osmotic regulator to called water vapor, is simply a gas like any other gas. Thus it is a variations in salinity in an estuary Blue crabs (Callinectes sapi- constituent of the atmosphere in the same way that other gases, dus) are abundant throughout the Chesapeake Bay and other similar such as O2, are. The partial pressure of water vapor—often called estuaries along the Atlantic and Gulf coasts of the United States. The the water vapor pressure—in the atmosphere is simply the portion graph shows the average osmotic pressure of the blood plasma of blue of the total atmospheric pressure that is exerted by the water vapor crabs as a function of the osmotic pressure and salinity of the ambient water during nonmolting periods of their lives, when they osmoregulate. 8 If you are unfamiliar with the concept of partial pressure, you should The dashed line is the isosmotic line. (After Kirschner 1991.) review pages 586–588 and Figures 22.1 and 22.3. Some semiarid 3 6 areas may be 4E becoming more desertlike. 1 2 5 Within complexes of desert communities, neighboring subparts Two years sometimes often differ greatly in pass without rain in aridity. hyperarid deserts. 7 5 Fog is almost the 4 only water input in some coastal deserts. Namib Desert beetles posture themselves 1 Sahara Desert 5 Namib Desert so the fog condenses on them. 2 Arabian Desert 6 Sonoran and Chihuahuan Deserts 3 Gobi Desert 7 Atacama Desert Figure 27.6 Deserts occur on all continents Sandy-colored 4 Great Victorian Desert areas are mostly deserts. Animals that have succeeded in living in deserts have had to evolve extreme adaptations to water stress, including extreme abilities to acquire water, conserve the water they have, and tolerate dehydration. 730 Chapter 27 Table 27.2 The saturation water vapor pressure Now we need to ask what is meant by the partial at selected temperatures pressure of water in an aqueous solution. Any particular aqueous solution (e.g., freshwater or seawater), if it is placed The saturation water vapor pressure is independent of other in contact with air in a closed system, will tend to establish gases; it does not depend on the composition of the air. This table also shows the mass of water per unit of volume when a characteristic, equilibrium water vapor pressure in the the saturation water vapor pressure prevails. air. That vapor pressure is the water vapor pressure of the aqueous solution. The water vapor pressure of pure Saturation water Mass of water per unit of Temperature vapor pressure air volume at saturation liquid water depends on the temperature of the water. (°C) (mg H2O/L) Specifically, the water vapor pressure of pure liquid water (mm Hg) (kPa) at a particular temperature is the same as the saturation 0 4.6 0.61 4.9 water vapor pressure of air at the same temperature; Table 10 9.2 1.23 9.4 27.2, therefore, can be used to look up the water vapor pres- 20 17.5 2.33 17.3 sure of pure liquid water. One way to think of the water 30 31.8 4.24 30.4 vapor pressure of liquid water is that it is a measure of the a tendency of the liquid water to inject water vapor into air. 37 47.1 6.28 43.9 Liquid water at 30°C has a much greater tendency to inject 40 55.3 7.37 51.1 water vapor into air than liquid water at 10°C. a Data for 37°C are included because 37°C is the usual deep-body In addition to depending on temperature, the water temperature of humans and other placental mammals. vapor pressure of an aqueous solution depends also on its solute concentration. The water vapor pressure is in fact a colligative property of a solution (see page 122). Raising the present. It can be calculated from the universal gas law (see Equa- concentration of dissolved entities in a solution lowers the water tion 22.1) and is independent of the partial pressures of the other vapor pressure of the solution. This effect is relatively small at the gases in the atmosphere. Humidity is an informal term referring concentrations of most animal body fluids; even a 1-Osm solution loosely to the water content of air. Although several measures of has a water vapor pressure that is 98% as high as that of pure humidity are in common use, the water vapor pressure is the most water. Sometimes, however, the effect of solutes is physiologically 9 useful for physiological analysis. important. For instance, if salt left behind from evaporated sweat Unlike other gases in the atmosphere, water vapor displays an is allowed to accumulate indefinitely on a person’s skin so that upper limit on its partial pressure: The water vapor pressure can newly secreted sweat becomes highly concentrated by dissolving rise only to a certain maximum in air of a particular temperature. accumulated salt, the water vapor pressure of the sweat can be The limit on water vapor pressure is a direct consequence of the reduced