Preventing hemodilution abolishes natriuresis of water immersion in humans.

The hypothesis was tested that hemodilution is one of the determinants of the water immersion (WI)-induced natriuresis. Eight males were subjected to 3 h of 1) WI to the midchest (Chest), 2) WI to the neck combined with thigh cuff-induced (80 mmHg) venous stasis (Neck + stasis), and 3) a seated time control (n = 6). Central venous pressure and left atrial diameter increased to the same extent during Chest and Neck + stasis (P < 0.05), whereas renal sodium excretion only increased during Chest from 77 +/- 7 to 225 +/- 13 micromol/min (P < 0.05). During Chest, plasma colloid osmotic pressure (COP) decreased from 27.7 +/- 0.7 to 25.1 +/- 0.7 mmHg (P < 0.05), and plasma volume (PV) increased from 3,263 +/- 129 to 3,581 +/- 159 ml (P < 0.05), whereas these variables remained unchanged during Neck + stasis. Plasma norepinephrine concentration decreased similarly during Chest and Neck + stasis by 45 +/- 7 and 34 +/- 4%, respectively (P < 0.05), whereas plasma renin activity decreased only during Chest (P < 0.05). In conclusion, during WI in humans 1) hemodilution (decrease in COP and increase in PV) is a pivotal stimulus for the natriuresis and 2) central blood volume expansion without hemodilution does not augment renal sodium output.