Osmoregulation and interstitial fluid pressure changes in humans during water immersion.

The aim of the present study was to determine the magnitude and direction of the shift of body fluids during water immersion of humans to the neck. Five healthy male subjects were studied lying in air for 1.5 h, sitting in 34 degrees C water to the neck for 1 h, and again lying in air for 1.5 h in two sets of experiments. For the first set, vasopressin (0.75 IU, sc) was injected before immersion. Blood and urine samples were drawn every 30 min in air and every 20 min in water. Urinary sodium, potassium, and osmolal clearances were significantly increased during immersion. When the mean maximum change during immersion was calculated for five subjects hematocrit fell by 1.1 U, plasma concentrations of sodium by 3.9 meq/l, chloride by 3.5 meq/l, potassium by 0.2 meq/l, osmolality by 7.9 mosmol/kg H2O, and proteins by 0.25 g/100 ml, whereas total plasma CO2 content increased by 1.33 mmol/l, threonine by 11.6%, proline by 9.0%, methionine by 14.0%, and alanine by 29%. Plasma volume increased 6.1%, and red blood cell volume calculated from hematocrit and hemoglobin increased 3.5%. In the second set of immersion experiments, without vasopressin injection, interstitial fluid pressures were measured with a cotton wick in PE-50 tubing inserted subcutaneously. A mean interstitial fluid pressure of -0.5 cmH2O was observed when the subjects were lying in air. Interstitial fluid pressure had started to decrease by 20 min of immersion, with a maximum decrease during immersion averaging 2.10 cmH2O. We conclude that hyposmotic fluid is mobilized into the blood from interstitial and other extravascular spaces during immersion.