Water and sodium balance in space

Area of Science:

• Space physiology
• Fluid and electrolyte balance
• Human adaptation to microgravity

Background:

• Fluid balances and body fluid regulation significantly differ between microgravity (microG) and Earth conditions.
• Initial adaptation to microG involves fluid redistribution to the upper body and increased vascular permeability, with unknown mechanisms.
• Major water balance components like evaporation, hydration, and urinary excretion are reduced in space flight.

Purpose of the Study:

• To investigate the mechanisms of body fluid regulation and sodium handling in microgravity.
• To understand the adaptation of fluid balance in weightlessness.
• To clarify the reasons behind reported absolute water loss during space flight.

Main Methods:

• Analysis of fluid balance and body fluid regulation data from space flight.
• Assessment of water balance components (evaporation, hydration, urinary excretion).
• Evaluation of sodium handling and endocrine system activation (renin-aldosterone, catecholamines) in microgravity.

Main Results:

• Cumulative water balance and total body water remain stable in microgravity under adequate hydration and nutrition.
• Absolute water loss during space flight is not a consequence of microgravity itself.
• Sodium handling is significantly affected, with reduced urinary excretion and retention in the extravascular space despite comparable intake.
• Sodium-retaining endocrine systems are more activated in microgravity.

Conclusions:

• Adaptation to weightlessness involves significant changes in sodium handling, with retention in the extravascular space.
• A large storage capacity for sodium in the extravascular space and a dissociated water and sodium handling mechanism contribute to fluid balance adaptation in microgravity.
• The findings challenge previous assumptions about water loss in space and highlight the complex physiological adjustments to microgravity.