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CHAPTER 44 - REGULATING THE INTERNALENVIRONMENT Introduction One of the most remarkable characteristics of animals is homeostasis , the ability to maintain physiologically favorable internal environments even as external conditions undergo dramatic shifts that would be lethal to individual cells. For example, humans will survive exposure to substantial changes in outside temperature but will die if their internal temperatures drift more than a few degrees above or below 37 o C. Another mammal, the arctic wolf, can regulate body temperature even in winter when temperatures drop as low as -50 o C. Three ways in which an organism maintains a physiological favorable environment include: Thermoregulation , maintaining body temperature within a tolerable range Osmoregulation , regulating solute balance and the gain and loss of water Excretion , the removal of nitrogen-containing waste products of metabolism such as urea. A. An Overview of Homeostasis 1. Regulating and conforming are the two extremes of how animals cope with environmental fluctuations An animal is said to be a regulator for a particular environmental variable if it uses mechanisms of homeostasis to moderate internal change in the face of external fluctuations. For example, endothermic animals such as mammals and birds are thermoregulators, keeping their body temperatures within narrow limits in spite of changes in environmental temperature. In contrast to regulators, many other animals, especially those that live in relatively stable environments, are conformers in their relationship to certain environmental changes. Such conformers allow some conditions within their bodies to vary with external changes. Many invertebrates, such as spider crabs of the genus Libinia , live in environments where salinity is relatively stable. These organisms do not osmoregulate, and if placed in water of varying salinity, they will lose or gain water to conform to the external environment even when this internal adjustment is extreme enough to cause death. Conforming and regulating represent extremes on a continuum. No organisms are perfect regulators or conformers. For example, salmon, which live part of their lives in fresh water and part in salt water, use osmoregulation to maintain a constant concentration of solutes in their blood and interstitial fluids, while conforming to external temperatures. Even for a particular environmental variable, a species may conform in one situation and regulate in another. Regulation requires the expenditure of energy, and in some environments that cost of regulation may outweigh the benefits of homeostasis. For example, temperature regulation may require a forest-dwelling lizard to travel long distances (and risk capture
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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