Animals need to maintain body temperature to prevent CELL damage caused by overheating or freezing: heat changes the structure of PROTEINS and kills neurons—even the high temperatures (HYPERPYREXIA) associated with FEVER can sometimes do this, while cold leads to ice formation in LIPID BILAYER MEMBRANE sites. Some species that live in extremely cold conditions have developed intracellular chemical processes (effectively the production of antifreeze molecules) to avoid this. Certain species in HIBERNATION can also withstand body temperatures below freezing, though how they do is not clear. To maintain body temperature, animals either recruit heat from the environment (they are known as poikilothermic or ectothermic—‘cold-blooded’ animals) or they generate heat (THERMOGENESIS) internally (they are endothermic or HOMOIOTHERMIC—‘warm-blooded’ animals). METABOLIC RATE and body size are important determinants of thermoregulation: small animals tend to have higher metabolic rates and to maintain higher body temperatures than larger animals, while low surface-to-volume ratio makes for more efficient thermoregulation.
Both endotherms and ectotherms use a variety of mechanisms to affect body temperature. (1) Altered heat exchange with the environment: increasing BLOOD flow (VASODILATION) of vessels near the body surface (in the SKIN) produces heat loss; VASOCONSTRICTION helps retain heat. A mechanism known as counter-current heat exchange is used by many endotherms—warmer blood in arteries travelling from the body core heats cooler blood in veins returning. In addition many animals have fur or feathers which insulate them: PILOERECTION (adjustment of the position of individual hairs) aids heat loss. (2) Evaporation: water is lost during BREATHING (either normal breathing or panting; dogs for example lose heat in this way) and by perspiration—SWEAT GLANDS in the skin allow evaporation of water which produces heat loss. (3) Behavioural responses: moving to warmer or cooler environments helps regulate temperature. SOCIAL BEHAVIOUR (for example huddling together to share and conserve heat) is also important. The construction of clothing or specialized dwellings (burrows, nests and so on) is also a behavioural adaptation to aid thermoregulation.
MIGRATION, HIBERNATION and AESTIVATION are all behavioural adaptations to enable animals to deal with changed environmental temperatures. All of these processes can be used by endotherms and ectotherms. (4) In addition, endotherms have another mechanism: endogenous heat production. Muscle activity is important in this, as is SHIVERING (which is caused by neural impulses causing muscles to contract desynchronously) a response triggered by the brain’s detection of lowered body temperature. NON-SHIVERING THERMOGENESIS is the production of heat by other endogenous means, principally through the action of BROWN ADIPOSE TISSUE (popularly known as BROWN FAT) and in the LIVER—some 20% of the body’s heat is generated here.
The neural mechanisms involved in thermo-regulation appears to depend on monitoring of blood temperature and skin. Thermoregulatory systems appear to be distributed across a large part of the CENTRAL NERVOUS SYSTEM and to be organized hierarchically: there appear to be mechanisms in the SPINAL CORD, BRAINSTEM and the HYPOTHALAMUS and PREOPTIC AREA, with increasing refinement as one moves higher up the system. As with other sensory systems, the higher level processes are more finely tuned, triggering physiological and behavioural changes in response to a narrower range of changes in temperature. It was once thought that body temperature was maintained to a strict SET POINT and a rather rigid homeostatic manner (see HOMEOSTASIS) but this does not appear to be quite the case: there is more flexibility in temperature regulation than had been supposed: a BOUNDARY MODEL probably provides a better account; see Satinoff (1983) for further discussion of this.
Satinoff E. (1983) A re-evaluation of the concept of homeostatic organization of temperature regulation. In: Handbook of Behavioral Neurobiology, vol. 6, Motivation, ed. E.Satinoff & P.Teitelbaum, pp. 443–472, Plenum Press: New York.
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