Regulation of body fluid compartments during short-term spaceflight

Area of Science:

• Space Physiology
• Human Physiology in Space
• Fluid and Electrolyte Balance

Background:

• Understanding fluid and electrolyte regulation is crucial for astronaut health during long-duration space missions.
• Previous studies suggest significant physiological adaptations occur in microgravity.

Purpose of the Study:

• To investigate body fluid, renal, and hormonal responses during Spacelab missions.
• To characterize fluid regulatory mechanisms in spaceflight.

Main Methods:

• Analysis of fluid balance, plasma volume, extracellular fluid volume, glomerular filtration rate, and key hormones (ADH, renin, aldosterone, ANP, cortisol) in seven subjects.
• Comparison of data collected during Spacelab Life Sciences-1 and -2 missions with preflight and postflight measurements.

Main Results:

• Total body water remained stable, but plasma volume and extracellular fluid volume decreased significantly within 21 hours of launch.
• Glomerular filtration rate increased, while fluid intake decreased during weightlessness.
• Antidiuretic hormone excretion increased, and fluid excretion decreased early in flight.
• Plasma renin activity and aldosterone levels showed initial decreases followed by later increases.
• Atrial natriuretic peptide was consistently lower, and urinary cortisol was elevated during flight.

Conclusions:

• Spaceflight induces significant fluid shifts and hormonal alterations, impacting fluid homeostasis.
• Increased capillary permeability may be a key factor in plasma volume reduction.
• The Gauer-Henry reflex might operate within the first 21 hours, followed by adaptation to a reduced plasma volume set point.