to be only 75% of that of pure water—an effect that can fact that (unlike the other atmospheric gases) water can exist as a significantly impair vaporization of the sweat. liquid (not just a gas) under ordinary atmospheric conditions. Air Having now discussed the partial pressure of water both in that has reached its maximum water vapor pressure is said to be air and in aqueous solutions, let’s now turn to the physical laws saturated, and its water vapor pressure is termed the saturation that govern the evaporation of water from terrestrial animals. As water vapor pressure. If a body of air has reached saturation with mentioned earlier, water always diffuses from regions of relatively water vapor, it cannot hold any more water in the gaseous state. high water vapor pressure to regions of lower water vapor pressure. Thus, if water vapor is added to such air from an outside source, the Thus water evaporates from an aqueous solution (e.g., a body fluid) excess water vapor promptly condenses out in the form of liquid if the water vapor pressure of the solution exceeds the water vapor water droplets (e.g., fog forms in this manner). The saturation water pressure of the air next to the solution. The rate of evaporation vapor pressure increases dramatically with the temperature of air, depends on the difference in water vapor pressure. Specifically, as Table 27.2 shows. if J is the net rate of evaporation per unit of solution surface area, To illustrate the immediate significance of these concepts, WVPs is the water vapor pressure of the solution, and WVPa is the consider that we humans exhale air at a temperature greater than water vapor pressure in the air, then 30°C, whereas a toad with a body temperature of 20°C breathes out WVPs − WVPa  air at 20°C. The air exhaled is essentially saturated with water vapor J=K (27.1) in both cases. Warmer air holds more water vapor when saturated, X however (see Table 27.2), and therefore humans lose more water where X is the distance separating WVPs and WVPa, and K is a than toads lose with each liter of air they exhale. proportionality factor. 9 From Equation 27.1, we see that the rate at which an animal In addition to being expressed simply as the water vapor pressure, the humidity of air is often expressed relative to the air’s temperature- loses water by evaporation in a terrestrial environment depends specific saturation water vapor pressure, discussed in the next paragraph. partly on the environmental humidity, expressed as WVPa. As we One expression of this sort is the saturation deficit, which is the difference know from everyday experience, lowering the water vapor pressure between the actual, prevailing water vapor pressure and the saturation water vapor pressure. Another such expression of hutmidity is the relative of the air (lowering the humidity) speeds evaporation if all other humidity, defined to be the ratio of the actual water vapor pressure over the factors are held constant. The rate of evaporation also depends on saturation water vapor pressure. the water vapor pressure of the body fluid from which evaporation  Water and Salt Physiology 731 is occurring, WVPs. Thus, for example, evaporation occurs faster however, because the lizards warm as time passes. As soon as they from body fluids that are warm than from ones that are cooler. If are as warm as the air, they start to lose water by evaporation.10 a mammal or bird—or a lizard basking in the sun—is exposed to the open air in a hot desert, the animal may face exceptional risks Summary of dehydration by evaporation because its body fluids are warm Natural Terrestrial Environments (meaning WVPs is high; see Table 27.2) while simultaneously the desert air is dry (WVPa is low). „„The water vapor pressure of air is the partial pressure A factor that is not immediately evident in Equation 27.1 is that of water vapor in the air and is the most useful expression of humidity for analysis of evaporation and the rate of evaporation also depends on the rate of air movement. condensation. If an animal is standing in still air, evaporation from the animal itself tends to humidify the air immediately next to its skin, creating „„The water vapor pressure of an aqueous solution is the equilibrium water vapor pressure the solution tends to a boundary layer of elevated WVPa near the skin, very similar to create in juxtaposed air if the solution and air are sealed the boundary layer depicted in Figure 5.3. The operative value of in a closed system. WVPa in Equation 27.1 is therefore raised, slowing evaporation. A wind, however, blows water-vapor-laden air away from the skin „„Water vapor diffuses from regions of high water vapor pressure to regions of low water vapor pressure. Thus surface, replacing it with drier air from the open atmosphere, thereby evaporation occurs