In vivo measurements of "apparent viscosity" and microvessel hematocrit in the mesentery of the cat

The arteriovenous (A-V) distribution of microvessel hematocrit (H_micro) was determined throughout successive microvascular divisions in cat mesentery from in vivo measurements of optical density. In vitro correlations of optical density and tube hematocrit in small-bore glass tubes permitted the computation of in vivo values of H_micro for luminal diameters ranging from 20 to 70 μm. For smaller-size vessels, H_micro was determined by microocclusion and red cell counting. The results demonstrate a monotonic fall in the ratio of H_micro H_systemic from 0.80 in the 70-μm arterioles to a minimum of 0.21 in the immediate postcapillaries (10 μm diameter) followed by a subsequent monotonic rise to 0.95 in the 70-μm venules. Conservation of red cell flux throughout the mesenteric network was partially demonstrated upon applying previously established in vitro relationships between discharge and tube hematocrits, the resulting disparity being attributed to the rheological behavior of blood and possible A-V shunting of red cells. Simultaneous measurements of pressure drop and red cell velocity in unbranched arterioles during systemic hemodilution facilitated a comparison of in vivo and in vitro (cone-plate viscometer) apparent viscosities (η). No significant differences between the two approaches were found for arteriole diameters ranging from 24 to 47 μm in the absence of leukocyte-endothelium adhesion, with 0 < H_micro < 36% and _vivo wall shear rates above 500 sec^-1. In situ measurements of η were found to increase 74% due to leukocyte adhesion during sustained flow reductions, in one illustrative case. Values of η computed from the A-V distribution of H_micro and the regression of η vs H_micro (established without leukocyte adhesion), in comparison with previous measurements of the A-V distribution of η with leukocyte adhesion present, suggest that leukocyte adhesion may result in overall increases in η ranging from 100 to 200%, throughout the mesenteric network.