Decreased production of red blood cells in human subjects exposed to microgravity.

The total-body red blood cell mass (RBCM) decreases during the first few days of spaceflight; however, the pathophysiology of "spaceflight anemia" noted on return to earth is poorly understood. In studies before, during, and after a 9-day mission we determined the rates of removal and replacement of RBCs by using chromium 51. The rate and efficiency of RBC production were assessed with iron 59. Serial measurements were made of plasma volume (PV), RBCM, serum ferritin level, and erythropoietin 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. Circulating RBCs disappeared at a normal rate during flight, but few new cells replaced those destroyed, resulting in a decrease in RBCM of 11% during the mission. After 22 hours in space, intramedullary formation of cells continued at near preflight levels as measured by erythron iron turnover. The coexistence of new cell formation in the bone marrow and failure of cells to be released into the blood is consistent with ineffective erythropoiesis. Microgravity causes blood located in gravity-dependent spaces to shift to a central volume. We conclude that the initial adaptation is a reduction in PV resulting in plethora. Increase in total body hematocrit causes a decrease in erythropoietin production. RBCM decreases because RBCs destroyed at a normal rate are not replaced.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular function in space flight.

Changes in orthostatic heart rate have been noted universally in Soviet and U.S. crewmembers post space flight. The magnitude of these changes appears to be influenced by mission duration, with increasing orthostatic intolerance for the first 7-10 days of flight and then a partial recovery in the orthostatic heart rate response. Fluid loading has been used as a countermeasure to this postflight orthostatic intolerance. Previous reports have documented the effectiveness of this technique, but it has also been noted that the effectiveness of volume expansion diminishes as flight duration exceeds one week. The response of carotid baroreceptor function was investigated utilizing a commercially available neck collar which could apply positive and negative pressure to effect receptor stimulation. Bedrest studies had validated the usefulness and validity of the device. In these studies it was shown that carotid baroreceptor function curves demonstrated less responsiveness to orthostatic stimulation than control individuals. Twelve Space Shuttle crewmembers were examined pre- and postflight from flights lasting from 4-5 days. Plots of baroreceptor function were constructed and plotted as change in R-R interval vs. carotid distending pressure (an orthostatic stimulus). Typical sigmoidal curves were obtained. Postflight the resting heart rate was higher (smaller R-R interval) and the range of R-R value and the slope of the carotid sigmoidal response were both depressed. These changes were not significant immediately postflight (L + O), but did become significant by the second day postflight (L + 2), and remained suppressed for several days thereafter. It is hypothesized that the early adaptation to space flight involves a central fluid shift during the initial days of flight, but subsequent alterations in neural controlling mechanisms (such as carotid baroreceptor function) contribute to orthostatic intolerance.