Effect of hemodilution and hemoconcentration on red cell flow velocity in the capillaries of the rat mesentery.

Erythrocyte velocities in capillaries of rat mesentery were measured during systemic normotension and hypotension over a range of hematocrit from 0.31 +/- 0.06 to 0.58 +/- 0.04. Velocity was measured according to the method of Tompkins, Monti and Intaglietta. The hematocrit was varied by isovolemic exchange of blood with 4.5 g% albumin solution (bovine) or packed red cells (Hct. approximately 0.9). Hypotension was induced by hemorrhage to a preselected pressure of about 35 mm Hg. Under normotension red cell velocity stayed almost constant over a range of Hct. from 0.31 +/- 0.07 to 0.58 +/- 0.09. However, during hypotension significant differences were found for control animals (Hct. 0.43 +/- 0.07), hemodiluted animals (Hct. 0.31 +/- 0.06), and hemoconcentrated animals (Hct. 0.58 +/- 0.04). Velocities amounted to 36 +/- 21%, 52 +/- 24%, and 29 +/- 27% respectively, compared to control velocity. These marked differences in red cell velocities are explained by the strong effect of the hematocrit levels on apparent blood viscosities. It is assumed that the post-capillary microvascular bed is the site where the hematocrit level will affect apparent blood viscosity most intensively.