Resistance to blood flow in microvessels in vivo.

TLDR

Blood flow resistance in peripheral vascular beds, governed by blood rheology in microvessels, strongly influences cardiovascular function and tissue transport. The study introduces a new method to calculate how blood rheology contributes to microvascular flow resistance. The method involved measuring vessel morphology, velocity, and hematocrit in rat mesenteric microcirculation via intravital microscopy, fitting a mathematical model that optimizes resistance dependence on diameter, hematocrit, and shear rate, and validating it with pressure–flow measurements across varied hematocrits. For vessels below ~40 µm, flow resistance is markedly higher and more hematocrit‑dependent than glass‑tube estimates, with 10‑µm vessels showing resistance four times that of glass tubes; these results, confirmed by pressure–flow experiments, suggest that interactions between blood components and vessel walls elevate resistance in small microvessels.

Abstract

Resistance to blood flow through peripheral vascular beds strongly influences cardiovascular function and transport to tissue. For a given vascular architecture, flow resistance is determined by the rheological behavior of blood flowing through microvessels. A new approach for calculating the contribution of blood rheology to microvascular flow resistance is presented. Morphology (diameter and length), flow velocity, hematocrit, and topological position were determined for all vessel segments (up to 913) of terminal microcirculatory networks in the rat mesentery by intravital microscopy. Flow velocity and hematocrit were also predicted from mathematical flow simulations, in which the assumed dependence of flow resistance on diameter, hematocrit, and shear rate was optimized to minimize the deviation between measured and predicted values. For microvessels with diameters below approximately 40 microns, the resulting flow resistances are markedly higher and show a stronger dependence on hematocrit than previously estimated from measurements of blood flow in narrow glass tubes. For example, flow resistance in 10-microns microvessels at normal hematocrit is found to exceed that of a corresponding glass tube by a factor of approximately 4. In separate experiments, flow resistance of microvascular networks was estimated from direct measurements of total pressure drop and volume flow, at systemic hematocrits intentionally varied from 0.08 to 0.68. The results agree closely with predictions based on the above-optimized resistance but not with predictions based on glass-tube data. The unexpectedly high flow resistance in small microvessels may be related to interactions between blood components and the inner vessel surface that do not occur in smooth-walled tubes.