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CHAPTER 44 REGULATING THE INTERNAL ENVIRONMENT Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section B1: Regulation of Body Temperature 1.Four physical processes account for heat gain or loss 2. Ectotherms have body temperatures close to environmental temperature; endotherms can use metabolic heat to keep body temperature warmer than their surroundings 3. Thermoregulation involves physiological and behavioral adjustments that balance heat gain and loss
Most biochemical and physiological processes are very sensitive to changes in body temperature. –The rates for most enzyme-mediated reactions increase by a factor of 2-3 for every 10 o C temperature increase, until temperature is high enough to denature proteins. –This is known as the Q 10 effect, a measure of the multiple by which a particular enzymatic reaction or overall metabolic process increases with a 10 o C increase in body temperature. –For example, if the rate of glycogen hydrolysis in a frog is 2.5 times greater at 30 o C than at 20 o C, then the Q 10 for that reaction is 2.5. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Because enzymatic reactions and the properties of membranes are strongly influenced by temperature, thermal effects influence animal function and performance. –For example, because the power and speed of a muscle contraction is strongly temperature dependent, a body temperature change of only a few degrees may have a very large impact on an animal’s ability to run, jump, or fly. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Although, different species of animals are adapted to different environmental temperatures, each animal has an optimal temperature range. –Within that range, many animals maintain nearly constant internal temperatures as the external temperature fluctuates. –This thermoregulation helps keep body temperature within a range that enables cells to function most effectively. –An animal that thermoregulates balances its heat budget over time in such a way that the rate of heat gain exactly matches the rate of heat loss. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Any homeostatic control system has three functional components: a receptor, a control center, and an effector. –The receptor detects a change in some variable in the animal’s internal environment, such as a change in temperature. –The control center processes the information it receives from the receptor and directs an appropriate response by the effector. Homeostasis depends on feedback circuits Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
One type of control circuit, a negative-feedback system, can control the temperature in a room. –In this case, the control center, called a thermostat, also contains the receptor, a thermometer. –When room temperature falls, the thermostat switches on the heater, the effector. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 40.9a
An organism, like any object, exchanges heat by four physical processes called conduction, convection, radiation, and evaporation. Four physical processes account for heat gain or loss by an animal to and from surrounding environment Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 44.3
Conduction is the direct transfer of thermal motion (heat) between molecules in direct contact with each other. –For example, a lizard can elevate a low body temperature with heat conducted from a warm rock. –Heat is always conducted from an object of higher temperature to one of lower temperature. –However, the rate and amount of heat transfer varies with different materials. Water is 50 to 100 times more effective than air in conducting heat. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Convection is the transfer of heat by the movement of air or liquid past a surface. –Convection occurs when a breeze contributes to heat loss from the surface of animal with dry skin. –It also occurs when circulating blood moves heat from an animal’s warm body core to the cooler extremities such as legs. –The familiar “wind-chill factor” is an example of how convection compounds the harshness of low temperatures by increasing the rate of heat transfer. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Radiation is the emission of electromagnetic waves by all objects warmer than absolute zero, including an animal’s body, the environment, and the sun. –Radiation can transfer heat between objects that are not in direct contact, as when an animal absorbs heat radiating from the sun. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Evaporation is the removal of heat from the surface of a liquid that is losing some of its molecules as gas. –Evaporation of water from an animal has a strong cooling effect. –However, this can only occur if the surrounding air is not saturated with water molecules (that is, if the relative humidity is less than 100%). –“It’s not the heat, it’s the humidity.” Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Although all animals exchange heat by some combination of the four mechanisms discussed in the previous section, there are important differences in how various species manage their heat budgets. Ectotherms have body temperatures close to environmental temperature; endotherms can use metabolic heat to keep body temperature warmer than their surroundings Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
An ectotherm has such a low basic metabolic rate that the amount of heat that it generates is too small to have much effect on body temperature. –Consequently, ectotherm body temperatures are almost entirely determined by the temperature of the surrounding environment. –Most invertebrates, fishes, amphibians, and reptiles are ectotherms. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In contrast, an endotherm’s high metabolic rate generates enough heat to keep its body temperature substantially warmer than the environment. –Mammals, birds, some fishes, a few reptiles, and numerous insect species are endotherms. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Figure 44.0 Fox in snow Figure 44.4 The relationship between body temperature and ambient (environmental) temperature in an ectotherm and an endotherm
