Study Guide for Exam 2 updated
Study Guide for Exam 2 updated BIL360
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1 Chapter 10: Thermal Relations Endothermy: animal’s tissues are warmed by its own metabolic production of heat. Ectothermy: external thermal conditions determine an animals body temperature Thermoregulation: maintenance of a relatively constant tissue temperature. Animals of this sort are termed ectotherms because the thermal conditions outside their bodies determine their body temperatures (ecto, “outside”). They are also called poikilotherms because they have variable body temperatures (poikilo, “variable”); their body temperatures are high in warm environments but low in cool ones. Most fish are excellent examples of ectotherms or poikilotherms; their tissues are not warmed metabolically and therefore are at essentially the same temperature as the environmental water in which the fish swim. When a poikilotherm displays thermoregulation and thus falls into the lower left category of our matrix, it does so by behavior: It keeps its tissues at a certain temperature by behaviorally choosing to occupy environments that produce that temperature in its body. Animals that exhibit endothermy—that is, animals that warm their tissues by their production of metabolic heat—are termed endotherms and fall on the right side of the matrix. Although endotherms may or may not be thermoregulators, most in fact exhibit thermoregulation (placing them in the lower right category of the matrix). Mammals and birds are outstanding examples of animals that exhibit both endothermy and thermoregulation. Many mediumsized and large insects, such as the bumblebees we have already discussed, also exhibit both endothermy and thermoregulation (in their flight muscles when they are flying). A homeotherm is an animal that thermoregulates by physiological means (rather than just by behavior). Mammals, birds, and insects such as bumblebees are homeotherms. Under many circumstances, the principal way they thermoregulate is by adjusting how rapidly they produce and retain metabolic heat: They thermoregulate by modulating their endothermy! Temperature is always a major factor in the lives of individual animals Ambient temperature largely determines metabolic rate Animal tissue temperature largely determines functional properties of tissues and constituents Temperature is always a major factor in the lives of individual animals, regardless of the particular thermal relations the animals exhibit. Whether animals are poikilotherms or homeotherms, for example, temperature is universally important in at least two ways: The environmental temperature—also known as ambient temperature—is a principal determinant of an animal’s metabolic rate and therefore the rate at which the animal must acquire food. The temperature of an animal’s tissues plays a principal role in determining the functional properties of the tissues and tissue constituents. For example, tissue temperature affects 2 whether protein molecules are in highperformance or low performance molecular conformations. Tissue temperature also affects the rates of biophysical processes (e.g., diffusion and osmosis), the rates of biochemical reactions, and the viscous physical states of cellular materials such as cell membrane phospholipids. Even for homeotherms, ambient temperature probably is the single most important factor determining geographical range In North America, for example, the northern limits of the winter ranges of birds often correlate well with particular winter temperatures. Eastern phoebes illustrate this pattern. The northern limit of their geographical range in winter corresponds closely with a line that connects all the places where the average minimum air temperature is –4°C Eastern phoebes in winter do not extend northward to a fixed latitude, mountain range, river, or other geographical limit. Instead, they extend northward to a relatively fixed severity of winter cold stress. Where winter nights average warmer than about –4°C, these birds are to be found. Where winter nights average colder than –4°C, they do not occur. Heat Transfer between Animals and their environments A living animal exchanges heat with its surroundings via: Conduction, Convection, Evaporation and Thermal Radiation. Conduction and convection are usefully discussed together because, in a sense, these two mechanisms of heat transfer define each other. What they have in common is that when heat moves through a material substance by either mechanism, the atoms and molecules of the substance participate in the transfer of heat. Conduction is the transfer of heat through a material substance that is macroscopically motionless. A familiar example of conduction is the transfer of heat through a block of copper. Conduction and convection both involve participation of atoms and molecules in the movement of heat. It is heat transfer by flow of material substance. Convection, in sharp contrast, is transfer of heat through a material substance by means of macroscopic motion of the substance. Fluid flow is required for convection. If a wind or water current is present, the macroscopic motion of matter carries heat from place to place. This transfer of heat is convection. It occurs when a material substance is macroscopically motionless. A critical difference between conduction and convection is that, for a given difference of temperature, heat transfer by convection is much faster than that by conduction. Consider, for example, a horizontal surface that is 10°C warmer than the surrounding air. If the air is moving at just 10 miles/hour (4.5 m/s), convection will carry heat away from the surface about 70 times faster than if the air is perfectly still! The acceleration of heat transfer by fluid movement is familiar from everyday experience. We all know, for instance, that a wind greatly increases the thermal stress of a cold day. Evaporation of body water from the respiratory passages or skin of an animal takes heat away from the animal’s body because water absorbs a substantial amount of heat whenever its