On the shear rate dependence of red cell aggregation in vitro

Non-Newtonian viscosity of blood, i.e., the rise in apparent viscosity at low flow, was believed to be a result of reversible aggregation of red cells at low velocity gradients (shear rate). By making a cone-plate viscometer transparent, direct observation was made possible of the blood flowing under defined shear rates. Red cell aggregates, occurring in all cases at low flow, were reversibly dispersed by increasing the shear rate. This behavior was independent of the addition of anticoagulants, but it could be altered by changing the plasma protein composition. Red cells in serum did not form aggregates; such nonaggregating samples did show an increase in viscosity at low shear rates. Since the sedimentation rate can be influenced by many parameters, it is not reliable in describing red cell aggregation. Aggregation of red cells is linked with a marked separation of plasma and cells. Such a separation is of considerable influence on cone-plate viscometry.