Respiratory control of body temperature: a theoretical model

The two-compartment model of a ventilatory CO2 chemostat of Grodins et al. (1954) is modified to describe a ventilatory thermostat fitting the requirements of panting. Allowance for the extra heat production during panting leads to a second order third degree differential equation which by introducing the ventilation/perfusion ratio can be reduced to the second degree. From the steady-state solutions calculated for dogs using experimentally determined constants it is concluded that: (1) the ventilation/perfusion ratio has no influence; (2) the ambient temperature leads to a marked panting only above 30 degree C; (3) the relative humidity of the air increases the panting; (4) due to the oxygen cost of panting, the regulation of the body temperature is possible only within limits. The limitation depends upon the energy requirements of the respiratory muscles, upon the set-point temperature and the controller gain. The unsteady-state solution is given by approximation. The ventilatory reaction to the heat stress appears to be slow with a half-time of about 15 min for a 20 kg dog. All properties of the model are mainly determined by the high specific heat of tissues relative to expired air.