The rate of heat production by a normal 70-kg person can vary from 75 to 80 kcal/hr when sitting still to more than 1400 kcal/hr at maximum rates of exercise. To maintain a constant body temperature, this rate of heat production must be matched by an equal rate of heat loss to the environment. Normally, body temperature regulation is accomplished by physiologic regulation of the rate of heat loss by vasomotor activity in the skin and the rate of sweating, as described earlier. However, the rate of heat production may also be varied to contribute to temperature regulation. Many of these changes in heat production are behavioral. When it is very hot, one naturally reduces one's level of activity and thereby decreases heat production. When it is cold, one can increase heat production by such common behavior as clapping hands and stomping feet.
Physiologically, the most important route of increasing heat production is through shivering. Shivering is an asynchronous contraction of the skeletal muscles resulting in increased muscle tone and tremors that occur at a rate of 10 to 20/s. Shivering is produced by descending neural pathways in the lateral columns of the spinal cord that facilitate the spindle stretch reflex arc of major muscle groups and can increase the rate of heat production up to five or six times normal. Shivering is an involuntary response to a fall in the body core temperature, but it can be reduced or stopped by voluntary pathways.
The rate of heat production also depends on the levels of endocrine hormones. Increases in some hormones can result in increased metabolic heat production, so-called chemical thermogenesis. Acute exposure to cold has been shown to stimulate catecholamine release from the adrenal medulla. Both circulating epinephrine and nore-pinephrine and that released by sympathetic nerve endings increase the metabolic rate by uncoupling oxidative phosphorylation and by releasing fat stores in the body. This mechanism can be activated rapidly, but it appears to be most important in animals having large quantities of brown adipose tissue. Humans, with the exception of infants, have little brown adipose tissue; thus, acute chemical thermogenesis can elevate body heat production by at most 10 to 15%.
In individuals who are constantly exposed to a cold climate, there is a chronic increase in thyroxin production that is accompanied by an increase in the size of the thyroid gland. This may account for an increased incidence of goiters in cold climates. Thyroxine also elevates heat production by uncoupling oxidative phosphorylation in many target tissues and can result in a substantial increase in the basal metabolic rate, but it has not been established how much of the total increase in the metabolic rate is due to this mechanism in the normal adaptation to cold.
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