Overview Platelet plug formation Always a balance between pro-aggregation and anti-aggregation forces Formation of a platelet plug occurs in four main stages Adhesion platelet Gp1b receptor binding to exposed subendothelial collagen login to view 1 more bullet Aggregation vasoconstriction results in decreased blood flow login to view 2 more bullets PGI2 and NO released by endothelial cells are natural aggregation inhibitors by vasodilating vessels Swelling inter-platelet binding via fibrinogen cross links to GpIIb/IIIa receptors on platelet membrane login to view 1 more bullet once this step happens the bleeding stops and is the end of the "bleeding time" test login to view 1 more bullet Maturation fibrinogen is converted to fibrin with cross-linking to strengthen plug Ca2+ also strengthens platelet plug