Effects of Reduced Hematocrit on Erythrocyte Velocity and Fluorescein Transit Time in the Cerebral Microcirculation of the Mouse

The red blood cell velocity and the arteriole-to-venule transit time of sodium fluorescein were measured in the pial microcirculation of 15 mice immediately before and after phlebotomy and hemodilution. The acute anemic state produced by the phlebotomy and hemodilution was accompanied by an increase in the velocity of the red blood cells and of the fluorescent plasma. Such increments in velocity must be attributable to either an increased blood pressure or a fall in cerebrovascular resistance. Increases in pressure were generally absent, as was vasodilation, one cause of a decreased resistance. On the other hand, decreased blood viscosity accompanies a fall in hematocrit and could therefore account for a decreased vascular resistance in every animal. These data are consonant with earlier reports in animals and man of an increased cerebral blood flow accompanying anemia. Moreover, the data support those workers who attributed this increase in flow at least partly to a decrease in blood viscosity.