Aggregation of human platelets in an annular vortex distal to a tubular expansion.

The aggregation of human platelets was studied in an annular vortex formed downstream of sudden expansions having a diameter ratio, outer to inner tube, of 3.33. Platelet-rich plasma (PRP), washed platelets in Tyrodes-albumin, as well as whole blood were investigated. In the smallest expansion of a 151- into a 504-μm tube, frequent interactions and collisions between the circulating cells were seen to result in the formation and growth of aggregates which migrated to the vortex center and were trapped. The measured rate and extent of aggregation in the vortex were greater in heparinized than in citrated PRP, and lowest in the washed platelet suspensions. These differences were paralleled by similar differences in the ADP- or thrombin-induced reaction in an aggregometer. In a given suspension and in steady flow, platelet aggregation in the vortex was observed only in a narrow range of Reynolds number between 4.5 and 17 and was markedly increased by the addition of subthreshold amounts of ADP or thrombin. In pulsatile flow, there was a significant decrease in the size of aggregates formed—from > 100 to 40 μm diameter, if already present in the suspension, were trapped in the vortex. The results have demonstrated for the first time that within a given range of shear stress in a vortex, platelets can aggregate without the addition of any agents.