The aggregation of blood platelets

Inthe circulating bloodtheplatelets are normally carried along separately from each other and there is no evidence that they adhere to normal vascular endothelium. When the wall of a blood vessel is injured platelets adhere to it immediately (Hugues, 1953) and to each other to form aggregates on the damaged intima (Bizzozero, 1882). When the vessel wall is broken aggregates of platelets tend to seal the opening and so help to arrest bleeding (Fulton, Akers & Lutz, 1953); such aggregates have been called 'haemostatic plugs' (Roskam, 1954). When diseases, for example, atherosclerosis, damage the intima platelets again adhere to it and to each other. The aggregates so formed are called 'platelet or white thrombi', and they may increase in size until they block the flow of blood. The aggregation of platelets is, therefore, one of the immediate causes of thrombosis. Why damage to the intima should cause platelets to adhere and to aggregate is still unknown. This paper describes a method by which the aggregation of platelets may be followed quantitatively, and results obtained with the method are described; some of these have been briefly reported earlier (Born, 1962a, b; Born & Cross 1963).