Rapidly recovered transient flow resistance: a newly discovered property of blood

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Abstract
Although blood flows in a pulsatile fashion, little consideration has been given in past studies to its instantaneous resistance to motion when onset and cessation of flow occur abruptly. Hemorheological studies have documented three kinds of blood flow properties. 1) Shear thinning is a fall in viscosity as shear rate rises. 2) Viscoelasticity is a transient shear stress variation due to elastic deformation of erythrocytes. Dilatancy is a viscoelasticity-modifying property attributed to high shear rate erythrocyte rigidity; viscoelasticity is prominent only at low shear rate. 3) Thixotropy is an initial extra flow resistance linked to developing orientation and disaggregation of erythrocytes. Thixotropy returns fully to blood over a period longer than 1 min. Measurements utilizing a fast response Couette viscometer have revealed an extra 10% transient flow resistance after a flow cessation shorter than that between heart beats. The rapidly recovered transient flow resistance has a temporal pattern similar to thixotropy. Its peak and duration are directly related to total shear strain (shear rate .times. time) over the 8-30 s-1 shear rate range studied. Transient behavior was essentially identical in analyses carried out using three different viscometer gaps. Numerical simulation to test the effect of the newly observed transient behavior on sudden onset tube flow shows that the developing pattern of pulsatile arterial flow can be affected by its presence.