BLOOD-VOLUME AND ERYTHROPOIESIS IN THE RAT DURING SPACEFLIGHT

A decreased red blood cell mass (RBCM) and plasma volume (PV) have bee n consistently found in humans after return from spaceflight. Rats flo wn on the Spacelab Life Sciences-1 mission were studied to assess:chan ges in RBCM, PV, erythropoiesis, and iron economy. The RBCM and PV inc reased in both ground control and flight animals as expected for growi ng rats. However on landing day, both the RBCM and PV, when normalized for body mass, were significantly decreased in the spaceflight animal s. During an 8-d postflight observation period, iron incorporation int o circulating red blood cells was diminished in the flight animals. Du ring the first 4 d postflight, increases in reticulocyte counts were s ignificantly smaller in the flight than the control animals. Fewer ery thropoietin-responsive progenitor cells were recovered from the bone m arrow of flight animals after landing than control rats. Serum erythro poietin (EPO) levels were the same in both groups. Thus, rats subjecte d to a 9-d spaceflight had less increase in RBCM than controls and dim inished erythropoiesis during an 8-d post-spaceflight observation peri od. The rat, like humans, appears to require a smaller blood volume in microgravity.