Diameter variability and microvascular flow resistance

Microvessels are known to exhibit irregular shapes, deviating substantially from an idealized cylindrical tube geometry. Such irregularities must be taken into account in calculating microvascular flow resistance and may add to the observation that flow resistance in living microvessels in vivo is about twice that predicted on the basis of tube flow studies in vitro. The present study was aimed at providing a comprehensive database describing the apparent diameter variability for all segments of a complete microvascular network in the rat mesentery and assessing the impact of this variability on segmental flow resistance and the pressure drop across the network. Diameters were estimated by intravital microscopy at axial intervals of 20 mu m along the 546 vessel segments of a mesenteric microvessel network, resulting in 6,319 separate diameter measurements. The amplitude of diameter variations in individual vessel segments decreased from similar to 15% of the mean vessel diameter in the smallest segments (similar to 5 mu m diam) to similar to 5% in the largest segments (similar to 60 mu m diam). Segmental hindrance was estimated to be 10-23% higher than calculated from arithmetic mean diameter, depending on the model used to estimate the hydrodynamically effective segment diameter. The overall pressure drop across the network calculated using a mathematical flow simulation was increased by 7-13.5%. This increase in flow resistance can explain similar to 10% of the observed discrepancy between flow resistance in vivo and in vitro.