Human thermoregulation in the heat: predictive models of physiological conductance and sweat rate

The literature pertaining to models of human thermoregulation is critically reviewed. It is concluded that none of the models investigated can be used to predict the physiological response of acclimatized men required to work in the heat for extended periods of time, the reasons being: (i) physiological conductance (the lumped parameter describing internal heat transfer) has not been completely described in terms of all the variables effecting it (ii) the dynamic nature of the sweat rate response has never been mathematically described. A composite block diagram model for human thermoregulation, based on a single cylinder core and shell model of the controlled system, is postulated as a feasible solution to the problem of predicting physiological responses to work in the heat. The controlling system, consisting of physiological conductance and sweat rate, is singled out for thorough investigation. Detailed experimentation was carried out on a single nude acclimatized man over a wide range of environments (13,6°C dry-bulb, 9,2°C wet-bulb to 38,5°C dry-bulb, 36,5°C wet-bulb) and work rates (resting, 60 W/m2 to exercising, 240 W/m2 ). Each experiment lasted four hours. Physiological conductance (K) is expressed as a sigmoidal function of a weighted body temperature driving signal (Tmb ) and metabolic rate (M), such that for the same Tmb, K increases with increasing M. The saturation level of K at low Tm b is expressed as a linear function of M, the saturation level of K at high Tmb, is expressed as an exponential function of M. Because of the dynamic nature of the sweat rate response to a step change in environmental conditions and/or work rate, basic engineering control theory is used tc formulate a block diagram model for thermoregulatory sweating. The model introduces a rational approach to combining Tmb, local effect of skin temperature, sweat gland fatigue, skin wettedness and their time histories.