Effects of Changes in Plasma Volume and Osmolarity on Thermoregulation during Exercise

Plateau values of deep body temperature were diminished during work after intake of 1 1/2 1 water, they were increased after intake of 2–3 % sodium chloride solution and after dehydration in a sauna by 1 1/2 kg. The shifts in plateau values were related to plasma osmolarity, not to plasma volume. In rest and during work at low temperature (7° C), when there was virtually no sweating, the effect of plasma osmolarity on body temperature disappeared. The rate of increase in sweating at the start of work was faster than normal after intake of water and slower after intake of sodium chloride solution and dehydration. The mechanism by which the changes in temperature were brought about is the increase in sweating at the start of work. Plasma osmolartiy may cause these changes either by a peripheral effect on the sweat glands or a central effect on the thermoregulatory centres.