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 pre ssure (COP) during immersion of humans (n = 8) for 6 h, and 2) the ext ent to which the natriuresis of water immersion is triggered by this d ecrease in COP. Irrespective of the depth, COPs were very similar duri ng 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 pre ssure rose instantly by similar to 12 mmHg (P < 0.05) and remained ele vated. Only a transient, marginal increase (1.6 +/- 0.7 mmHg) occurred during hip immersion. Cumulated sodium excretion during seated contro l, hip immersion, and neck immersion, respectively, differed significa ntly (30 +/- 5, 45 +/- 5, and 101 +/- 6 mmol). It is concluded that th e decrease in COP during immersion is primarily due to fluid shifts oc curring 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 .