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 44.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 44.8
Our own body temperature is kept close to a set point of 37 o C by the cooperation of several negative-feedback circuits that regulate energy exchange with the environment. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 40.9b
One mechanism by which humans control body temperature involves sweating as a means to dispose of metabolic heat and cool the body. –If the thermostat in the brain detects a rise in the temperature of the blood above the set point, it sends nerve impulses directing sweat glands to increase their production of sweat. This lowers body temperature by evaporative cooling. –When body temperature drops below the set point, the thermostat in the brain stops sending the signals to the glands and the body retains more of the heat produced by metabolism. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 44.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig
For endotherms and for those ectotherms that thermoregulate, the essence of thermoregulation is management of the heat budget so that rates of heat gain are equal to rates of heat loss. –If the heat budget gets out of balance, the animal will either become warmer or colder. Thermoregulation involves physiological and behavioral adjustments that balance heat gain and loss Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
(1) Adjusting the rate of heat exchange between the animal and its surroundings. –Insulation, such as hair, feathers, and fat located just beneath the skin, reduces the flow of heat between an animal and it environment. –Other mechanisms usually involve adaptations of the circulatory system. –Vasodilation, expansion of the diameter of superficial blood vessels, elevates blood flow in the skin and typically increases heat transfer to a cool environment. –Vasoconstriction reduces blood flow and heat transfer by decreasing the diameter of superficial vessels. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Another circulatory adaptation is a special arrangement of blood vessels called a countercurrent heat exchanger that helps trap heat in the body core and reduces heat loss. –For example, marine mammals and many birds living in cold environments face the problem of losing large amounts of heat from their extremities as warm arterial blood flows to the skin. –However, arteries carrying warm blood are in close contact with veins conveying cool blood back toward the trunk. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
–This countercurrent arrangement facilitates heat transfer from arteries to veins along the entire length of the blood vessels. –By the end of the extremity, the arterial blood has cooled far below the core temperature, and the venous blood has warmed close to core temperature as it nears the core. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 44.5
–In essence, heat in the arterial blood emerging from the core is transferred directly to the returning venous blood, instead of being lost to the environment. In some species, blood can either go through the heat exchanger or bypass in other blood vessels. The relative amount of blood that flows through the two different paths varies, adjusting the rate of heat loss as an animal’s physiological state or environment changes. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Circulatory adaptations that reduce heat loss enable some endotherms to survive the most extreme winter conditions. –For example, arctic wolves remain active even when environmental temperatures drop as low as C. –Thick fur coats keep their bodies warm. –By adjusting blood flow through countercurrent exchangers and other vessels in the legs, wolves can keep their foot temperatures just above 0 0 C - cool enough to reduce heat loss but warm enough to prevent frostbite. At the same time, wolves can loose large quantities of heat through their feet after long-distance running. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
(2) Cooling by evaporative heat loss. –Terrestrial animals lose water by evaporation across the skin and when they breathe. –Water absorbs considerable heat when it evaporates. –Some organisms can augment this cooling effect. For example, most mammals and birds can increase evaporation from the lungs by panting. Sweating or bathing to make the skin wet also enhances evaporative cooling. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
(3) Behavioral responses. –Both endotherms and ectotherms use behavioral responses, such as changes in posture or moving about in their environment, to control body temperature. –Many terrestrial animals will bask in the sun or on warm rocks when cold or find cool, shaded, or damp areas when hot. –Many ectotherms can maintain a very constant body temperature by these simple behaviors. –More extreme behavioral adaptations in some animals include hibernation or migration to a more suitable climate. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
( 4) Changing the rate of metabolic heat production. –Many species of birds and mammals can greatly increase their metabolic heat production when exposed to cold. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Hibernation is long-term torpor that evolved as an adaptation to winter cold and food scarcity. –During torpor or hibernation, body temperature declines, perhaps as low as 1-2 o C or even lower. –Because metabolic rates at these temperatures are so low, the energetic demands are tremendously reduced, allowing organisms to survive for long periods of time on energy stored in body tissues or as food cached in a burrow. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig
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