The elastic yield stress of human blood.

The elastic yield stress is a key parameter in the way human blood flows, because after yielding the microstructure formed by the red cells undergoes a dramatic change and the blood is transformed to a superfluid state. This yielding of the structure is revealed in measurements of the progressive changes in viscoelasticity with increasing amplitude of oscillatory flow. Measurements of the magnitude and phase of the pressure differential along the tube length, and the volume flow through the tube, are performed with sinusoidal oscillatory flow in a cylindrical tube at a frequency near that of the normal pulse rate. This gives the shear rate, shear strain, and the viscous and elastic components of shear stress at the tube wall, and from these parameters the viscosity and elasticity of the blood are calculated. The instrumentation (Vilastic Scientific Inc.) used to perform these measurements produces graphs of the viscous and elastic shear stress versus the shear strain. Near unit strain the elastic stress reaches a maximum, marking the point at which the quiescent aggregated cell structure yields and reassembles into a layered structure. Examples are given to show how factors such as red cell aggregation and cell deformability affect the elastic yield stress.