Heat production is an essential feature of life. As has already been indicated, the heat loss from the body is a measure of its energy exchange and is a result of the chemical changes which are an essential part of the living process. In the warm – blooded animal this production of heat also serves to maintain the body at a constant temperature (36 – 37⁰ C; 97 – 99⁰ F) which is usually above that of the environment. The maintenance of this constancy is one of the important homeostatic mechanisms of the body. Since the heat production exceeds that which is necessary for maintaining the body temperature, the excess is dissipated by radiation and particularly by the cooling effect of the evaporation of water from the skin. The latter as sweating is particularly important where the temperature of the environment exceeds that of the body.

The regulation of body temperature is brought about by reflex changes involving the sweating mechanism and vasomotor changes which are activated by the stimulation of coetaneous temperature nerve endings and through are regulatory centers in the hypothalamus.

Variations

The body temperature reflects the metabolic rate and is normally subject to only small variations. Thus during sleep and the early hours of the morning, when the basal metabolism is at its minimum, the body temperature likewise is lower then it is during the day. This diurnal variation in body temperature about 3⁰ F In hypothyroidism, old age, cachexia, and other conditions where the metabolic rate is depressed, the body temperature likewise is reduced. On the other hand, in hypothyroidism, leukemia, and other conditions associated with an increased metabolic rate it is elevated. The most notable rise in body temperature occurs in fever secondary to infection, neoplastic processes, or other conditions in which the normal hypothalamic control of the body temperature is set at a higher level.