physical state changes from a liquid to a gas. 3 The amount of heat required to vaporize water, called the latent heat of vaporization, depends on the prevailing temperature. It is 2385–2490 J (570–595 cal) per gram of H O 2 at physiological temperatures. These are large values. Whereas heating a gram of liquid water from 0°C to 100°C requires 100 cal, changing a gram from a liquid to a gas requires 570–595 cal—almost six times as much. The enormous heat absorption that occurs when water vaporizes means that evaporation can be a highly effective cooling mechanism for an animal. The heat is absorbed from the body surface where the vaporization occurs, and it is carried away with the water vapor. Loss of heat is absorbed from surface of evaporation and carried away with water vapor. Thermal radiation One of the dominant mechanisms of heat transfer for terrestrial animals Beams of radiant energy emitted by all objects Objects exchange heat at a distance Wavelengths emitted depends on surface temperature (energy) of emitting object Poikilothermy (ectotherms) Poikilothermy is by far the most common type of thermal relation exhibited by animals. Amphibians, most fish, most nonavian reptiles, all aquatic invertebrates, and most terrestrial invertebrates are poikilotherms. The defining characteristic of poikilothermy is that the animal’s body temperature is determined by equilibration with the thermal conditions of the environment and varies as environmental conditions vary. Poikilothermy and ectothermy are the same thing. The two terms simply emphasize different aspects of one phenomenon; whereas poikilothermy emphasizes the variability of body temperature, ectothermy emphasizes that outside conditions determine the body temperature Poikilotherms may behaviorally thermoregulate, but are limited by thermal opportunities of environment. Body temperature (Tb) determined by external thermal conditions, and varies as environmental conditions vary. Investigators have worried a lot about the question of documenting true behavioral thermoregulation. They thus have compared living animals with inanimate model animals. In one study, living lizards in a natural setting on a Mediterranean island were found to exhibit far less variable body temperatures than lizard models placed widely in the same environment. Such evidence documents that real lizards do not simply position themselves at random, but behave in ways that keep their body temperatures within a relatively narrow preferred range. Eurythermal: wide range of temperatures. For these and other reasons, poikilotherms must typically be thermal generalists: They must be capable of functioning at a variety of different body temperatures. Species differ in how wide a range of body temperatures is acceptable. Some species, termed eurythermal, can function over wide ranges of body temperature; goldfish, for instance, maintain normal body orientation, feed, and swim at 4 body temperatures of 5–30°C. Stenothermal: narrow ranges of body temperature. Other poikilotherms, termed stenothermal, have comparatively narrow ranges of body temperature over which they can function. Poikilotherms – acute responses to changing temperature The acute responses are those that individual animals exhibit promptly after their body temperatures are altered. After that we address the chronic responses A poikilotherm’s resting metabolic rate increases roughly exponentially with body temperature. One simple way to describe an exponential relation between metabolic rate (or any other physiological rate) and temperature is to specify the multiplicative factor by which the rate increases when the body temperature is increased by a standardized increment of 10°C. This factor is called the temperature coefficient, Q 10 Q↓10=R↓T /R↓(T−10) Where R iT the rate at any given body temperature T, and R (T – 10) is the rate at a body temperature 10°C lower than T. To illustrate, if the resting metabolic rate of an animal is 2.2 J/min at a body temperature of 25°C and 1.0 J/min at 15°C, the Q 10 is 2.2. As a rough rule of thumb, the Q for the metabolic rates of poikilotherms is usually between 10 2 and 3. If metabolic rate were a truly exponential function of body temperature, you could calculate the Q of an animal from data for any two body temperatures that are 10 10°C apart and always get the same value. Because metabolic rate is not a truly exponential function of temperature, however, the Q of an animal in fact varies with 10 the particular range of body temperatures considered. Poikilotherms – chronic responses to changing temperature When an animal is kept at a new body temperature for weeks, acute relationship between metabolism and temperature changes and such a change is an example of acclimation. In the experiment, a group of lizards named the “33 degrees Celsius acclimated” group, was maintained for 5 weeks at 33 degrees Celsius. Another group of lizards where maintained at 16 degrees Celsius. Then after after both groups were maintained in these environments for 5 weeks at the end of this chronic exposure, the lizards were exposed acutely to three different body temperatures. In the graphs we see that the lizards that were acclimated to lower temperatures have higher metabolic rates than those that were acclimated to higher temperatures. The acute relationships between metabolism and temperatures is altered when these lizards live at 33 degrees vs. 16 degrees. There are differences between acclimation and acclimatization where acclimation refers to a change in one variable and acclimatization refers to overall changes more natural changes like you see in changes in seasons or environment. Acclimation reduces the effect of body temperature on metabolism. 