Influence of spaceflight on erythrokinetics in man.

A significant postflight reduction in the circulating red cell mass has been observed in both the American and Soviet manned programs. The mechanism and etiology of this loss were studied in blood samples from the four payload crewmen of Spacelab 1 taken before, during, and after flight. These samples and samples from control groups on the ground were analyzed for selected hematological and biochemical parameters, which were chosen on the basis of data previously collected, the restraints imposed by the use of human subjects, and the guidelines established for the first Spacelab mission. Twenty-two hours after weightless exposure, there was an increase in hemoglobin and hematocrit. On day 7 in flight, the hemoglobin and hematocrit remained high and there was a slight decrease in reticulocyte number. On landing, red cell mass, plasma volume, hematocrit, and reticulocyte number were decreased. Throughout the 2-week postflight sampling period, hemoglobin, hematocrit, and reticulocyte number remained below the preflight value. Since this crew was not exposed to 100 percent oxygen these results are viewed as evidence that other spaceflight factors cause the measured red cell mass reduction.