Plasma volume, red blood cell mass, and erythrocyte survival determinations were performed for the Gemini astronauts before and after Gemini orbital flights IV, V, and VII. Plasma volume decreases similar to those seen after bed rest and water immersion were found in Gemini IV and V. While each of these flights was affected by different stresses and the data accumulated are not as complete or extensive as desired, the results suggest that the plasma volume changes associated with the shorter flights may have been compensated for in the longer Gemini VII flight. These preliminary studies showed decreases in the survival of erythrocytes and red blood cell mass (both studied with sodium chromate Cr 51) in Gemini V and VII. No compensation for the RBC mass decrease was found even in the longer flight. Data obtained indicate decreased RBC mass is a result of mild hemolysis of unknown cause.
Fischer CL, Johnson PC, Berry CA. Red Blood Cell Mass and Plasma Volume Changes in Manned Space Flight. JAMA. 1967;200(7):579–583. doi:10.1001/jama.1967.03120200057007
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