5 What is the significance of this acclimation response? As the animals cool from a body temperature of 33°C to 16°C during the first hour, their average metabolic rate will decline along the acuteresponse line for 33°Cacclimated animals, following the thin arrows from x to y. Immediately after the lizards have cooled fully to 16°C, their average metabolic rate will be y, the metabolic rate of 33°C acclimated lizards at 16°C. Note that the drop of body temperature causes a profound fall in metabolic rate. What will happen to the average metabolic rate of the lizards during the following 5 weeks at 16°C? The answer is that the metabolic rate will rise from y to z because during those 5 weeks the lizards will become 16°Cacclimated animals! At the end of the 5 weeks, they will have the metabolic rate of 16°Cacclimated animals at 16°C This increase in Metabolic Rate that we see after chronic exposure to this new temperature is what we call compensation. What acclimation is doing is reducing the effect of temperature on the animal’s metabolic rates, so it starts to bring the animals metabolic rate to what it was before. We do have partial compensation after a drop in body temperature, the metabolic rate rises during acclimation but does not return to its original level. In other words, the compensation if partial if the rate returns only partially to its original level. During acclimation, what responses occur in the biochemistry and molecular biology of metabolism? Cells modify their amounts of key, ratelimiting enzymes notably enzymes of the Krebs cycle and the electrontransport chain. During acclimation to cold temperatures, greater amounts of these enzymes are synthesized. Poikilotherms – evolutionary responses to changing temperature. In an evolutionary time scale animals might evolved special adaptations in order to live at particular temp. Dramatic examples are provided by animals that live in different geographical regions. On the graph, we see that the body temperatures at which 19 species of iguanid lizards are able to sprint fastest correlate well with the behaviorally regulated preferred body temperatures of the species. One of the important mechanisms that animals use to become better adapted to different body temperatures is this molecular specialization. Species that have long evolutionary history and different environments will have synthesized different molecular forms, important protein molecules, cell membrane phospholipids and many of the important biological macromolecules that contributes to metabolic function. The final 3D conformation of proteins is maintaining by relatively weak bonds and these are subjects to changes in temperature. The same protein under different temperatures might assume different final 3D structure which will influence the function of that protein. Molecular specialization: Crystallin proteins of eye lenses are found in different molecular forms across related species that are suited to particular thermal conditions of each species. The pictures illustrate the eye lenses of a cow (homeotherms not poikilotherms), a coral reed soldier fish and an Antarctic tooth fish. On the left we see that the lenses of these 6 animals are at their natural environmental temperatures. On the right side illustrates that the lenses of the cow and soldier fish develop cold cataracts which would blind the animals with only shortterm exposure to 0 degrees Celsius. Conversely the Antarctic tooth fish has always the lenses at 2 degrees without undergoing denaturation. In brief, all these vertebrates have crystallin proteins, but they have different molecular forms of the proteins: forms differentially suited to the distinct temperatures at which their eye lenses function. This is a theme that is repeated throughout the study of proteins and other macromolecules. Optimum temperature for sprinting wellcorrelated with preferred body temperature across lizards. Poikilotherms: dealing with freezing temperature If poikilotherms are exposed to temperatures even slightly colder than those necessary to freeze water, they face a threat of freezing. Because of the animal body fluids have lower freezing points than pure water and this is why we put salt on the winter when is snowing because the salt will lower the freezing point of water and helps to get rid of the ice. Nonetheless, animals will still freeze when you go to temperatures below 0 They have Behavioral avoidance when freezing conditions exists and an example of this is when frogs, turtles in the winter they will inhabit the bottom of lakes instead of the surface because the surface freeze but not typically all the way to the bottom. There are other poikilotherms that have to deal with these conditions physiologically. Physiological mechanisms: o Antifreeze compounds: dissolved substances that lower the freezing point of body fluids. o Colligative antifreezes. Some antifreezes lower the freezing point of the body fluids strictly by colligative principles: They affect the freezing point by increasing the total concentration of solutes in the body fluids, not by virtue of their particular chemical properties. The most common of these colligative antifreezes are polyhydric alcohols, especially glycerol, sorbitol, and mannitol. o Noncolligative antifreezes. Some antifreezes lower the freezing point of the body fluids because of specialized chemical properties. Certain proteins and glycoproteins produced by a variety of insects and marine fish are the best understood antifreezes of this sort. They are believed to act by binding (through weak bonds such as hydrogen bonds) to nascent ice crystals in geometrically specific ways, thereby suppressing growth of ice by preventing water molecules from freely joining any crystals that start to form. The noncolligative antifreezes can be quite dilute and yet highly effective because they depress the freezing point hundreds of times more than can be accounted for by simple colligative principles. The noncolligative antifreezes, however, do not depress the melting point any more than colligative principles explain. Thus solutions containing these antifreezes exhibit the unusual property—termed thermal hysteresis—that 7 their freezing points are substantially lower than their melting points. The non colligative antifreezes are usually called thermal hysteresis proteins (THPs) or antifreeze proteins. o Supercooling: Takes place in animate as well as inanimate worlds and animals