Temperature Regulation
Temperature Regulation
Temperature regulation, or thermoregulation, is the ability of an organism to maintain its body temperature when other temperatures surround it. An organism maintains homeostasis, a steady internal state, only if its body temperature stays within prescribed limits. Cellular activities require an optimum amount of heat.
They depend on enzyme action, and enzymes function within a narrow range of temperature. For this reason, living things can only tolerate a limited rise or drop in temperature from the optimum. Mechanisms exist that regulate body temperature to stay within its survival limits.
Cells require a source of heat for their activities. The heat source can be external or internal. The sun is an external source of heat for living things. Cellular metabolism is an internal heat source. During cellular respiration, the chemical energy in food is converted to high-energy phosphate groups in adenosine triphosphate, ATP. In the process, cells lose about 60% of the released energy as heat. Plants and most animals give up this heat to the environment. Birds, mammals, and some fish, however, make use of some of the heat that metabolism yields.
Organisms exchange heat with the environment by four major physical processes. First, there is conduction, the direct transfer of heat from a warmer to a cooler object. Humans cool off during a swim because the heat of the body is conducted to the cool water. During another process called convection, a breeze passes over a surface and brings about heat loss. This is why a fan cools people on a hot day. Radiation, the emission of electromagnetic waves, transfers heat between objects that are not in direct contact. Animals absorb heat that the sun radiates. Finally, evaporation, the physical change of liquid to gas removes heat from a surface. Sweating and panting are cooling processes. Perspiration evaporates from the skin, and panting increases evaporation from the respiratory system. Both procedures take heat away from the body.
Externally heated animals
Ectotherms are animals that warm their bodies by absorbing heat from their surroundings. In most ectotherms, the body temperature fluctuates with changes in the surrounding temperature. The body temperature of snakes, for example, cools in cold weather and warms up in hot weather. Most marine fish and invertebrates, however, live in water that stays the same temperature. Their body temperature, therefore, does not change. In everyday language, scientists say that these animals are cold-blooded. Amphibians, reptiles, most fish, and invertebrates are considered cold-blooded. This term is misleading, however, because many lizards have high body temperatures. External temperature plays a major role in the activity rate of ectothermic animals. When the weather is warm, they become active. They slow down when the temperature drops.
Certain ectotherm behaviors help regulate body temperature. To warm up, reptiles find sunny places, and stretch out for maximum exposure. If it gets too warm, lizards alternate between sun and shade. Amphibians warm up by moving into the sun or diving into warm water. They cool off by entering the shade. In cold weather, honeybees huddle together to retain heat. Bees and large moths build up heat before takeoff by contracting their flight muscles without moving their wings.
In addition to behaviors, physiological adaptations help ectotherms regulate temperature. Diving reptiles conserve heat because their blood circulates inward toward the body core during a dive. The skin of bullfrogs secretes more mucus when it is hot, allowing more cooling by evaporation. Many ectotherms exist at a lower temperature during torpor, a state of slowed metabolism. This helps them survive a food shortage. If the food supply increases, they come out of torpor in a few hours.
KEY TERMS
Cooling center —Thermoregulating area of the hypothalamus whose impulses result in temperature-lowering activities of various parts of the body.
Ectotherm —A cold-blooded animal, whose internal body temperature is similar to that of its environment. Ectotherms produce little body heat, and are dependent on external sources (such as the sun) to keep their body temperature high enough to function efficiently.
Endotherm —An animal that uses its metabolism as a primary source of body heat and uses physiological mechanisms to hold its body temperature nearly constant.
Heating center —Thermoregulating area of the hypothalamus whose impulses result in temperature-raising activities of various parts of the body.
Hibernation —A type of torpor in which an animal’s body temperature drops, thereby enabling it to withstand prolonged cold and little food.
Internally heated animals
Endotherms are animals that warm their bodies mainly from their own metabolism. Mostly birds and mammals, they maintain a constant body temperature regardless of changes in the surrounding temperature. Scientists commonly call them warm-blooded, because the internal temperature they generate is usually higher than the environmental temperature. Endotherms have certain advantages. Their consistent higher temperature allows them to be active at all times. It also gives them the freedom to live in varied habitats. It helps them survive on land, where the air temperature changes more drastically than water temperature.
Although most fish are ectothermic, mackerals, tuna, swordfish, marlins, and some sharks are endotherms; although in swordfish and some mackeral, warming occurs only in the central nervous system and the retina of the eye. Endothermic fish are more closely related to their ectothermic relatives than they are to each other. Thus, endothermy evolved independently in these different groups, as an adaptation that allowed the animals to expand their habitats into waters of varying temperatures.
Certain adaptations accompany the maintenance of a constant body temperature. A high metabolic rate helps supply heat. An efficient circulatory system conducts and distributes heat around the body. Fat layers, fur, and feathers insulate the body and retain heat. Shivering muscles contract to increase body heat, and hormones such as epinephrine and thyroxin increase the rate of metabolism. Under unfavorable conditions some endotherms experience torpor, for example, hibernation, in which the body temperature drops. Hibernation enables animals to survive long periods of cold and lack of food.
Receptors in various parts of the body, such as the skin, deliver information to the hypothalamus of the brain about the body’s temperature. The hypothalamus acts as a thermostat. It contains two thermoregulating areas that stimulate nerve impulses. One area, the heating center, sends out impulses that raise the body temperature. It causes blood vessels near the surface of the body to contract, thus preventing heat loss. It also causes fur to stand erect and become more insulating. It causes shivering by muscles that produce heat when they contract, and it stimulates hormonal production. The other area, the cooling center, sends out impulses that bring about a temperature drop. It causes surface blood vessels to expand thereby releasing heat. It brings about sweating or panting.
Resources
BOOKS
Burshtein, A.I. Introduction to Thermodynamics and Kinetic Theory of Matter. Weinheim, Germany: Wiley-VCH, 2005.
Germann, William J. Principles of Human Physiology. San Francisco, CA: Pearson Benjamin Cummings, 2005.
Hall, John E., ed. Guyton & Hall Physiology Review and Problems of Human Anatomy and Physiology. Philadelphia, PA: Elsevier Saunders, 2006.
Roy, Bimalendu Naravan. Fundamentals of Classical and Statistical Thermodynamics. West Sussex, UK, and New York: John Wiley, 2002.
Turns, Stephen R. Thermodynamics: Concepts and Applications. New York: Cambridge University Press, 2006.
Van De Graaff, Kent M., and R. Ward Rhees, eds. Human Anatomy and Physiology: Based on Schaum’s Outline of Theory and Problems of Human Anatomy and Physiology. New York: McGraw-Hill, 2001.
Bernice Essenfeld