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etiology of cold injuries, these physiological responses may alter the metabolism of persons living and working in cold climates. This chapter reviews the human physiological responses elicited by cold exposure and then considers some factors accounting for differences in response among individuals. The purpose is to provide a basis for considering how physiological responses influence performance and nutritional requirements of soldiers exposed to cold.

HUMAN HEAT BALANCE IN THE COLD

Biophysical Factors

Body temperature reflects the summated effects of internal heart production and heat transfers between the body and ambient environment. The heat balance equation describes the relationship:

where M represents metabolic heat production, and Wk represents energy leaving (positive for concentric work) or entering (negative for eccentric work) the body as external work.2 Heat exchange between the body and environment occurs via evaporation (E), radiation (R), convection (C), and conduction (K), with W/m2 being watts per square meter. The sum of these processes is heat storage (S), which represents heat gain by the body if positive or heat loss from the body if negative. The biophysics of human thermal balance is considered in detail elsewhere (Santee and Gonzalez, 1988).

In humans exposed to environments colder than body temperature, heat flows from the body core toward the environment, primarily via dry (i.e., conductive and convective) heat-loss mechanisms. Wind increases convective heat loss from the body surface (Santee and Gonzalez, 1988), thus providing the basis for the concept of wind chill (Siple and Passel, 1945). Because water has a much higher thermal capacity than air, convective heat transfer is greater (perhaps 70-fold) during immersion in water than in air of the same temperature (Gonzalez, 1988). Clothing provides insulation between the body and the environment, thus limiting convective and conductive heat loss, but wet clothing provides considerably less insulation than dry. Thus, environmental characteristics besides temperature influence the potential for heat loss and the resulting physiological strain of defending body temperature.

2  

During concentric work, the muscle shortens as it develops tension; during eccentric work, the muscle lengthens as it develops tension.



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