Physical factors governing the aggregation of human platelets in sheared suspensions

The effect of shear rate on the ADP-induced aggregation of human blood platelets in flow through tubes was studied over the full physiologically significant range. The extent of single platelet aggregation at 0.2 $ mu$M ADP in citrated platelet-rich plasma, PRP, was greatest at mean tube shear rate, G = 314 s$ sp{-1}$; however, aggregate size steadily decreased from G = 39.3 to 1800 s$ sp{-1}$. At 1.0 $ mu$M ADP the rate of aggregation increased up to G = 1800 s$ sp{-1}$ where virtually no unaggregated platelets remained after 43 s of flow, although, aggregate size was still limited by shear rate. A shear-dependent delay in the onset of aggregation and an increase in collision efficiency with time suggest the existence of a time and shear-dependency in the expression of bonds mediating aggregation. Greater aggregation of platelets from female donors than male donors was due to differences in the ionized calcium concentration, (Ca$ sp{2+}$), in the plasma of donors of different hematocrit when the chelating agent citrate is used as anticoagulant. At physiological (Ca$ sp{2+}$) aggregation was much greater in heparinized and hirudinized plasma than in citrated plasma and no sex difference was present. Aggregation in whole blood was much greater than in PRP due to a shear-dependent increase in the frequency of collision between activated platelets caused by the motion of red cells