Beneficial and Detrimental Effects of Thermal Adaptation

Abstract

The heat balance of the human body depends on the one hand on the amount of energy produced by metabolism, mainly in organs and muscles, and on the other hand on the amount of heat transferred to or from the environment. In basic conditions (70... 100 W metabolic heat production) and in a thermoneutral environment (around 22°C, low humidity and wind speed, light clothing) the heat balance of the body will yield a mean body temperature between 36 and 37°C. The body as a thermal system without activated thermoregulatory control, described by the physical laws of heat transfer, is generally called the “passive (thermal) system” (see Fig. 1), because it would passively accept thermal changes induced by the environment (external cold/heat) and by muscle activity (endogeneous heat). No optimal functioning of the internal processes around a core temperature of 37°C, not even survival, could be expected, because in a “passive system” the thermoregulatory effector mechanisms, metabolic heat production, sweat production and vasomotor activity, would not be adjusted to thermal needs of the body. Fortunately, body temperatures are sensed by spatially distributed thermal receptors and fed back to parts of the central nervous system which may be called the “active (thermoregulatory) system” (see Fig. 1), because it activates effector mechanisms reducing mean body temperature deviations: mainly vasoconstriction and shivering as cold defense mechanisms or vasodilatation and evaporative heat loss (in humans via sweat production) as heat defense. The neuronal outputs of the active system are transformed into heat gain or heat loss mechanisms as inputs of the passive system compensating, at least partially, the “disturbing” inputs from environment and muscle exercise. This is achieved by the basic control loop of thermoregulation.