Total Heat Exchange with the Environment

To calculate the total heat exchange between the body and the environment, the effects of conduction, radiation, and evaporation can be added by combining Eqs. [2], [4], and [5]:

This equation is particularly useful because it demonstrates the primary determinants of the exchange of heat with the environment that are regulated physiologically. The last term in the equation is the heat loss due to evaporation. As noted earlier, the rate of heat loss by evaporation is inversely related to the relative humidity. The first term on the right combines the heat exchange by conduction and radiation because both are proportional to the difference between the temperature of the environment (the ambient temperature) and that of the skin (Ta - Ts). The constant of proportionality K includes several factors that determine heat exchange by the two processes, including the thickness (L) of insulating layers on the surface of the skin, the thermal conductivities of these layers, and their radiative characteristics and those of the surroundings. Convection will increase both terms in the equation by increasing the evaporation of water (JH2O) and by increasing K because of a decrease in the effective thickness of insulating layers of air and clothing. Convection is normally the greatest contributor to heat loss, especially in the absence of adequate clothing, because it markedly increases both the conductive and evaporative components of heat exchange.

At a given ambient temperature, when other conditions, such as the type of clothing, wind velocity, and relative humidity are constant, Eq. [6] shows that the rate of heat exchange will depend on the temperature of the skin surface (Ts) and the rate of water loss (JH2O). As discussed in the next sections, the temperature of the skin surface is under physiologic control by regulation of cutaneous blood flow, and the loss of water from the body above the normal insensible rate is determined by the rate of sweat production, which is under autonomic control. Thus, the body regulates heat loss or gain with the environment by controlling two variables: the cutaneous blood flow, which determines the skin temperature, and the rate of sweating, which, at a given relative humidity, is directly proportional to the rate of evaporative heat loss.

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