do not cause themselves to supercool but they can modify their probabilities of spontaneous freezing during supercooling. Altering quality/quantity of ice nucleating agents in their bodies is how animals do this. o Tolerance of freezing: Animals like certain barnacles, mussels and snails actually freeze and survive some tolerate solidification of 6080% of their body water as ice. Allowing extracellular freezing of fluids and is used by species that have evolved to these temperatures and do so by allowing the extracellular freezing of fluids. o For freezingtolerant animals, whereas intracellular freezing is destructive, extracellular freezing is safe and helps prevent intracellular freezing. o Antifreeze and Supercooling are both used to prevent freezing and used by species that if their body would freeze they would die. Homeothermy in mammals and birds Homeothermy, the regulation of body temperature by physiological means, gives mammals and birds a great deal more independence from external thermal conditions than is observed in lizards, frogs, or other poikilotherms. Mammals and birds independently evolved the fullfledged forms of homeothermy they exhibit today. Although the extent of convergence in their physiology of homeothermy is remarkable, they also exhibit consistent differences, one being in their average body temperatures. Placental mammals typically maintain deepbody temperatures averaging about 37°C when they are at rest and not under heat or cold stress. Birds maintain higher temperatures under similar conditions: about 39°C. These temperatures can fluctuate over the course of the day and what this means is that cellular function can be specialized to operate at that particular temperature so if your core temperature is always at 37 degrees then all of your enzymes and proteins and membrane components can be specialized to work during these temperatures which is an advantage of homeothermy. Now the downside of Homeothermy is that is more metabolically expensive than poikilothermy in that you have to generate this internal metabolic heat in order to maintain that constant temperature. The detection of body temperature in a mammal or bird occurs in multiple parts of the body; thermosensitive neurons of importance are found in the skin, spinal cord, and brain, and sometimes also in specialized locations such as the scrotum. So if core temperatures drop below those control centers in the brain will activate mechanisms in the brain to activate heat production in the body and if too high mechanisms that institute heat voiding in the body. These control centers are located in the hypothalamus and the associated preoptic regions 8 of the brain. The resting metabolic rate of a mammal or bird typically varies with ambient temperature, as shown in the graph. Within a certain range of ambient temperatures known as the thermoneutral zone (TNZ), an animal’s resting metabolic rate is independent of ambient temperature and constant. The lowest ambient temperature in the TNZ is termed the lowercritical temperature; the highest is the uppercritical temperature. The lowercritical and uppercritical temperatures depend on the species, and they can also be affected by acclimation or acclimatization. An animal’s basal metabolic rate (BMR) is its metabolic rate when resting and fasting in its thermoneutral zone. Below lowercritical temperature and above uppercritical temperature, metabolic rate increases as a result of physiological work to keep body temperature constant. Beyond that uppercritical temperature, we see an increase in metabolic rate as temp increase and below the lower critical temperature, we see an increase in the metabolic rate as temperature decreases. These increases in metabolic rate in both cold and warm environments arise from the animal’s need to perform physiological work to keep its deepbody temperature constant regardless of whether the ambient temperature is low or high. Shivering and vasoconstriction of cutaneous vascular beds are activated because the set point of the thermo regulatory control system is above body temperature. Sweating and vasodilation of cutaneous vascular beds are activated because the set point is below body temperature The example of the graph is of the whitetailed ptarmigan and this is an animal that is adapted to pretty cold temperatures so we see a pretty dramatic raise of metabolic rate at warmer temperatures. They are pretty well adapted to cold. Dry heat transfer helps us understand this graph of the relationship between metabolism temperature and this concept is “defined to be heat transfer that does not involve the evaporation (or condensation) of water.” Dry heat transfer occurs by conduction, convection and thermal radiation. They can be lumped in this circumstance because, in all three cases, the rate of heat transfer between an animal and its environment tends to increase approximately in proportion to the difference in temperature between the animal’s body and the environment (T – T ). By lumping the three together, we can study dry heat transfer as B A a whole (because dry heat transfer = conduction + convection + thermalradiation heat transfer). In a uniform thermal environment, if factors other than temperature are held constant, Rate of dry heat transfer ∝ T B T A Heat moves out of an animal’s body by dry heat transfer when T exceeds T ; B A 9 conversely, heat moves into the body when T is less than T . The rate of dry heat B A transfer is proportional tB (T A T ) in either case, and tB s (A – T ) can be thought of as being the “driving force” for dry heat transfer. So the bigger that difference the faster the rate of heat transfer is going to be happen. Under such conditions, in which an animal is losing heat to its environment, the only way the animal can maintain a constant body temperature is to make heat metabolically at a rate that matches its rate of heat loss. Accordingly, if M is the animal’s metabolic rate, M must equal the animal’s rate of heat loss. Thus, if we assume that the equation describes the rate of heat loss at ambient temperatures within and