Chapter 15

Despite exposure to wide fluctuations of environmental temperatures, human body temperature is maintained within a narrow range.23,131 Elevation or depression of body temperature occurs when (1) thermoregulatory mechanisms are overwhelmed by exposure to extremes of environmental heat or cold; (2) endogenous heat production is either inadequate, resulting in hypothermia, or exceeds the physiologic capacity for dissipation, resulting in hyperthermia; or (3) disease processes or xenobiotic effects interfere with normal thermoregulatory responses to heat or cold exposure.

Heat is transferred to or away from the body through radiation, conduction, convection, and evaporation. Radiation involves the transfer of heat from a body to the environment, and from warm objects in the environment, for example, from the sun to a body. Conduction involves the transfer of heat to solid or liquid media in direct contact with the body. Water immersion conducts significant amounts of heat away from the body. This effect facilitates cooling in a swimming pool on a hot summer day, or may lead to hypothermia despite moderate ambient temperatures on a rainy day. The amount of heat lost through conduction and radiation depends on the temperature gradient between skin and surroundings, cutaneous blood flow, and insulation such as subcutaneous fat, hair, clothing, or fur in lower animals.148 In the respiratory tract, heat is lost by conduction to water vapor or gas. In animals unable to sweat, this represents the primary method of heat loss. The amount of heat lost through the respiratory tract depends on the temperature gradient between inspired air and the environment, as well as the rate and depth of breathing.148Convection is the transfer of heat to the air surrounding the body. Wind velocity and ambient air temperature are the major determinants of convective heat loss. Evaporation is the process of vaporization of water, or sweat. Large amounts of heat are dissipated from the skin during this process, resulting in cooling. Ambient temperature, rate of sweating, air velocity, and relative humidity are important factors in determining how much heat is lost through evaporation. On a very humid day, sweat may pour off, rather than evaporate from a person exercising in a hot environment, thereby accomplishing little heat loss. In very warm environments, thermal gradients may be reversed, leading to transfer of heat to the body by radiation, conduction, or convection.164

In the normal human, stimulation of peripheral and hypothalamic temperature-sensitive neurons results in autonomic, somatic, and behavioral responses that lead to the dissipation or conservation of heat. Thermoregulation is the complex physiologic process that serves to maintain hypothalamic temperature within a narrow range of 98.6 ± 0.8°F (37 ± 0.4°C) known as the set point.131 This hypothalamic set point is influenced by factors including diurnal variation and the menstrual cycle. Maintaining, raising, or lowering the set point results in many outwardly visible physiologic manifestations of thermoregulation such as sweating, shivering, flushing, or panting. In the central nervous system, thermosensitive neurons are located ...

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