Pathophysiology

Body temperature is dependent on the balance between heat production from metabolism and heat loss (or gain) to the environment. The heat-balance equation is

+ [+ conduction + radiation + convection - evaporation]

Through the four mechanisms of radiation, convection, conduction, and evaporation, the body is able to maintain core temperature within a very narrow range. Radiation, conduction, and convection are considered sensible or "dry" heat-exchange mechanisms. Heat can be transferred in either direction by these three methods: heat can be gained or lost, depending on the environment and the body. Radiation, which is heat transferred by electromagnetic waves, is the primary mechanism of heat loss when the air temperature is lower than body temperature, accounting for 65 percent of cooling in such an environment. Convection is heat exchange between a surface and a medium, usually the air water molecules circulating around the body, and is usually a minor mechanism of heat loss, accounting for only 10 to 15 percent. Once environmental air temperature exceeds body temperature, however, convection can be a source of heat gain. Convection is greatly affected by winds: as wind speed increases, the amount of heat loss through convection also increases. Conduction, which is heat exchange between two surfaces in direct contact with each other, accounts for only 2 percent of heat loss under normal circumstances. However, the conductivity of water is 25 percent times greater than air; thus, heat can be lost very quickly with immersion in cool water.

Evaporation is considered insensible or "wet" heat exchange. Unlike the "dry" or sensible heat exchanges in which heat can be gained or lost, evaporation can only result in heat loss. Evaporation is the conversion of liquid to a gaseous phase at the expense of energy. Humans primarily disperse heat by evaporation through the mechanism of sweating; the respiratory component of evaporation does not contribute significantly to heat loss. Sweating can dissipate a tremendous amount of heat, and in an environment in which the air temperature is greater than the body temperature, sensible exchange mechanisms cease to become a source of heat gain, and evaporation becomes the only means of heat dispersal. Sweat that drips to the ground or is wiped off does not result in significant heat loss. Actual evaporation must take place for heat loss to occur. Conditions of high humidity block evaporation, and conditions of both high humidity and high ambient temperatures are particularly hazardous.

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