SPECIAL ARTICLE

Abstract

Blood viscosity is dependent on the shear rate at which it is measured. Reversible red cell aggregation is largely a function of fibrinogen-red cell interaction at normal hematocrits. Red cell aggregation is the basis for the anomalous rheological behavior of blood, since a considerable fraction of the shear stress applied to blood flowing at low shear rates is required to break up the aggregates accounting for the greater shear stress required to produce a given shear rate of low magnitude. Erythrocyte sedimentation rate is also a measure of red cell aggregation and has been used as an indicator of the "suspension stability" of blood. The tendency for red cell aggregation is greatest during periods of low flow states and is maximum when the blood is standing still. At such times, a certain shear stress, the yield stress, must be applied to overcome the reversible red cell-fibrinogen bond, causing the blood to "yield" or begin to flow. Manipulation of blood viscosity and yield shear stress, by hemodilution or fibrinogen dilution are readily accomplished, and experimental studies indicate that these rheological alterations have circulatory and metabolic significance.