During which stage do platelets form a plug?

Platelets form a platelet plug during the aggregation stage of hemostasis. Hemostasis proceeds through a sequence of events: first, vasoconstriction reduces blood flow to the injured area; then, platelet adhesion occurs when platelets bind to exposed subendothelial collagen via von Willebrand factor; next, platelet activation occurs as adhered platelets release ADP, thromboxane A2, and other mediators from their granules; finally, aggregation takes place when these chemical signals recruit additional platelets from the circulation to pile onto the site of injury, forming the primary platelet plug. Vasoconstriction is the initial mechanical response that limits blood loss but does not itself form the plug. Adhesion is the step where individual platelets first attach to the injured vessel wall but have not yet accumulated into a plug. Activation is the step where platelets change shape and release granule contents, which are necessary prerequisites to aggregation but do not constitute plug formation themselves. Fibrinolysis is the later phase in which plasmin dissolves the fibrin clot after healing has occurred, the opposite of plug formation. Understanding the sequential stages — vasoconstriction, adhesion, activation, aggregation, and coagulation — helps students explain how antiplatelet drugs like aspirin (which inhibit thromboxane A2) interfere specifically with platelet aggregation.