Effects of simulated microgravity (HDT) on blood fluidity.

Exposures to microgravity and head-down tilt (HDT) produce similar changes in body fluid. This causes an increase in hematocrit that significantly affects hemorheological values. Lack of physical stimulation under bed rest conditions and the relative immobility of the crew during spaceflight also affects the blood fluidity. A group of six healthy male subjects participated as volunteers, and blood samples were collected 10 days before, on day 2 and day 9, and 2 days after the HDT phase. Blood rheology was quantified by plasma viscometry, red cell aggregability, and red cell deformability. A reduced red cell deformability, an indication of the diminished quality of the red blood cells, was measured under HDT conditions that finally led to the so-called "space flight anemia." Enhanced red cell membrane fragility induced by diminished physical activity and an increase in hemoglobin concentration are responsible for this effect. Plasma viscosity is reduced as a result of diminished plasma proteins. However, despite the reduction in plasma proteins, including fibrinogen, alpha 2-macroglobulin, and immunoglobulin M, red cell aggregation was enhanced, principally because of the increase in hematocrit. Our results of hemorheological alterations under HDT conditions may help to elucidate the formerly documented hematologic changes during spaceflight.