Effects of clinostatism and orthostatism on blood viscosity

: The present study was planned to study the magnitude of the effects of body position upon blood viscosity. We compared blood rheological properties in nine healthy subjects after 1 h in a horizontal position and after 1 h in a vertical position. After orthostatism, blood viscosity was 38%, 41%, and 22% higher at 0.05, 1, and 20 s-1 shear rates, respectively. This result was the consequence of the increase of hematocrit (41.2% +/- 2.3% vs 44.2% +/- 2.7) and plasma viscosity (1.55 +/- 0.09 mPa.s vs 1.67 +/- 0.08 mPa.s) induced by orthostatism in all the subjects. The rise of the different plasma proteins and fibrinogen was the cause of the plasma viscosity increase. Blood viscosity at standard hematocrit (45%) was not significantly altered by orthostatism. Rheological studies at standard hematocrit showed that body position did not modify red cell deformability and aggregability. As suggested by the increase in hematocrit and plasma proteins and by the absence of alteration of blood rheological properties at standard hematocrit, the large effect of body position upon blood viscosity is the consequence of the hemoconcentration without alteration of erythrocyte deformability and aggregability.