STRUCTURE AND HEMODYNAMICS OF MICROVASCULAR NETWORKS - HETEROGENEITY AND CORRELATIONS

The objective of this study was to quantify the heterogeneity of topol ogical, morphological, and hemodynamic parameters in microvascular net works and to identify functionally relevant correlations among these p arameters. Seven networks in the rat mesentery (383-983 vessel segment s per network) were examined, and measurements were made of segment ge neration, diameter, length, and hematocrit in all segments (n = 3,129) and of flow velocity (only in 3 networks, 1,321 segments). In additio n, hematocrit, flow rate, and pressure were derived for all segments f rom a mathematical simulation. All parameters obtained exhibit heterog eneous distributions with coefficients of variation ranging from 0.28 (capillary diameter) to > 1.5 (volume flow and pressure gradient). Sev eral strong correlations exist between parameters, e.g., discharge hem atocrit increases with vessel diameter, and shear rate increases with intravascular pressure. Because of such correlations, the extrapolatio n from average values for ''typical vessels'' to network properties ca n lead to substantial errors. For example, the mean network transit ti me estimated based on averaged quantities is 6.5 s, which is about 60% higher than the true value (4.08 s). Simplified models of the vascula r bed may therefore be inadequate to describe functional properties of the microcirculation.