below the TNZ, then (at those ambient temperatures) M (T – T ). We can rewrite this expression as an equation by B A introducing a proportionality coefficient (C): M C (T –T ) ∝ B A This equation, which is a famous equation for analyzing a mammal’s or bird’s thermal relations, is called the linear heattransfer equation, also described sometimes as Newton’s law of cooling or Fourier’s law of heat flow. The coefficient C, which is termed the animal’s thermal conductance, is a measure of how readily heat can move by dry heat transfer from an animal’s body into its environment. An animal with a high C can be thought of as having a low resistance to dry heat loss. Conversely, an animal with a low C can be thought of as having a high resistance to dry heat loss. Physiologists, accordingly, define an animal’s resistance to dry heat loss to be the inverse of C: 1/C. The resistance to dry heat loss is often cal ed insulation (I). Thus I = 1/C. The linear heattransfer equation can therefore also be written as: 1 M= (TI−T B A The greater the insulation, the less your MR is going to change. According to this equation metabolic rate is a function of either conductance or insulation and the difference between body temperature and ambient temperature. When T BT →A heat moves out of the body When T BT →A heat moves into the body Modulation of insulation against a background of constant metabolic heat production is the principal means by which a mammal or bird thermoregulates in its thermoneutral zone. As the ambient temperature is lowered in the TNZ and (B – TA) accordingly becomes greater, a mammal or bird responds by increasing its insulation, I. This increase in the animal’s resistance to heat loss counterbalances the increase in the driving force for heat loss, (T – T ), so that the animal’s actual rate of heat loss remains constant (or nearly so). B A The animal’s rate of metabolic heat production, therefore, can also remain constant. In the TNZ, as T decreases and (T – T ) therefore increases, I is increased in a precisely A B A counterbalancing way so that the ratioB T –AT )/I remains constant. The metabolic rate of the animal, M, can therefore be constant. 10 Regulating body temperature in the TNZ: modulating insulation One means of varying insulation is erection or compression of the hairs or feathers. Each hair or feather can be held upright or allowed to lie flat against the skin by the contraction or relaxation of a tiny muscle at its base, under control of the sympathetic nervous system. These responses are termed pilomotor (referring to hairs) responses in mammals and ptilomotor (referring to feathers) responses in birds. If the ambient temperature declines within the TNZ, the hairs or feathers are erected to an increased degree. In this way the pelage or plumage is fluffed out and traps a thicker layer of relatively motionless air around the animal, thereby increasing the resistance to heat transfer through the pelage or plumage. Another mechanism of modulating insulation is the use of vasomotor responses in blood vessels responses that alter the rate of blood flow to the skin surface and other superficial parts of the body. Arterioles supplying superficial vascular beds are constricted by vasoconstriction at cool ambient temperatures because of stimulation by the sympathetic nervous system. This response retards transport of heat to the body surfaces by blood flow, keep body more internal and conserves heat. Conversely, vasodilation at warm ambient temperatures enhances blood transport of heat to body surfaces where the heat is readily lost and released across the body. Insulation may also be modified by postural responses that alter the amount of body surface area directly exposed to ambient conditions. At low ambient temperatures, for example, mammals often curl up, and some birds tuck their heads under their body feathers or squat so as to enclose their legs in their ventral plumage. Many birds hold their wings away from their bodies when ambient temperatures are high. Regulating body temperature below the TNZ: When a mammal or bird is below its lowercritical temperature, it must increase its rate of heat production as the ambient temperature declines. Although all metabolic processes produce heat as a byproduct, mammals and birds have evolved mechanisms, termed thermogenic mechanisms, that are specialized to generate heat for thermoregulation. One of these, shivering, is universal in adult mammals and birds. Shivering: unsynchronized contraction, relaxation of skeletal muscle in highfrequency rhythm. Either mode of contraction uses ATP and liberates heat. When a muscle shivers, the conversion of ATPbond energy to heat becomes the primary function of contraction because no useful mechanical work is accomplished. Nonshivering thermogenesis (NST): placental mammals, young of some birds. In mammals the primary site of NST is brown adipose tissue and this is a specialized type of adipose tissue often reddish brown and under cold norepinephrine is released in BAT and binds to the betaadrenergic receptors in the cell membranes of the BAT cells and it response by releasing these stored lipids which fuels for mitochondrial oxidation. The brown fat oxidizes stored lipids at a high rate, resulting in heat production. We see these in cold acclimated or acclimatized adult mammals as well as in hibernators and in new born individuals. 