if the water vapor pressure of an decreasing the WVPa next to the skin and speeding evaporation. aqueous solution exceeds that of the surrounding air. A wind in a desert can be extremely dehydrating. Evaporation takes place at a rate proportional to the In an animal that is losing water by evaporation through its difference in vapor pressure. integument (skin), K in Equation 27.1 is the permeability of the integu- ment to water. A low value of K represents the chief physiological defense that animals can marshal to protect themselves from high Organs of Blood Regulation rates of evaporative desiccation. A low value of K—signifying a Animals living in their natural environments routinely experience low integumentary permeability to water—slows the loss of body conditions that tend to change their blood composition. A mammal water by evaporation. exposed to the dryness of desert air, for example, loses water by To complete our discussion of the diffusion of water vapor in evaporation, and its loss of water tends to raise the osmotic pres- terrestrial animals, let’s return to a point made at the start: Animals sure of its blood, concentrate ions in its blood, and decrease the tend to lose water to the atmosphere, not gain water from it. Put volume of its blood (challenges to osmotic regulation, ionic regu- another way, animals rarely gain water by condensation. lation, and volume regulation). Another example would be a fish When is condensation possible? For most types of animals, that migrates from the lower part of the Chesapeake Bay (where condensation can occur only when the body surface is cooler than the salinity is high) into the upper part (where the salinity is near the air. Under such circumstances—which are uncommon—ani- that of freshwater) (see Figure 27.4). In the dilute water, the fish will mals can function like glasses of iced tea. We are all familiar with take on water at an accelerated rate by osmosis, and the influx of the fact that gaseous water from the atmosphere condenses into water will tend to lower the osmotic pressure of its blood, lower its water droplets on the outside of a cold glass of iced tea on a humid blood ion concentrations, and expand its blood volume, as we saw summer day. This process depends in part on the physics of the earlier. Animals such as mammals and fish are regulators of blood initiation of water-droplet formation, an advanced topic in physical composition and respond to such challenges in negative feedback chemistry. However, after minute (invisibly tiny) water droplets have fashion (see Box 1.1). Certain of their organs act to reverse changes been initiated, their growth follows the principles of water diffusion in blood composition, keeping their blood characteristics stable we are discussing. Water will diffuse from the water vapor in the despite the environmental challenges they encounter. atmosphere into a droplet of liquid water—causing the droplet to Among the organs involved, the kidneys play particularly grow—only if the water vapor pressure in the atmosphere exceeds important roles, not only in mammals and fish but also in most the water vapor pressure of the liquid water in the droplet. A cold other types of animals. Indeed, the most fundamental function of water droplet has a relatively low water vapor pressure (see Table 10 27.2). If the water vapor pressure in the air is simultaneously high, Animals in saturated air could, in principle, gain water at a low rate while at the same temperature as the air, because the solutes in body fluids lower water will move from its gaseous state in the air into the liquid the body-fluid water vapor pressure slightly. However, in reality, when state in the droplet. In this way, minute droplets will grow into animals are in saturated air, their metabolic heat production raises their big, visible droplets. temperature to be at least slightly above air temperature, forcing the water vapor pressure gradient to favor evaporation. Thus, without a cooling For the most part, the body surfaces of animals are not cooler mechanism, water is not gained by diffusion from the air. Of course, than the air, explaining why animals usually lose water by evapora- evaporation of body fluids can cool an animal’s body surfaces. However, tion rather than gaining it by condensation. Occasionally, however, thermodynamics dictates that evaporative cooling cannot be sufficient to cause simultaneous condensation (one process cannot cause evaporation animals in special circumstances have cool body surfaces. For ex- and condensation simultaneously). Thus, for water to be obtained by ample, when lizards that have spent the night in chilly underground diffusion from air because of body cooling, the cooling must be caused by burrows emerge into the open air in the morning, their skin can be some process other than simultaneous evaporation. Some insects, ticks, and other terrestrial arthropods do not follow the physical rules we are cooler than the air; when the air is humid, water droplets can then discussing here and can gain water steadily from atmospheric water vapor; form on their skin and be ingested. Such condensation is transitory, these cases are discussed in Chapter 28 (see page 772). 