Effect of Systemic Hematocrit on Blood Velocity Profiles in Arterioles

Abstract

Systemic hematocrit (Hctsys) is an important determinant of cardiac output and flow in the peripheral circulation. The cell free layer, by virtue of its location at the interface between the wall and the blood stream, very likely plays a key role in this relationship. We aimed to determine blood's velocity profile in arterioles with an emphasis on the hemodynamics near vessel walls at different Hctsys. The velocity profiles of 0.5 μm fluorescent labeled microspheres were assessed in rat cremaster arterioles (~35 μm) during systemic hemodilution and hemoconcentration with high speed video intravital microscopy. Hctsys was increased by 5% from 40 to 75% or decreased by 5% from 40 to 20%. Velocity was determined using single frame analysis of multiple images obtained with a gated imaging system. Velocities were filtered, normalized, and fit to a linear regression to calculate the parabolic coefficient (K). The data showed a positive correlation between Hct and the K value, where increased Hctsys resulted in blunted velocity profiles (K>2) across the flow stream. Analysis of profiles indicates a velocity discontinuity near the vessel wall, plasma only boundary layer and a microsphere free layer near the wall. These results confirm the presence of a finite size structure between the vascular endothelium and the flowing plasma, suggesting a hemodynamic function for the endothelium glycocalyx.