DECREASED PRODUCTION OF RED-BLOOD-CELLS IN HUMAN-SUBJECTS EXPOSED TO MICROGRAVITY

The fetal-body red blood cell mass (RBCM) decreases during the first f ew days of spaceflight; however, the pathophysiology of ''spaceflight anemia'' noted on return to earth is poorly understood. In studies bef ore, during, and after a 9-day mission we determined the rates of remo val and replacement of RBCs by using chromium 51. The rate and efficie ncy of RBC production were assessed with iron 59. Serial measurements were made of plasma volume (PV), RBCM, serum ferritin level, and eryth ropoietin level. PV decreased within hours, resulting in an increased total body hematocrit during the first few days of the mission. Serum erythropoietin level decreased within 24 hours and remained low. Circu lating RBCs disappeared at a normal rate during flight, but few new ce lls replaced those destroyed, resulting in a decrease in RBCM of 11% d uring the mission. After 22 hours in space, intramedullary formation o f cells continued at near preflight levels as measured by erythron iro n tumover. The coexistence of new cell formation in the bane marrow an d failure of cells to be released into the blood is consistent with in effective erythropoiesis. Microgravity causes blood located in gravity -dependent spaces to shift to a central volume. We conclude that the i nitial adaptation is a reduction in PV resulting in plethora. Increase in total body hematocrit causes a decrease in erythropoietin producti on. RBCM decreases because RBCs destroyed at a normal rate are not rep laced. The normal erythron iron tumover and the rapidity of decrease i n RBCM indicate that reduction in the release of new RBCs results from ineffective erythropoiesis. On return to 1 g, the gravity-dependent b lood spaces are reestablished, resulting in ''anemia of spaceflight.''