732 Chapter 27 kidneys is to regulate the composition of the blood plasma by removing < 1, the urine is hyposmotic to the plasma. If U/P > 1, the urine is water, salts, and other solutes from the plasma in controlled ways. Other hyperosmotic to the plasma. The kidneys of an animal typically organs also play major roles in the regulation of blood composition. have control over the U/P ratio and can adjust it within a species- For example, the gills of aquatic animals are typically important specific range. Humans, for example, can have an osmotic U/P ratio organs of blood regulation, and salt glands (discussed in Chapter as high as 4 or as low as 0.1. 28) are important in certain birds, lizards, turtles, and other reptiles. To explore the interpretive value of the U/P ratio, let’s start by In this chapter we emphasize the kidneys, and more specifically considering a freshwater fish. As discussed in Chapter 5 (see Figure we emphasize a conceptual (rather than a mechanistic) understand- 5.19), the body fluids of a freshwater fish have an osmotic pressure ing of their function.11 Two reasons for taking a conceptual approach far higher than the osmotic pressure of freshwater. That is, the are paramount. First, some of the most important concepts of kidney blood plasma is strongly hyperosmotic to freshwater. Suppose that function apply almost universally—providing insights that pertain a fish takes a quantity of pure water—H2O—into its body fluids regardless of the specifics of various kidney types. Second, the by osmosis from the pond or stream in which it lives. This water concepts that apply to kidney function can often be applied as well will dilute the fish’s blood and reduce its plasma osmotic pressure. to other organs (e.g., salt glands) that regulate blood composition. Can the fish restore its original plasma osmotic pressure (i.e., can Kidneys are fluid-processing organs: They start with blood it osmoregulate) by producing urine? A bit of reflection will reveal plasma and produce urine. Many of the effects of kidney function that the answer is yes only if the fish is able to produce urine that is on blood composition can be analyzed by comparing the output more dilute than its plasma—that is, hyposmotic urine (U/P < 1). of the kidneys with their input—that is, by comparing the urine A urine that is hyposmotic to the blood plasma preferentially to the blood plasma. This sort of comparison is usually carried out voids water. By this we mean that, relative to the blood plasma, by use of U/P ratios: ratios of urine (U) composition over plasma urine of this sort is richer in water and poorer in dissolved solutes. (P) composition. Therefore, when the urine is excreted, it disproportionally depletes the blood plasma of water. Because of this preferential removal of The osmotic U/P ratio is an index of the action water from the plasma—and the converse, the preferential reten- of the kidneys in osmotic regulation tion of solutes in the plasma—voiding the urine acts to elevate the The osmotic U/P ratio is the osmotic pressure of the urine divided osmotic pressure of the plasma toward its original level. One way to by the osmotic pressure of the blood plasma. For example, if an see this point is to contrast this outcome with what would happen if animal’s urine osmotic pressure is 150 mOsm and its plasma os- a fish’s urine were always isosmotic to its blood plasma. A fish with motic pressure is 300 mOsm, its osmotic U/P ratio is 0.5. Urine may, a U/P ratio of 1 merely excretes water and solutes in the same ratio in principle, be isosmotic, hyperosmotic, or hyposmotic to the blood at which they exist in its blood plasma. Thus, if the fish’s plasma plasma.12 The osmotic U/P ratio reflects this relative osmoticity were too dilute, it would remain too dilute regardless of how much of the urine. If U/P = 1, the urine is isosmotic to the plasma. If U/P isosmotic urine (U/P = 1) the fish might excrete. 