Human physiological responses to immersion into water of different temperatures

To differentiate between the effect of cold and hydrostatic pressure on hor mone and cardiovascular functions of man, a group of young men was examined during 1-h head-out immersions in water of different temperatures (32 degr ees C, 20 degrees C and 14 degrees C). Immersion in water at 32 degrees C d id not change rectal temperature and metabolic rate, but lowered heart rate (by 15%) and systolic and diastolic blood pressures (by 11%, or 12%, respe ctively), compared to controls at ambient air temperature. Plasma renin act ivity, plasma cortisol and aldosterone concentrations were also lowered (by 46%, 34%, and 17%, respectively), while diuresis was increased by 107%. Im mersion at 20 degrees C induced a similar decrease in plasma renin activity , heart rate and systolic and diastolic blood pressures as immersion at the rmo-neutrality in spite of lowered rectal temperature and an increased meta bolic rate by 93%. Plasma cortisol concentrations tended to decrease, while plasma aldosterone concentration was unchanged. Diuresis was increased by 89%.. No significant differences in changes in diuresis, plasma renin activ ity and aldosterone concentration compared to subjects immersed to 32 degre es C were observed. Cold water immersion (14 degrees C) lowered rectal temp erature and increased metabolic rate (by 350%), heart rate and systolic and diastolic blood pressure (by 5%, 7%, and 8%, respectively). Plasma noradre naline and dopamine concentrations were increased by 530% and by 250% respe ctively, while diuresis increased by 163% (more than at 32 degrees C). Plas ma aldosterone concentrations increased by 23%. Plasma renin activity was r educed as during immersion in water at the highest temperature. Cortisol co ncentrations tended to decrease. Plasma adrenaline concentrations remained unchanged. Changes in plasma renin activity were not related to changes in aldosterone concentrations. Immersion in water of different temperatures di d not increase blood concentrations of cortisol. There was no correlation b etween changes in rectal temperature and changes in hormone production. Our data supported the hypothesis that physiological changes induced by water immersion are mediated by humoral control mechanisms, while responses induc ed by cold are mainly due to increased activity of the sympathetic nervous system.