Analysis of the hematocrit distribution in the mesenteric microcirculation.

The hematocrit in the microcirculation of the rat's mesentery was determined either by using measurements of optical density (in 56 arterioles and 52 venules) or by counting the number of red cells per unit vessel length (in 63 capillaries) from photographs. The systemic hematocrit (Hsys) averaged 0.42 +/- 0.025. The mean microvessel hematocrit (Hmicro) decreased and its standard deviation increased with decreasing vessel diameter; in capillaries, Hmicro/Hsys ranged between almost zero and 0.86 with an average of 0.36 +/- 0.23. In order to correct Hmicro for the Fahraeus effect, additional experiments were carried out in vitro, in which the tube hematocrit was obtained by centrifugation of glass tubes (I.D. 13-95 micron) perfused with rat's blood of different feed hematocrit. Even after correction for the Fahraeus effect the in vivo data seem to suggest a 'loss of red cells' in the microcirculation. To reconcile the observed hematocrit distribution with the law of mass conservation, model calculations were performed, in which the presence of shunt vessels was assumed on the one hand, and a proportionality between single vessel hematocrit and flow rate on the other. The experimental data are best described by a model which combines the flow dependence of hematocrit and a small shunt component. The positive correlation between single vessel flow rate and Hmicro which is contained in this model also explains an increase of average capillary hematocrit resulting from a more homogeneous flow distribution associated with vasodilatation.