11 The mechanisms of operation and the anatomy of kidneys are discussed From our analysis of the freshwater fish, we arrive at two general in Chapter 29. principles of kidney function (Figure 27.7): 12 If A and B are two solutions and A has a higher osmotic pressure than „„The production of urine isosmotic to blood plasma (U/P = B, then A is hyperosmotic to B, whereas B is hyposmotic to A, as explained 1) cannot serve directly to change the osmotic pressure of in Chapter 5. If two solutions have the same osmotic pressure, they are the plasma or bring about osmotic regulation of the plasma. isosmotic to each other. Implications for excretion U/P ratio Effects on water excretion Effects on solute excretion Effects on composition of blood plasma U/P = 1 Water is excreted in the same Solutes are excreted in the same The formation of urine leaves the ratio of (isosmotic urine) relation to solutes as prevails in relation to water as prevails in solutes to water in the blood plasma the blood plasma. the blood plasma. unchanged, thus does not alter the plasma osmotic pressure. U/P < 1 Water is preferentially excreted. Solutes are preferentially held The ratio of solutes to water in the plasma is (hyposmotic urine) Urine contains more water back from excretion. Urine shifted upward. The osmotic pressure of the relative to solutes than plasma. contains less solutes relative to plasma is raised. water than plasma. U/P > 1 Water is preferentially held back Solutes are preferentially The ratio of solutes to water in the plasma is (hyperosmotic urine) from excretion. Urine contains excreted. Urine contains more shifted downward. The osmotic pressure of less water relative to solutes solutes relative to water than the plasma is lowered. than plasma. plasma. Figure 27.7 The interpretive significance of the osmotic U/P ratio The terms solute and solutes refer to total numbers of osmotically effective dissolved entities.  Water and Salt Physiology 733 „„The production of hyposmotic urine (U/P < 1) aids osmotic The effects of kidney function on ionic regulation of the blood plasma if an animal’s plasma regulation depend on ionic U/P ratios has become too dilute and the plasma osmotic pressure The action of an animal’s kidneys in ionic regulation can be analyzed therefore needs to be raised. in ways closely analogous to the analysis of osmotic regulation (see As one might expect, most freshwater animals have evolved kid- Figure 27.7). For each ion, an ionic U/P ratio can be computed; it is neys that have the capacity to make urine that is hyposmotic to the concentration of that ion in the urine divided by the concen- their plasma. We humans, as well as most other terrestrial animals, tration of the ion in the blood plasma. The sodium U/P ratio, for also have that capacity, which serves us well after an evening of example, is the urine Na+ concentration divided by the plasma Na+ too much iced tea or beer. concentration. To see the interpretive value of an ionic U/P ratio, If the plasma osmotic pressure of an animal has been raised let’s continue with Na+. If the sodium U/P ratio is greater than 1, the to abnormally high levels, urine that is more concentrated than urine contains more Na+ per unit of water volume than the plasma; the plasma must be produced to correct the problem. Such urine thus the excretion of urine preferentially voids Na+ and lowers the preferentially voids solutes (and preferentially retains water), plasma Na+ concentration. Conversely, if the sodium U/P ratio is thereby lowering the ratio of solutes to water in the plasma. From less than 1, the excretion of urine acts to retain Na+ preferentially this analysis, we arrive at a third principle of kidney function (see in the body and raise the plasma Na+ concentration. Figure 27.7): The kidneys can play a role in ionic regulation even when not playing any direct role in osmotic regulation. In this way, the kidneys „„The production of hyperosmotic urine (U/P > 1) aids illustrate that ionic regulation is a distinct concept from osmotic osmotic regulation of the blood plasma if an animal’s regulation. Marine teleost (bony) fish are good examples of animals plasma has become too concentrated and the plasma in which the kidneys participate in ionic regulation but not osmotic osmotic pressure therefore needs to be lowered. regulation. These fish are hyposmotic to the seawater in which they The ability to produce urine that is hyperosmotic to the blood plasma live. Therefore they lose water osmotically to their environment is not nearly as widespread as the ability to produce hyposmotic while simultaneously they gain ions by diffusion from the seawater. urine. The greatest capacities to concentrate the urine are found in Both of these processes tend to raise the osmotic pressure and the mammals, birds, and insects—all primarily terrestrial groups that ion concentrations of their blood plasma. The marine teleost fish frequently face risks of dehydration. produce a urine that is isosmotic to their plasma (osmotic U/P = 1); their urine, therefore, can play no direct role in solving their osmotic The effects of kidney function on regulatory problem. However, their urine differs dramatically from volume regulation depend on the their blood plasma in its solute composition. In