Control of red blood cell mass in spaceflight

The effect of spaceflight on red blood cell mass (RBCM), plasma volume (PV) erythron iran turnover, serum erythropoietin, and red blood cell (RBC) production and survival and indexes were determined for six astronauts on two shuttle missions, 9 and 14 days in duration. respectively. PV decreased within tile first day. RBCM decreased because of destruction of RBCs either newly released or scheduled to be released from the bone marrow. Older RBCs survived normally. On return to Earth, plasma volume increased, hemoglobin concentration and RBC count declined, and serum erythropoietin increased. We propose that entry into microgravity results in acute plethora as a result of a decrease in vascular space. FV decreases, causing an increase in hemoglobin concentration that effects a decrease in crythropoietin or other growth factors or cytokines. The RBCM decreases by destruction of recently formed RBCs to a level appropriate ibr the microgravity environment. Return to Earth results sequentially in acute hypovolemia as vascular space dependent on gravity is refilled, an increase in plasma volume, a decrease in hemoglobin concentration (anemia), and an increase in serum erythropoietin.