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PHYSIOLOGY OF TEMPERATURE REGULATION AND HEAT TRANSFER

Healthy humans are able to self-regulate their body temperature within a narrow range (37±0.4°C or 98.6±0.8°F), despite wide variations in ambient temperature. Temperature regulation is performed by the thermoregulatory center in the preoptic anterior hypothalamus, which integrates impulses transmitted by the peripheral warm and cold receptors of the skin and the central thermoreceptors located in the hypothalamus, spinal cord, viscera, and great veins. Normally, the body maintains a balance between the heat generated by the muscles and liver and the heat lost through the skin and lungs. During exercise, there may be as much as a 4-fold increase in heat production. In febrile states, homeostatic mechanisms such as vasoconstriction and shivering raise the body temperature by increasing heat production and decreasing heat dissipation until a new elevated set point is reached. When the illness abates, the set point returns to the original lower level. Lowering of the body temperature is heralded by the onset of sweating.

There are 4 methods of heat transfer to and from the body: conduction, radiation, convection, and evaporation. Conduction helps transfer heat from the body to a solid or liquid medium that is in contact with it, and is the primary mechanism of cooling during ice or water immersion, which, if prolonged, can lead to hypothermia, especially in cold climates. In the lungs, heat is lost by conduction to water vapor and air. Radiation transfers heat from a warmer body to a colder body via electromagnetic waves, and is a principal mechanism of heat elimination in temperate environments. The amount of heat transferred through conduction or radiation is dependent on the temperature gradient between the skin and surroundings, as well as skin blood flow and insulation characteristics (adiposity, hair, clothing). In convection, heat is transferred from the body to the surrounding air, according to the ambient air temperature and wind speed. Evaporation removes heat from the body when sweat (or water that is in contact with the body) vaporizes from its liquid state, and the rate of evaporation is dependent on the relative humidity, ambient air temperature, wind velocity, and degree of sweat production. The evaporation of sweat is a principal mechanism for heat elimination through body heat production or when the ambient temperature rises.

The adaptation to heat stress relies on both a greater rate of sweating at a lower degree of exercise and a lower electrolyte content of sweat. Sweating and subsequent evaporation are very effective mechanisms of temperature control, provided that the environmental humidity is low enough to permit evaporation. This does not occur when the air becomes saturated with water vapor at humidity levels of 90% to 95% and is reduced at humidity levels greater than 75%. In addition, sweating can cause a substantial water and electrolyte loss occurring at up to 1 L/hour/m2 of body surface area.

When the ambient temperature drops, cold-sensitive neurons in the ...

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