Platelets adhere to collagen on the injured basement membrane. This initiates a series of changes in the platelets themselves, including a change in shape and release of ADP and other substances. ADP release stimulates further platelet aggregation. The coagulation cascade is triggered off, and the action of thrombin leads to the formation of fibrin, which converts the loose platelet plug into a firm, stable wound seal. The role of platelets is less important in large vessel injury because platelet aggregates are of insufficient strength and size to seal the defect. The coagulation system along with vascular contraction is important here.
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