The Pressure-Flow Relation for Plasma in Whole Organ Skeletal Muscle and Its Experimental Verification

Abstract

The whole-organ pressure-flow relation in resting rat skeletal muscle is examined for the flow of plasma. Due to the small size of the blood vessels in this organ, inertia and convective forces in the blood are negligible and viscous forces dominate. Direct measurements in the past have shown that skeletal muscle blood vessels are distensible. Theoretical formulations based on these measurements lead to a third order polynomial model for the pressure-flow relation. The purpose of the current study is to examine this relation experimentally in an isolated muscle organ. A high precision feedback controlled pump is used to perfuse artificial plasma into the vasodilated rat gracilis muscle. The results indicate that the pressure-flow curve in this tissue is nonlinear in the low flow region and almost linear at physiological flow rates, following closely the third order polynomial function. Vessel fixation with glutaraldehyde causes the curves to become linear at all pressures, indicating that vessel distention is the primary mechanism causing the nonlinearity. Furthermore, the resistance of the post-fixed tissue is determined by the pressure at which the fixative is perfused. At fixation pressures below 10 mmHg, the resistance is three times higher than in vessels fixed at normal physiological pressures. Dextran (229,000 Dalton) is used to obtain Newtonian perfusates at different viscosities. The pressure-flow relation is found to be linearly dependent on viscosity for all flow rates. Skeletal muscle has multiple arterial inflows. Separate perfusion of the two major arterial feeders in the rat gracilis muscle show that for low pressures the flow at each feeder is dependent on the pressure at the opposite feeder, whereas at normal pressures the flow becomes independent of the opposite feeder pressure. The hemodynamic resistance in plasma perfused vasodilated skeletal muscle depends on vessel distensibility, plasma viscosity, and can be closely modeled by a third order polynomial relation.