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CHAPTER 44 CONTROLLING THE INTERNAL ENVIRONMENT Most animals can survive environmental fluctuations that are more extreme than any of their individual cells could tolerate. This is possible because mechanisms of homeostasis maintain internal environments within ranges tolerable to body cells. Homeostatic mechanisms include: - Adaptation to the thermal environment (thermoregulation) - Adaptation to the osmotic environment (osmoregulation) - Strategies for the elimination of waste products of protein catabolism (excretion) They are long-term adaptations that evolved in populations facing environmental problems. They include cellular mechanisms and short-term physiological adjustments. I. Regulation of Body Temperature Metabolism and membrane properties are very sensitive to changes in an animal's internal temperature. - Each animal lives in, and is adapted to, an optimal temperature range in which it can maintain a constant internal temperature when external temperatures fluctuate. - Maintaining the body temperature within a range that permits cells to function efficiently is known as thermoregulation. A. Four physical processes account for heat gain or loss An organism exchanges heat with its environment by four physical processes: 1. Conduction is the direct transfer of thermal motion (heat) between molecules of the environment and a body surface. - Heat is always conducted from a body of higher temperature to one of lower temperature. - Water is 50 to 100 times more effective than air in conducting heat. - For example, on a hot day, an animal in cold water cools more rapidly than one on land. 2. Convection is the transfer of heat by the movement of air or liquid past a body surface. - For example, breezes contribute to heat loss from an animal with dry skin. 3. Radiation is the emission of electromagnetic waves produced by all objects warmer than absolute zero. - It can transfer heat between objects not in direct contact. - For example, an animal can be warmed by the heat radiating from the sun. 4. Evaporation is the loss of heat from a liquid's surface that is losing some molecules as gas. - Production of sweat greatly increases evaporative cooling. - Can only occur if surrounding air is not saturated with water molecules Evaporation and convection are the most variable causes of heat loss. Terrestrial animals are affected by all four of the above processes; aquatic animals are impacted minimally by radiation and not at all by change of state. B. Ectotherms derive body heat mainly from their surroundings; endotherms derive it mainly from metabolism Animals may be classified as either ectotherms or endotherms depending on their major source of body heat. Ectotherm
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This note was uploaded on 04/05/2008 for the course BIO G 006 taught by Professor Macneill,a. during the Spring '08 term at Cornell University (Engineering School).

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