DETERMINATION OF CAPILLARY-TUBE HEMATOCRIT DURING ARTERIOLAR MICROPERFUSION

Intracapillary hematocrit is known to be substantially lower than arte rial hematocrit. We hypothesized that capillary hematocrit might be in fluenced by interactions between plasma macromolecules and the endothe lial cell surface. Microvessel perfusion pipettes were inserted in sec ond- or third-order vessels, and capillaries were perfused with three different artificial bloods composed of 50% red cells plus the followi ng suspension media: fetal calf serum (group I), serum albumin plus se rum globulins (fractions II and III; group II), and bovine serum album in plus dextran (group III). The mean hematocrits of the pipette-perfu sed capillaries averaged close to 50% of the systemic value with all p erfusion fluids and were not different from the hematocrits of the cap illaries perfused by the animal. These data suggest that bifurcations proximal to the pipette location did not contribute to the reduction i n mean tube hematocrit normally seen in the animal. Furthermore, inter actions between the plasma macromolecules and the endothelial cell sur face do not appear to contribute to the low intracapillary hematocrit. Analysis of the data indicate that the capillary Fahraeus effect, the network Fahraeus effect in terminal vessels of the arterial tree, and intracapillary events all contribute to the reduction in intracapilla ry hematocrit.