Human Responses to Thermal Stress.

The body of a homeotherm may be divided into a core, whose temperature is maintained within narrow limits, regardless of environmental temperature, and a peripheral shell, whose temperature is strongly influenced by the environment. Homeotherms regulate core temperature in order to provide a stable physical-chemical environment for metabolic and other physiological processes. Heat content of the body depends on the balance between metabolic heat production and heat exchange with the environment by convection, radiation, and evaporation which, in humans, is largely evaporation of sweat. Heat exchange by convection and radiation depends on the the temperature difference between skin and environmental temperatures, heat exchange by evaporation of sweat depends on the wetness and temperature of the skin and the ambient water vapor pressure, and both convection and evaporation depend on air movement. Animals control heat exchange behaviorally, through the willed, conscious use of any means available and physiologically, through responses which ordinarily function independently of consciousness. Thermoregulation is the name given to these processes for controlling body heat balance. Physiological means of thermoregulation include control of metabolic heat production, control of skin temperature through skin blood flow, and control of skin wetness through sweating. The physiological control of these responses according to core and skin temperatures, and the effects of various physiological processes--including circadian rhythms, the menstrual cycle, exercise and acclimatization to heat and cold--on the control are discussed. Clothing affects heat by creating a microenvironment between the clothing and the skin. Finally there is a discussion of clinical and pathological issues.