Humans are homeotherms, i.e. they maintain their central body temperature constant despite variations in environmental temperatures. In warm surroundings heat is dissipated, whereas in a cold environment heat loss is prevented and heat production increases.
Complex mechanisms are involved in thermoregulation. Thermoreceptors are located throughout the skin, although predominantly in the face and hands. In addition, cold thermosensors have been identified in the upper gastrointestinal tract, the tongue, and the respiratory system. In the presence of cold, skin receptors are stimulated and impulses are sent through the dorsal nerve roots to the lateral spinothalamic tracts, the hypothalamus, and the cortex. Cortical stimulation results in a conscious sensation of cold. The hypothalamus receives information from the entire body, senses the arterial blood temperature directly, and co-ordinates mechanisms to maintain temperature. The posterior part of the hypothalamus plays a major role in controlling peripheral vasoconstriction, shivering, and metabolic rate. It also inhibits the anterior part of the hypothalamus, which is sensitive to warmth and is responsible for heat dissipation.
Thus, when humans are in cold surroundings, a conscious perception of this external threat prompts them to put on more clothes, find shelter, increase physical activity, and move elsewhere (behavioral adaptation).
Immediate vasoconstriction due to the activation of both the sympathetic nervous system and local cutaneous reflexes shunts peripheral venous blood to the inner part of the body, thus decreasing heat loss. This central pooling of venous blood beneath warm arterial blood circulating in the opposite direction forms a countercurrent exchange system, thus warming blood returning to the heart.
Heat is lost predominantly in the outer layers of the body through the four physical processes of conduction, convection, radiation, and evaporation. Conduction (transfer of heat by direct contact) is predominant during immersion, since the thermal conductivity of the body in water is 32 times higher than that in air. Convection (transfer of air heated by the body surface) is enhanced by wind, water currents, or large temperature gradients. Radiation (heat transfer by non-particulate means) occurs from unprotected skin surfaces in the presence of large temperature gradients. For example, about 50 per cent of total heat production can be lost from the uncovered head. Evaporation of water occurs primarily on skin and airway surfaces. It may account for a heat loss of 0.6 kcal/g of evaporated water.
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