Role of hemodilution on renal responses to water immersion in humans

The present experiments were designed to elucidate 1) the role of the lower extremity capillary bed in decreasing plasma colloid osmotic pressure (COP) during immersion of humans (n = 8) for 6 h, and 2) the extent to which the natriuresis of water immersion is triggered by this decrease in COP. Irrespective of the depth, COPs were very similar during the immersion procedures, varying between 25.3 +/- 0.5 and 26.4 +/- 0.6 mmHg, which was significantly lower than during control (28.3 +/- 0.3 and 28.6 +/- 0.3 mmHg). During neck immersion, central venous pressure rose instantly by approximately 12 mmHg (P < 0.05) and remained elevated. Only a transient, marginal increase (1.6 +/- 0.7 mmHg) occurred during hip immersion. Cumulated sodium excretion during seated control, hip immersion, and neck immersion, respectively, differed significantly (30 +/- 5, 45 +/- 5, and 101 +/- 6 mmol). It is concluded that the decrease in COP during immersion is primarily due to fluid shifts occurring in the capillary bed of the legs and that this may account for up to 25% of the immersion-induced increase in renal sodium excretion.