Redistribution of fluid towards head and torse - 10% less in legs
Distention of baroceptors reduces renin-angiotensin and causes release of ANP
This leads to increased salt and water excretion and reduction in plasma volume
In first 24hr of spaceflight - 17% reduction in plasma volume
Haemocrit rises which causes a decrease in erythropoietin (so total RBC mass is decreased)
Overall blood volume decreased by about 10%
Post flight - aerobic capacity decreased due to reduced stroke volume, and the restoration of blood can make astronauts light headed and prone to fainting
Countered by:
1. Exercise during microgravity
2. Negative pressure suits on re-entry to prevent blood pooling in head
3. Isotonic fluids to bulk out plasma volume and positive pressure for lower body
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