11 Regional heterothermy: allowing cooling of some tissues while keeping other warm. Appendages such as legs, tails, and ear pinnae present particular thermal challenges when mammals and birds are below thermosneutrality. The appendages are potentially major sites of heat loss because they have a great deal of surface area relative to their sizes, are often thinly covered with fur or feathers, and exhibit (because of their dimensions) intrinsically high rates of convective heat exchange. A mammal or bird can limit heat losses across its appendages in cool environments by allowing the appendage tissues to cool. Allowing the appendage to cool toward ambient temperature reduces this driving force, in effect compensating for the appendage’s relatively low resistance to heat loss. Cooling of the appendages, a type of regional heterothermy, is in fact very common. When the ambient temperature is low, the tissues of appendages—especially their distal parts—are often 10–35°C cooler than tissues in the core parts of an animal’s thorax, abdomen, and head. The example is the opossum’s ear which is very cold to reduced the amount of heat lost and this is done by restricting blood flow and therefore restricting that area of heat. Countercurrent heat exchange: To understand this process, we have to look at the leg of a wolf where in the first figure, we see blood flow without countercurrent heat exchange and the arteries are carrying blood up to the paw and we see that as the blood approaches the paw is losing heat to the external environment and then as the blood returns back to the core of the body it continues to lose heat so we have this very cold blood returning. What’s happening is that we are basically shortcircuiting the flow of heat into the appendage. Now Blood flow with countercurrent heat exchange. As the arteries carry the blood to the paw, there is also a close association with the vein so some of that heat is moving into these and some of the heat lost from the arterial blood enters the venous blood. The temperature of the venous blood thus rises as the blood travels towards the body. Regulating body temperature above the TNZ using nonevaporative mechanisms. Animals adapted to hot, dry environments first use nonevaporative mechanism of cooling as a defense of those high temperatures. Sweating, panting, and other modes of actively increasing the rate of evaporative cooling are so easy to observe when they occur that they are often thought to be the principal or only means by which mammals and birds cope with high environmental or metabolic heat loads. Evaporation, however, has a potentially lethal price: It carries body water away. Although evaporative cooling may solve problems of temperature regulation, it may create problems of water regulation. For many mammals and birds, especially species that have long evolutionary histories in hot, arid climates, active evaporative cooling is in fact a last line of defense against heat loading. Other defenses are marshaled preferentially, and only when these other defenses have done as much as they can is body water used actively to void heat. In this section we discuss the nonevaporative defenses. When these defenses are employed as the preferential or firstline defenses, they act as waterconservation mechanisms. 12 Behavioral defenses are one set of commonly employed nonevaporative defenses. Desert rodents, for instance, construct burrows, which they occupy during the day and most emerge on the desert surface only at night. They thus evade the extremes of heat loading that could occur in deserts. Cycling of body temperature: deep body temperature of camels at night = 34 degrees Celsius and at day = 40 degrees Celsius. A dehydrated dromedary in summer permits its deepbody temperature to fall to 34–35°C overnight and then increase to more than 40°C during each day. Its body temperature therefore cycles up and down by about 6°C. The advantage of such cycling is that it permits some of the heat that enters the body during the intensely hot part of each day to be temporarily stored in the body and later voided by nonevaporative rather than evaporative means. When dawn breaks on a given day, a camel’s body temperature is at its lowest level. As the day warms and the sun beats down on the camel, the animal simply lets heat accumulate in its body, rather than sweating to void the heat, until its body temperature has risen by 6°C. Controlled hyperthermia: Many mammals and birds employ controlled, profound hyperthermia as a principal nonevaporative, water conserving mechanism of coping with hot environments. A high body temperature in and of itself holds advantages for water conservation and a high Tb impedes heat gain from the environment by decreasing the driving force (Ta Tb) that favors heat influx, and thus the high Tb reduces the rate at which body water must be evaporated to void the incoming heat. Among mammal’s profound hyperthermia typically occurs only in species with long evolutionary histories in hot, arid climates like certain antelopes native to the deserts and dry savannas of Africa. Regulating body temperature above the TNZ with Active evaporative cooling Sweating (only in some mammals): fluid secreted to skin surface During sweating, a fluid called sweat is secreted, by way of the ducts of sweat glands, through the epidermis of the skin onto the skin surface where it can evaporate and take with it lost of heat and therefore cools the body down. Gular fluttering (only seen in birds): vibration of gular area which is the floor of the buccal cavity while holding their mouth open and this process increases the evaporation from those moist respiratory membranes by increasing the flow of the air. Panting (mammals and birds): increased breathing rate to increase evaporative cooling because water in the warm moist membrane in the respiratory tract is released into the air so we have evaporation also. Increasing respiratory rate, you will increase this evaporative cooling. 