particular, the U/P amount of urine produced ratios for Mg2+, SO42–, and Ca2+ are far greater than 1. The excretion The kidneys help regulate the quantity of water in an animal’s of urine by these fish therefore serves the important ionic regula- body—that is, they aid volume regulation—by voiding greater or tory role of keeping down the internal concentrations of these ions, lesser amounts of water as required. We ourselves provide a familiar which the fish tend to gain from the seawater. example: We make a lot of urine after drinking a lot of water, but we make little urine if we are short of water. Summary The kidneys, in fact, can play a critical role in volume regulation Organs of Blood Regulation even when not playing any direct role in osmotic regulation. In this respect, kidney function illustrates that volume regulation and „„The effects of kidney function on the composition of the osmotic regulation are distinct processes, as stressed previously. To blood plasma are analyzed using osmotic and ionic U/P ratios. Figure 27.7 summarizes the interpretation of illustrate these points, let’s consider freshwater crabs. These are U/P ratios. species of true crabs that live in rivers and lakes, mostly in tropi- cal and subtropical parts of the world. Freshwater crabs provide „„Osmotic regulation, volume regulation, and ionic regulation are separable kidney functions in the sense striking examples of animals in which the kidneys participate that the kidneys can participate in volume regulation in volume regulation but not osmotic regulation. The crabs are while simultaneously not aiding osmotic regulation, or dramatically hyperosmotic to the freshwater in which they live they can carry out ionic regulation independently of and thus experience a steady osmotic flux of water into their body osmotic regulation. fluids. To meet this challenge to volume regulation, the crabs produce a substantial flow of urine; each day, their kidneys excrete the same amount of water as they gain by osmosis. However, at least in the species that have been investigated, the kidneys of Food and Drinking Water freshwater crabs are unable to produce urine that is more dilute The specific composition of food and drinking water often has major than the blood plasma. Their urine is always isosmotic to the implications for the water–salt physiology of animals living in their plasma (U/P = 1). Consequently, the production of urine by the natural environments—illustrating once again that physiology and crabs does not alter their plasma osmotic pressure. Although the ecology are intimately related. To start our discussion of this topic, kidneys of freshwater crabs help them dispose of their excess let’s focus on the relative osmoticities of predators and their prey. volume of water, other organs must maintain the high osmotic When one animal captures and eats another, the water–salt pressure of their blood. composition of the prey animal—not just its nutrient content—may 734 Chapter 27 be significant for the predator. Consider, for example, preda- major part of the diet of desert sand rats (Psammomys obesus) and tor–prey relations in the ocean. Marine mammals and teleost fish are consumed in large quantities at times by dromedary camels. are dramatically hyposmotic to seawater. However, most marine The total salt concentration in some halophytes exceeds that of invertebrates are approximately isosmotic to seawater. Recogniz- seawater by as much as 50%. Many of the halophytes are succulent ing these properties, when a mammal or fish consumes a meal of plants with juicy leaves. Animals that eat them obtain considerable invertebrates, the body fluids of its prey are markedly more con- water, but they obtain a large salt load as well. centrated in salts than its own body fluids are. The predator must Analytically, the salt levels of salty plants pose much the same therefore eliminate excess salts to maintain its normal body-fluid problems for animals as the problem posed by salty drinking water. composition. In contrast, consider a mammal or fish that consumes Suppose, for example, that the Na+ concentration in a halophyte’s a meal of fish. In this case, the body fluids of the prey are similar tissues is five times that in mammalian blood plasma. A mammal in salt concentration to those of the predator. Thus the fish-eating would then require kidneys that can produce a sodium U/P ratio predator incurs little or no excess salt load when it eats, in contrast greater than 5 to be able to excrete the Na+ and obtain a net gain of to the large salt load incurred by the invertebrate-eating predator. A H2O from the plants.13 Most mammals cannot achieve such a high fish-eating predator benefits from the work that its prey performed sodium U/P ratio. Sand rats, however, have kidneys with legendary