Circulation, kidney function, and volume-regulating hormones during prolonged water immersion in humans

Abstract

To investigate whether prolonged water immersion (WI) results in reduction of central blood volume and attenuation of renal fluid and electrolyte excretion, these variables were measured in connection with 12 h of immersion. On separate days, nine healthy males were investigated before, during, and after 12 h of WI to the neck or during appropriate control conditions. Central venous pressure, stroke volume, renal sodium (UNaV) and fluid excretion increased on initiation of WI and thereafter gradually declined but were still elevated compared with control values at the 12th h of WI. Atrial natriuretic peptide (ANP) concentration in plasma initially increased threefold during WI and thereafter declined to preimmersion levels, whereas plasma renin activity, plasma aldosterone, and norepinephrine remained constantly suppressed. It is concluded that, compared with the initial increases, central blood volume (central venous pressure and stroke volume) is reduced during prolonged WI and renal fluid and electrolyte excretion is attenuated. UNaV is still increased at the 12th h of WI, whereas renal water excretion returns to control values within 7 h. The WI-induced changes in ANP, plasma renin activity, plasma aldosterone, and norepinephrine may all contribute to the initial increase in UNaV. The results suggest, however, that the attenuation of UNaV during the later stages of WI is due to the decrease in ANP release.

Mechanisms of increase in cardiac output during acute weightlessness in humans

“…Acute weightlessness did not decrease HR to below the 1-G control value, as expected compared with effects of simulation experiments, such as an upright-to-supine body posture change, head-down tilt, or head-out water immersion (4,9,13,14,17,26,28,29). One might speculate that a possible mechanism for the lack of a decrease in HR during 0 G could be that the preceding 1.8 G stimulated HR by activation of the sympathetic nervous system through inhibition of the baroreflexes, and that this activation counteracted the effects of the subsequent short-term 0-G phase.…”

Body fluid regulation in µ-gravity differs from that on Earth: an overview

“…Hypervolemia, through an acute isotonic saline infusion (2 l within 25 min), led to a longlasting (>24 h) natriuresis and diuresis, closely paralleled by increased renal release of urodilatin; however, the plasma ANP concentrations remained unchanged [7]. Long-term head-out water immersion -which is comparable to the above-described balloon experiments -stimulates both the atrial release of ANP [35] and the renal release of urodilatin [31], in parallel with the long-lasting natriuretic and diuretic profile.…”

“…The acute assumption of a supine body position (C. Drummer, C. Hesse, A. Börger, M. Herten, M. Heer The effect of posture on the humoral and renal response to an oral sodium load. Manuscript submitted for publication), head-down tilt experiments [9,14], lower body positive pressure stimulation [37], water immersion to the neck [31,35], and an acute isotonic saline infusion [7] are typical hypervolemia models in human physiology. Animal experiments additionally allow the inflation of a cardiac balloon [19].…”

Haemodynamic changes after prolonged water immersion

“…Since the density of water is 800 times that of air, the ambient pressure increases linearly with depth. During head-out water immersion, the hydrostatic pressure on the body surface promotes an immediate increase in intrathoracic blood volume by shifting peripheral venous blood towards the thorax (Gabrielsen, Johansen, & Norsk, 1993;Risch, Koubenec, Beckmann, Lange, & Gauer, 1978;Stadeager et al, 1992), which in turn increases cardiac preload and stroke volume (Christie et al, 1990;Norsk, Bonde-Petersen, & Christensen, 1990). Hydrostatic pressure also triggers a shift of interstitial fluid into plasma (Miki, Hajduczok, Hong, & Krasney, 1986).…”