Physiology Of Temperature Homeostasis

Body temperature may fall as a result of heat loss by conduction, convection, radiation, or evaporation. Conduction is the transfer of heat by direct contact, down a temperature gradient, e.g., from a warm body to the cold environment. Since the thermal conductivity of water is approximately 30 times that of air, the body loses heat rapidly when immersed in water, producing a rapid decline in body temperature.

Convection is the transfer of heat by the actual movement of heated material, e.g., wind disrupting the layer of warm air surrounding the body. Convective heat loss increases in windy conditions, a particular hazard for outdoors enthusiasts.

Heat also may be lost by radiation to the environment (primarily from noninsulated body areas) and by evaporation of water. Evaporation of the water contained in exhaled, water-saturated air occurs over a wide range of ambient temperatures and may be prevented by inhalation of warmed, humidified air.

Opposing the loss of body heat are the mechanisms of heat conservation and gain. In general, these are controlled by the hypothalamus; thus hypothalamic dysfunction may cause an impairment in temperature homeostasis. Heat is conserved by peripheral vasoconstriction and, importantly, by behavioral responses. If behavioral responses such as putting on clothing or coming indoors from a cold environment are impaired for any reason (e.g., drug intoxication or trauma), the risk of hypothermia is increased.

Heat gain is effected by shivering and by "nonshivering thermogenesis." The nonshivering component of heat production consists of an increase in metabolic rate brought about by increased output from the thyroid and adrenal glands.

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