Effect of hydration on plasma volume and endocrine responses to water immersion

The effect of hydration status on early endocrine responses and on osmotic and intravascular volume changes during immersion was determined in humans undergoing successive periods of dehydration, immersion, rehydration, and immersion. Immersion caused an isotonic expansion of plasma volume, as well as suppression of plasma renin activity and aldosterone, which all occurred independently of hydration status. On the other hand, the concentration of plasma vasopressin (PVP) was found to decrease during dehydrated immersion, but not during rehydrated immersion. It is concluded that plasma tonicity is not a factor influencing PVP suppression during water immersion.