Are physiological changes in capillary tube hematocrit related to alterations in capillary perfusion heterogeneity?

Average capillary tube hematocrit is hypothesized to be reduced below the mean systemic value as a result of heterogeneous distribution of blood flow among microvessels. Furthermore, capillary tube hematocrit changes with vasomotor state and these have been proposed to be due to concomitant changes in perfusion heterogeneity. We reasoned that if alterations in average capillary tube hematocrit were related to changes in perfusion heterogeneity, then mean capillary tube hematocrit should be inversely related both to the heterogeneity of capillary blood flows and to the heterogeneity of capillary hematocrit. These inferences were tested by comparing estimates of average capillary tube hematocrit (Ht) and the heterogeneities of capillary blood flow and Ht in vasoconstricted and vasodilated hamster tibialis anterior muscles. Ht was estimated from the number of red blood cells per unit capillary length (n/l); heterogeneity of capillary blood flow and heterogeneity of Ht were estimated by calculating the coefficient of variation of capillary red blood cell velocities (vrbc) and n/l, respectively. Average Ht varied with vasomotor state. However, there was no correlation between average Ht and the heterogeneity of vrbc, our index of capillary blood flow: in fact, the heterogeneity of vrbc was constant under all experimental conditions. The heterogeneity of Ht varied with vasomotor state, but not in inverse proportion to average Ht as was expected. From these observations we conclude that alterations in average capillary tube hematocrit are not due to concomitant alterations in perfusion heterogeneity.