13 1 Introduction to Oxygen and Carbon Dioxide Physiology Chapter 22 Properties of gases in gas mixtures and aqueous solutions The modern study of gases in the gas phase traces back to John Dalton (1766–1844), who articulated the law of partial pressures. According to this concept, the total pressure exerted by a mixture of gases (such as the atmosphere) is the sum of individual pressures exerted by each of the several component gases in the mixture. The individual pressure exerted by any particular gas in a gas mixture is termed the partial pressure of that gas. The diagram shows a container surrounding a body of dry atmospheric air at sea level. Data on the four most abundant constituents of dry air are shown which are Oxygen, Nitrogen, Argon and Carbon dioxide. The air exerts a total pressure of 1 atmosphere, which is the sum of the partial pressures. Each constituent would exert its same partial pressure even if the other constituents were absent. The partial pressure will measure how these gases behave. We are going to be talking about partial pressure with respect when talking about carbon dioxide and how they move into and out of and across the membrane of animals bodies. Gases are going to dissolve, diffuse and react according to their partial pressure and NOT according to their concentration either in gases mixtures or in liquids. The partial pressure of each gas can be calculated from the universal gas law: PV=nRT, where P= volume, V= total volume, n = number of moles of gas, R = universal gas constant and T = absolute temperature. When we are thinking about gases, we are thinking about partial pressures. In order to calculate the partial pressure of a particular gas in a gas mixture, one sets n equal to the molar quantity of the particular gas of interest and V equal to the volume occupied by the gas mixture as a whole. This next is a rearrangement of the equation where we can see that the partial pressure and the concentration of a gas are proportional to each other at some given temperature. P= n RT (V) Gases in aqueous solution When gases dissolve into an aqueous solution, they disappear into that aqueous solution. If there are bubbles, that gas is out of the solution and this is different from when gases are in the solution because they are not visible in any way. The partial pressure of a gas dissolved in an aqueous solution is defined to be equal to the partial pressure of the gas in a gas phase with which the solution is at equilibrium. To illustrate, consider what happens i2 O free water is brought into contact with air containing O2 t a partial pressure of 0.21 atm. Let’s assume, specifically, that the volume of 2 air is so great that as2 dissolves in the water, there is essentially no change in the 2 concentration of the air, and thus the partial pressure of 2 in the air remains 0.21 atm. Oxygen will dissolve in the water until equilibrium is reached with the air. Then the partial pressure of O in the aqueous solution will be 0.21 atm. If this solution is later exposed to air 2 that contains O2 t a partial pressure of 0.19 atm, the solution will lose O2 o the air until a new equilibrium is established. The partial pressure and the concentration of a gas in an aqueous solution are proportional to each other. Henry’ law is the fundamental law that relates partial pressure and concentration in aqueous solutions. C x AP x In gas phases, where C is also proportional to P , the equation relating C and P is x x x x essentially identical for all gases (because all adhere to the universal gas law). In aqueous solutions, however, the proportionality constant A varies a great deal not only from one type of dissolved gas to another, but also from one solution of a particular gas to another, depending on the temperature and salinity of the water. Absorption coefficient, A, varies for different gases (CO2 > O2): The coefficient A, is a measure of gas solubility and having a high value of this signifies high solubility meaning that a lot of gas will dissolve at any particular partial pressure. Different gases will have different values of this absorption coefficient like Carbon dioxide has a higher value of A than Oxygen saying that CO2 is more soluble in aqueous solutions than Oxygen is. Solubility of gases decrease with increases temperature and this goes with all gases and this goes with the example of the soda when it gets warmer all of that CO2 leaves and you will have coca cola that is no longer carbonated. So as temperature increases, the solubility of gases decreases. solubilities of gases in aqueous solutions decrease with increasing salinity. Increasing The the salinity of an aqueous solution tends to drive gases out of solution by decreasing the solubilities of the gases, a phenomenon called the saltingout effect. Diffusion of gases Gases diffuse from areas of high partial pressure to areas of low partial pressure and this is true for gas mixtures, gases with an aqueous solution and across gaswater interfaces. Gases diffuse more readily through gas phases (like air) than though aqueous solutions like water and this is in particularly important for animals who buried their eggs in soil or sand. An interesting application of these principles is provided by analyzing the O supply to mice 2 3 in underground burrows, or the O s2 ply to eggs of sea turtles buried in beach sand. If the soil or sand is dry and porous, O2 s often supplied chiefly or entirely by diffusion through the soil or sand, that is, diffusion through the network of minute gasfilled spaces among the soil or sand particles. If the porous air spaces among the soil or sand are filled with water and this provokes that the rate of oxygen diffusion drops by a factor of about 200000 and the consequences can be drastic for underground animals or eggs. On the graph For the first 50 days, when the sand was mostly dry, the O p2 tial pressure in the nest was high but the O 2 rtial pressure fell to zero for 2 days when the sand became temporarily saturated with water. Most eggs were dead when hatching occurred 10 days later. Diffusion through water can meet the O r2 uirements of living tissues only if the distances to be covered are about 1 millimeter or less! This rule has many important applications. A dramatic application concerns the consequences of liquid accumulation in a person’s lungs: Just a small accumulation of body fluids in the terminal air spaces of a person’s lungs immediately creates a dire medical emergency because of the small diffusion distance that is tolerable with water present. The difference between air and water in the ease of diffusion of CO is less than that for O , 2 2 but still substantial. 