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Plasma colloid osmotic pressure increases in humans during simulated microgravity
S T Hsieh1, R E Ballard, G Murthy
1Gravitational Research Branch, NASA-Ames Research Center, Moffett Field 94035-1000, USA.
Aviation, Space, and Environmental Medicine
|March 7, 1998
Summary
Astronauts experience plasma volume loss and increased plasma colloid osmotic pressure (COP) during simulated microgravity. This dehydration and elevated COP may contribute to orthostatic intolerance after spaceflight.
Area of Science:
- Space physiology
- Human physiology
- Fluid shifts
Background:
- Microgravity exposure causes significant plasma volume reduction in astronauts.
- This fluid loss is a potential factor in post-flight orthostatic intolerance.
Purpose of the Study:
- To investigate the effect of simulated microgravity on plasma colloid osmotic pressure (COP).
- To determine if whole-body dehydration during head-down tilt (HDT) increases plasma COP.
Main Methods:
- Seven healthy males underwent 16 days of 60° head-down tilt (HDT).
- Plasma COP was measured using a colloid osmometer.
- Plasma volume was assessed using a modified Evans blue dye technique.
Main Results:
- Plasma COP significantly increased by day 14 of HDT (29.9 mm Hg vs. 23.1 mm Hg pre-HDT).
- Plasma volume decreased concurrently with elevated COP.
- Following HDT, plasma volume remained low and COP stayed elevated compared to pre-HDT levels.
Conclusions:
- Simulated microgravity leads to reduced plasma volume and elevated plasma COP.
- These changes likely indicate a significant loss of extracellular fluids during HDT.
- Findings suggest a physiological response to dehydration in a microgravity analog.