Intravascular volume and tonicity as factors in the regulation of body temperature.

The effect of alterations in intravascular volume and tonicity on thermoregulatory and cardiovascular responses to heat and exercise have been compared in four subjects. Core temperatures were found to be significantly higher during dehydration, and when dehydration was prevented by administration of 1% saline, than when dehydration was prevented by water administration. These higher temperatures were associated with elevated levels of plasma [Na] and osmolarity, but no consistent relationship between temperature and changes in intravascular volume could be demonstrated. Relationships observed between core temperature and plasma tonicity were consistent with the hypothesis that the adverse effects of dehydration on thermoregulation can be attributed to an inhibition of sweating mediated by an increase in either plasma osmotic pressure or plasma [Na]. In separate experiments the heart rate response to exercise was shown to be reduced by saline, compared with water and dehydration, and this may be explained by the smaller reduction in intravascular volume which occurs during exercise following administration of hypertonic saline. It is concluded that the effects of reduced intravascular volume, and increased intravascular tonicity on physical work capacity may be distinguished by the adverse effect on the cardiovascular system of the former, and on the thermoregulatory system of the latter.