Effect of Hematocrit and Inertial Losses on Pressure-Flow Relations in the Isolated Hindpaw of the Dog

Abstract

The effect of hematocrit on pressure-flow relations was studied in the perfused isolated hindpaw of the dog. Blood and suspensions of erythrocytes (RBC) in albumin-Ringer solution, hematocrit range of 0 to 80%, were used as perfusates. Flow resistance was found to increase markedly with hematocrit. No significant difference was found between pressure-flow data obtained with blood and RBC in albumin-Ringer solution when comparisons were made at comparable hematocrits, indicating negligible effect of RBC aggregation when blood was the perfusion fluid. Computations of flow resistance relative to cellfree perfusate indicated that non-Newtonian viscosity, as well as vessel distensibility, contributed to the nonlinearity of pressure-flow curves. Perfusion of paws with two Newtonian fluids of different viscosities allowed computation of inertial pressure losses under conditions of steady flow. At physiological perfusion pressures, inertial losses accounted for about 40% of the total pressure drop for cell-free albumin-Ringer solution, and dropped to about 5% for a hematocrit of 50. The results suggest that inertial losses, rather than a Fåhraeus-Lindqvist effect, play an important role in pressure-flow relations in the hindpaw.