Whole Body Cooling Efficiency
Humans dissipate excess body heat mostly by radiation and evaporation from the surface of their mostly hairless bodies. Cooling by evaporation of sweat is the primary means of heat dissipation when environmental temperatures are high, when a person is undertaking strenuous physical activity, or both. Whole-body cooling by evaporation of sweat produced by eccrine sweat glands is an uncommon method of thermoregulation among mammals, but it is observed in all catarrhine primates (Old World monkeys and apes including humans). Modern humans have the highest density of eccrine sweat glands of any mammal. Evaporation of eccrine sweat from the surface of the skin cools the skin itself including the contents of its small blood vessels (capillaries). Blood flowing through the venous capillaries back to the heart from the cooled surface is slightly cooler than core temperature. After the blood is oxygenated in the lungs, it is pumped by the heart to the periphery, including the brain. The mammalian brain is heat-sensitive, and can tolerate only 3-5C elevation above normal body temperature before its functions are seriously impaired. Thus, evaporation of eccrine sweat from the body’s surface helps to keep the brain cool and maintain continuity of normal behaviour and cognition, even under extremes of temperature and muscular exertion. Evolution of a whole-body cooling mechanism characterized by a high density of eccrine sweat glands helped to release an essential thermal constraint on the evolution of increased brain size in the genus Homo.
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