Adhesion of human platelets to collagen on the walls distal to a tubular expansion.

The adhesion of washed human platelets to collagen fibers on glass was measured in, and downstream of, an annular vortex formed at the sudden expansion of an 0.917- into a 3.00-mm tube at Reynolds numbers Re between 28 and 113. Washed human platelets in Tyrodes-albumin containing apyrase and red cells were used. Hematocrits varied from 0 to 50% and platelet concentrations from 1 to 5 × 103 μl−1. After flow for 1 or 3 min, the adhering platelets were fixed, stained, and counted under a high-power microscope. At all but the highest Re, there was a pronounced peak in platelet number density N in the vortex, followed by a minimum at the reattachment point, and a smaller secondary peak downstream, after which N asymptoted to a steady value. At Re = 38, N increased 6× as the hematocrit increased from 0 to 40%, and at 20% hematocrit, N increased 3.3 × as the platelet concentration rose from 1 to 5 × 103 μl−1. The asymptotic N was almost independent of Re, remaining at 0.52 and 2.5 cells/103 μm2 at 0 and 20% hematocrit, respectively. In the vortex, however, the mean platelet adhesion decreased by 32% over the same range of Re, accompanied by a 54% lowering of the peak N. In pulsatile flow at the same mean Re, the minimum and secondary peaks were almost levelled off. The primary peak remained and, as in steady flow, decreased in value with increasing mean Re. The results showed that both diffusion and surface reaction rate control the initial rate of adhesion, and that the adhesion peaks on either side of the reattachment point may be explained by curvature of the streamlines carrying platelets to the wall.