1 Chapter 24 Continuation The Oxygenbinding characteristic of respiratory pigments To understand the function of respiratory pigments, we need to understand the oxygen equilibrium curve. The oxygen equilibrium curve: the respiratory pigment is saturated when O2 partial pressure high enough that all binding sites are oxygenated. In the graph we see the partial pressure of O2 in blood on the x axis and the percentage of heme groups oxygenated on the y axis (to how many of the binding sites of the respiratory pigments are bound to oxygen) Carrying capacity: amount of O2 carried per unit of volume at saturation (e.g. human blood 20 ml O2/100 ml so when all of the biding sites for oxygen of the hemoglobin in the blood are saturated, the blood is carrying 20 ml of oxygen/ 100 ml of blood. We can also show the oxygen equilibrium curve as blood oxygen concentration as a function of oxygen partial pressure. On the another graph, we have again on the x axis the partial pressure of oxygen in blood but on the y axis we have the mL of oxygen per 100mL of blood. If we know the carrying capacity of blood, we can move between these two different graphs very easily. In this second one instead of having percentages saturation, we are converting into actual volumes of oxygen that are in the blood using that knowledge of the carrying capacity at saturation. The transport of oxygen in the human blood using a oxygen equilibrium curve the O 2 rtial pressure in the alveolar gases of our lungs at about 13.3 kPa (100 mm Hg) the highest. The blood arriving at the alveoli has a lower partial pressure of oxygen because we have to have that diffusion gradient of a lower partial pressure so oxygen diffuses from the alveoli into the blood. So the blood that’s leaving the lungs, that is picking up oxygen has a slightly lower partial pressure of oxygen around 12.012.7 kPa. This slight difference doesn’t really affect the oxygen content because we really have a plateau at this point on the oxygen equilibrium curve. Now, the saturation partial pressure of hemoglobin is very well matched to this alveolar partial pressure, so this area were we get the plateau on the curve is very close to the oxygen partial pressure that we see at the lungs. Here at this point of the partial pressure of oxygen at the lungs, we see that the blood becomes saturated with oxygen and its reaching its carrying capacity. After oxygenated blood leaves the lungs, its going to flow to the left side of the heart and be pump out to the nonrespiratory tissues of the bodies (muscle, other organs, etc.) In these tissues, mitochondria are using up oxygen, they are combining it with electron and protons to form water so in these systemic tissues mitochondria are removing oxygen from solution and the oxygen partial pressure near mitochondria in cells, in the tissues goes down and this is going to favor oxygen moving from the blood into these systemic tissues because they necessarily are going to have lower partial pressure of oxygen because they are using up oxygen 2 and taking it out of the solution. Oxygen partial pressure is going down in the systemic tissues is going to cause the partial pressure of oxygen in the blood to fall as the blood is giving up oxygen to these tissues. We can measure how much oxygen is released by the blood into the tissues by looking at how much of a difference we see on that y axis. The arrows are representing how much oxygen volume is released to the tissues after leaves the lungs. When people are at rest, the oxygen partial pressure of this venous blood is about 5.3kPa so it contains less oxygen about 5mL of oxygen /100 mL of blood released to the tissues dropping the oxygen content of the blood by about 5mL/100mL of blood. The blood release of oxygen from the blood is often expressed as the blood oxygen utilization coefficient and is the percentage of the arterial oxygen that is released to the systemic tissues. If we are thinking of 5mL of oxygen/ 100 mL of blood that it has been released 5mL at the carrying capacity of 20mL and that’s around 25% of oxygen utilization. During exercise, there is a much higher utilization coefficient. The venous partial pressure of oxygen drops much lower so in the example of the graph we see a drop of 2.7 kPa. At this point in this oxygen equilibrium curve, the oxygen content on venous blood that is returning back towards the heart and the lungs from tissues during exercise is going to be much more deprived of oxygen. The oxygen utilization for skeletons at work is around 65% or higher. As the partial pressure of the oxygen decreases, we see lower concentrations of the oxygen in the blood representing the oxygen been released to the systemic tissues to a greater extend when they are being very active on using that oxygen during exercise. Systemic tissues during exercise are in the very steep portion of the curve and this has to do with cooperativity and the binding site of hemoglobin. The shape of the oxygen equilibrium curve depends on O2binding site cooperativity Blood hemoglobin transports respiratory pigments in the blood, carrying oxygen through the blood and delivers it to other tissues. We also have the oxygen equilibrium curve for myoglobin, which is another molecular form of hemoglobin, but it differs in its molecular structure because myoglobin has a single subunit while hemoglobin has 4. Myoglobin contains a single O2binding site because it contains only one single subunit, each binding site functions independently of one another. As the partial pressure of oxygen increases, more myoglobin is going to become bound to that oxygen until all of the myoglobin molecules are saturated with O2. As PO 2 increases, we have greater thermodynamic activity of oxygen and until myoglobin approaches saturation. It has a Hyperbolic relationship Vertebrate blood hemoglobins have 4 O2binding sites within each molecule of the 4 subunits, which show cooperativity and this means that the binding of oxygen at 1 or 2 of the sites enhances affinity of remaining sites for O2. Partially oxygenated molecules is more likely than a deoxygenated one to bind additional molecules of oxygen. The curve is a Sigmoidal relationship. 3 Cooperativity affects oxygenation and deoxygenation of respiratory pigments. During?
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