Body heat balance in man subjected to endogenous and exogenous heat load

The body heat balance, measured by a thermometric method, was investigated in humans subjected to endogenous and exogenous heat load. The purpose of the present study was to test the concept of heat exchange by a servomechanism in human thermoregulation. Two series of experiments were performed on male volunteers. In series I 15 subjects performed physical exercise (50% VO2 max) for 60 min at a constant ambient temperature of 25 degrees C. In series II 16 subjects rested in a climatic chamber where the ambient temperature was elevated over 30 min from 22 to 42 degrees C and kept stable at this level during the subsequent 60 min. It was found that in both series of experiments the sweating rate followed an exponential curve exhibiting an inertial course. Heat was stored in the body mainly at the beginning of experiment. In series I the net body heat load of 125 W/m2 was equalized by sweat evaporation, beginning after 40 min of the exercise. In series II the net body heat load of 80 W/m2 was equalized in the same way, starting after 35 min of the constant high ambient temperature. In both series of experiments the amount of heat stored in the body calculated from the body heat balance was quite close to the amount of heat calculated from the calorimetric equation. It is concluded, that under the present experimental conditions, heat loss from the body by sweat evaporation seems to be a regulated variable in the human thermoregulatory system. The observed increase in rectal temperature may result from an inertial course of the sweating reaction.