Regional blood flows and cardiac output distribution in rats during acute anemia or polycythemia

Radioactively labelled microspheres (15 micron diameter) were used to measure cardiac output (CO) distribution and blood flows in spleen, kidneys, and skeletal muscle before and after normovolemic anemia or polycythemia in anesthetized rats. Hematocrits were changed from 45 to 33% or from 45 to 59% by an exchange transfusion of homologous plasma or packed cells. Anemia was accompanied by a 39% increase in CO while polycythemia showed a 25% decrease. Following hemodilution the spleen as well as skeletal muscle received greater than normal fractions of CO and in each the flow increase was greater than expected from the fall in viscosity. The renal fraction of CO was unchanged. Following hemoconcentration "greater-than-normal" fractions of CO were distributed towards spleen and kidney. In these tissues the changes in flow were significantly greater than the change in resistance due to viscosity. Skeletal muscle flow changes appeared to have been due mostly to increased viscosity. These observations imply that during acute, isovolemic changes in hematocrit, the flow changes of individual vascular beds cannot be explained by viscosity changes alone but the importance of nervous control or of local metabolic factors remains to be investigated.