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Changes in body fluid compartments during a 28-day bed rest.
S M Fortney1, K H Hyatt, J E Davis
1Space Biomedical Research Institute, NASA Johnson Space Center, Houston, TX 77058.
Aviation, Space, and Environmental Medicine
|February 1, 1991
Summary
Simulated space-flight (bed rest) caused significant fluid loss in all body compartments, including plasma volume and total body water. Extracellular volume decreased early, with intracellular deficits developing later during the study.
Area of Science:
- Space physiology
- Human physiology
- Fluid dynamics
Background:
- Extravascular fluid shifts during spaceflight are not fully understood.
- Simulated microgravity provides a model to study these effects.
Purpose of the Study:
- To quantify body fluid compartment responses during prolonged simulated microgravity (bed rest).
Main Methods:
- Serial isotope measurements were used to assess fluid volumes in 10 healthy men.
- Study included controlled diet, ambulatory periods, 28 days of bed rest, and recovery.
Main Results:
- Significant decreases in plasma volume (PV) and extracellular volume (ECV) were observed by day 2 of bed rest.
- Total body water (TBW) and red cell volume (RCV) also decreased, with TBW loss shifting from ECV to intracellular compartments over time.
- No restoration of ECV was evident after 1-2 weeks of bed rest.
Conclusions:
- Prolonged bed rest leads to significant fluid loss across all body compartments.
- Fluid shifts occur from extracellular to intracellular spaces during simulated microgravity.
- Bed rest impairs the body's ability to restore extracellular fluid volume.