Phenomena that occur when blood contacts a foreign surface are controlled by transport processes in the bloodstream immediately above the surface and by reactions at the surface itself. All constituents of blood that attach to a surface may leave the surface and be replaced by other constituents. Some of the constituents may attach in superposition. This turnover of constituents at the surface is influenced by the nature of the substrate and the shear stress between the blood and the surface. The turnover process is easy to demonstrate with platelets. When platelets depart from a surface, the site they vacate has a higher attractiveness for adhesion of other platelets than the surrounding surface that has not had a cell attached to it, but this enhanced attractiveness diminishes in a few minutes if a new occupant does not occupy the site. As data accumulate, it is beginning to appear that platelet turnover differs between different blood donors but remains relatively constant for each individual unless dietary or pharmacological changes that affect the turnover rate are introduced.