Red blood cell half-life was not shortened on the Apollo missions, although the crews of both Apollo and Gemini missions were exposed to at least four hours of 100% oxygen prior to launch and during flight. It was not clear whether inhibition of erythropoiesis (decreased production of red blood cells) caused by hyperoxia, or hemolysis of red blood cells (increased destruction) was the causative agent for the decrease in red blood cell mass. The Skylab missions offered the opportunity to rule out the hyperoxia hypothesis while testing whether changes in red blood cell mass are progressive with longer periods of weightlessness. Skylab experiment M113 was designed to (1) determine the effect of Earth-orbital missions on plasma volume and red blood cell populations (particularly red cell mass, life span, and production and destruction rates), and (2) provide baseline data for correlation with data from other hematologic and immunologic experiments.
Most measurements in this experiment were obtained by the tracer kinetics method. The red blood cell mass and half-life were determined using 51chromium (51Cr) as the tracer. A sample of 12.5 milliliters of blood was drawn and mixed with 2.5 milliliters of anticoagulant acid citrate dextrose (ACD) solution and 25 microcuries of 51Cr. Ten milliliters of 51Cr-tagged cells from the sample were reinfused into the bloodstream after incubating the cells for 4 minutes at room temperature and adding 50 milligrams of ascorbic acid. The red blood cell mass was determined by averaging the radioactivity of 30- and 31-minute red blood cell samples. Red blood cell chromium half-life was estimated from the measured activity profile of 51Cr. Plasma volume was measured by injecting 2 microcuries of 125iodine human serum albumin each time the red blood cell mass was determined.
To track the red blood cell lifespan, 50 microcuries of 14carbon-glycine was injected intravenously 30 days prior to launch. The 14C radioactivity was followed for 125 days on Skylab 2, 131 days on Skylab 3, and 141 days on Skylab 4. In general, blood was drawn weekly throughout this period, including the time in space. Radioactivity was determined by extracting heme, igniting the dried extract, and determining microcuries of 14C per milligram heme. Iron turnover was determined by injecting 2 microcuries of 59iron-citrate at recovery and using blood samples drawn at 30 and 31 minutes, and 2-3 hours later. Iron reappearance was obtained from blood samples drawn 1,3,7 and 14 days after recovery. Reticulocyte counts were made from weekly preflight and postflight blood samples.
The recovery mean value for the red blood cell mass of all Skylab astronauts was lower (1843 milliliters) than their preflight mean (2075 milliliters) and different from the control postflight mean (2046 milliliters). While the postflight decrease in mean red blood cell mass was 232 milliliters for the Skylab crewmembers, the corresponding value for the controls was only 7 milliliters. There was no significant difference between pre- and postflight crew mean values and control mean value either for the 51Cr red blood cell half-life or for the 14C-glycine red blood cell mean life span; neither was there any statistically significant difference between the control subjects and the Skylab crews in appearance or turnover of iron, indicating that the rate of erythropoiesis was the same for both groups. However, postflight reticulocyte counts measured at recovery following each mission were low. The postflight counts were greater than preflight values in only one crewmember of the 28-day mission 2 weeks after recovery, in all three crewmembers of the 59-day mission 1 week after recovery, and in all three crewmembers of the 84-day mission 1 week or less after recovery. The control subjects, on the other hand, did not develop a change in reticulocyte count, indicating that the changes observed in the Skylab crews were not due to blood draw amounts. Inhibited bone marrow was thought to be a possible explanation for the low red blood cell masses and low postflight reticulocyte counts, and normal iron turnover was thought to be caused by a postflight rebound in the bone marrow. As for the plasma volume, the mean percent decrease was less in the Skylab 2 crew than in the crews of the longer Skylab 3 and 4 missions, but still greater than the decrease observed during Apollo missions, which suggested that the plasma volume did not return to normal even after prolonged flights.