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

Area of Science:

• Physiology
• Cardiovascular Physiology
• Renal Physiology

Background:

• Water immersion (WI) is known to affect fluid balance and cardiovascular dynamics.
• Understanding the long-term physiological responses to WI is crucial for various applications, including space travel and clinical settings.

Purpose of the Study:

• To investigate the effects of prolonged 12-hour water immersion on central blood volume and renal fluid and electrolyte excretion.
• To explore the hormonal mechanisms, including atrial natriuretic peptide (ANP), plasma renin activity, aldosterone, and norepinephrine, that mediate these responses.

Main Methods:

• Nine healthy males underwent 12 hours of head-out water immersion (WI) or control conditions on separate days.
• Measurements included central venous pressure, stroke volume, renal sodium and fluid excretion (UNaV), plasma ANP, plasma renin activity, plasma aldosterone, and norepinephrine levels before, during, and after immersion.

Main Results:

• WI initially increased central venous pressure, stroke volume, and UNaV, which then gradually declined but remained elevated compared to controls at 12 hours.
• Plasma ANP levels initially tripled then returned to baseline, while plasma renin activity, aldosterone, and norepinephrine remained suppressed throughout WI.
• Renal water excretion normalized within 7 hours, but UNaV remained elevated at 12 hours.

Conclusions:

• Prolonged WI leads to a reduction in central blood volume and attenuated renal fluid and electrolyte excretion compared to initial responses.
• Hormonal changes, particularly the initial rise and subsequent decrease in ANP, play a significant role in regulating renal responses during WI.
• The findings suggest that decreased ANP release contributes to the attenuation of sodium excretion in later stages of prolonged WI.