Elastic effects in pulsatile blood flow

Abstract

The persistence of pulsation into the microcirculation raises questions concerning the roles of the steady and oscillatory pressure components in determining blood flow. To resolve these questions, measurements have been made on precisely controlled flows in small, rigid tubes. Principal measurements are with 46% hematocrit human blood in an 0.086 cm diam, tube for steady flow, sinusoidal oscillatory flow at 2 Hz, and for the superposition of these to produce pulsatile flow. For steady flow the pressure is linearly related to the volume flow for low flows, and is followed by a nonlinear relation at higher stress levels. For oscillatory flow, the pressure is resolved into a component in phase with the volume flow (an energy dissipative component) and a one in quadrature (a recoverable energy component). For the purpose of comparison, measurements were also made with a Newtonian, viscous glycerol solution. With glycerol solution, both pressure components are linearly proportional to the volume flow amplitude, the quadrature component being inertial-like. However, with blood, both components exhibit nonlinear effects and at small flow amplitudes the quadrature component is elastic, i.e., spring-like. With increasing amplitude, this elastic component vanishes, then becomes inertial-like. These effects are moderated in pulsatile flow by the balance between the steady and oscillatory flow components. This pressure-flow behavior is explained by the viscoelastic property of blood and the shear degradation of RBC aggregates.