The viscosity and viscoelasticity of blood in small diameter tubes

Abstract

Both steady and oscillatory flow of blood are studied in small rigid tubes having radii from 0.02 to 0.2 cm. This is done for rates of flow extending from higher values where nonlinear effects are evident down to very low values where the pressure to flow relations are linear. The data are analyzed using the parameters of Poiseuille theory for steady tube flow and of the linear, viscoelastic theory for oscillatory tube flow. In the low flow, linear region the apparent values of the steady flow viscosity and the oscillatory flow viscosity at 2 Hz are dependent upon the tube radius, this being in contraction with the assumptions of the theories. Another theoretical analysis is then made assuming that the blood in a boundary zone at the tube wall has modified viscous and viscoelastic properties. The measurements are in good agreement with this analysis. For low rates of flow, the steady flow is pluglike, the blood in the core moving as a solid. The pressure to flow relation is then controlled by the boundary zone for all tube radii. In this case of oscillatory flow, the core undergoes viscoelastic deformation and thus flow occurs in both the core and the boundary zone. For larger tubes, the boundary zone effects became insignificant. Under this condition the oscillatory pressure to flow relation may be used to obtain the viscoelasticity of the blood, free from the boundary layer artifacts which dominate the steady flow.