Key Points

• The normal body core temperature (about 37°C, 98.6°F) is maintained despite wide variations in environmental temperature and with changes in metabolic heat production. Body temperature has a diurnal variation of about 0.6°C, being lowest in the morning and highest in the late afternoon. With ovulation, the body temperature rises 0.2 to 0.5°C and remains elevated in the second half of the menstrual cycle. The body temperature may rise 2 to 3°C with heavy exercise.

• Heat is transferred to or from the body by the processes of radiation, conduction, and evaporation. Convection enhances the latter two mechanisms. The heat transferred by conduction and radiation is proportional to the difference between the temperature of the skin and the surroundings, thus either mechanism can result in heat loss or gain. Evaporation is solely a heat loss mechanism, resulting in 580 kcal heat loss per 1 L of water evaporated from the skin surface.

• Heat production varies from about 80 kcal/hr at rest to more than 1400 kcal/hr with maximal exertion. Heat production may be increased by shivering or thyroid hormone and catechol-amines, which accelerate metabolism.

• Cutaneous blood flow is regulated in the range of 0 to 30% of the cardiac output in order to match body heat loss to heat production. The resulting change in skin temperature regulates the rate of heat exchange by conduction and

Essential Medical Physiology, Third Edition

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