Changes in limb volume during head-out water immersion in humans.

In order to investigate the contribution of fluid shift from the legs to immersion-induced diuresis, 7 normal and 2 legless (both legs disarticulated at the hip) individuals were tested to measure changes in leg volume (normal subjects) and urinary excretion of solutes and water during a 3-h head-out water immersion (HOI) in thermoneutral water (34.5 +/- 0.5 degrees C) with a 1-h control period before and after immersion. On a separate day, a 5-h time control (TC) experiment on the same subjects was carried out by having them sit in thermoneutral air (29 +/- 0.1 degrees C, relative humidity 60%). The leg volume decreased by 192 +/- 20 ml during the 3-h HOI, which accounted for 3.5% of the average leg volume of the preimmersion period. During TC, however, the leg volume increased by 110 ml in the same time course. An increase in urine volume and sodium excretion was observed during HOI in both normal (P less than 0.05) and legless subjects. Net urine volume induced by HOI (urine volume during HOI minus urine volume during corresponding TC) in normal subjects was 494 +/- 89 ml in 3 h, which was close to a calculated volume reduction of the lower extremities during HOI (508 +/- 53 ml). The average net urine volume during HOI in the legless subject was 183 +/- 48 ml/min. The above results may suggest that the increase in renal water output is largely accounted for by a fluid shift from the lower extremities during HOI, however, the fluid other than that from the extremities should be accounted for as the urine source.(ABSTRACT TRUNCATED AT 250 WORDS)