to maintain body fluids more dilute than seawater—an intriguing concentrating abilities. Therefore, after scraping off and discarding lesson in ecological energetics. the saltiest parts of the leaves, sand rats are able to eat halophytes without ill effects from the salt they ingest. Accordingly, they can Salty drinking water may not provide H2O eat foods that other desert rodents must avoid. When animals drink water rich in salts, the water may not serve as a useful source of H 2O. Whether an animal can gain H 2O Air-dried foods contain water by drinking salty water (e.g., seawater) depends on whether Many terrestrial animals consume air-dried seeds or other dry plant the animal can eliminate the salts from the salty water using matter. These air-dried foods contain moisture, even though they less H 2O than was ingested with them. This principle, which are ostensibly dry. The moisture they contain is significant, particu- applies to all animals, is a critical consideration when people larly for animals that live where drinking water is difficult to find. suffering from dehydration are presented with the option of Air-dried foods equilibrate with air moisture. Accordingly, they drinking salty water. vary in their water content as the humidity varies. Whereas “dry” We have all heard Coleridge’s famous line from The Rime of barley grain, for example, contains almost 4 g of water per 100 g the Ancient Mariner, “Water, water, everywhere, nor any drop dry weight at 10% relative humidity, its water content is five times to drink.” Sailors desperate for water discovered long ago that higher at 76% relative humidity. When air-dried plant material is drinking ocean water was worse than drinking no water at all: exposed to an altered air humidity, its moisture content changes Drinking the seawater paradoxically dehydrated them. We now within hours. Two humidity patterns are of importance to animals know that a key consideration in understanding this paradox is in this regard. First, the relative humidity of the air tends to rise that the maximum Cl– concentration that the human kidney can at night, and second, it tends to be higher belowground than produce in the urine is lower than the concentration of Cl– in aboveground. Animals that get water from air-dried food can seawater. Therefore, if people drink seawater, the Cl– they ingest often increase their water intake by feeding at night or by storing can be excreted only by voiding more H 2O than was taken in the food in burrows prior to ingesting it. with the Cl–. That is, such people not only must use all the H 2O ingested with the seawater to excrete the Cl–; they must also draw Protein-rich foods can be dehydrating for on other bodily reserves of H2O, thereby dehydrating their tissues. terrestrial animals Some animals are able to excrete salts at higher concentrations Because carbohydrates and lipids consist primarily of carbon, than humans can and thus are able to gain H2O by drinking salty hydrogen, and oxygen, their oxidation during metabolism results solutions such as seawater (by excreting the salts in less H2O than mostly in formation of CO2 and H2O. The CO2 is exhaled into the was ingested with them). atmosphere, and the H2O contributes to an animal’s water resources. Proteins, by contrast, contain large amounts of nitrogen, and their Plants and algae with salty tissue fluids pose catabolism results in nitrogenous wastes. challenges for herbivores The products of protein catabolism can affect a terrestrial Some plants in terrestrial environments—particularly ones native animal’s water balance when they must be excreted in solution to deserts—have very salty tissue fluids. If herbivores eat such in the urine. In mammals, for example, the principal nitrogenous plants, they receive a substantial salt load along with the food waste is urea, a highly soluble compound voided in the urine. The value of the plants. amount of urinary water required to void urea depends on the The soils in some desert regions are very saline. One reason urea-concentrating ability of an animal’s kidneys. When a mam- for this condition is that salts tend to accumulate over eons of mal is producing urine with as high a urea concentration as it can, time in the places where rain settles in deserts; the evaporation of a high-protein meal often forces the animal to void more water (to rain water leaves the salts it contains behind in the soil, and each get rid of the urea) than a low-protein meal. rainfall adds to the salts left by preceding rainfalls. Plants called halophytes (“salt plants”) root in these saline soils and often have 13 This is just a rough calculation in the case of the plants because the high salt concentrations in their tissue fluids. Such plants form a organic constituents of plants must also be considered.