Overview 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 mediated by vWF Aggregation vasoconstriction results in decreased blood flow mediated by activated platelets which synthesize TxA2 by cyclooxygenase I enzymes COXI inhibited by aspirin which results in ↓ aggregation 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 GpIIb/IIIa inserted on membrane by binding of ADP to platelet ADP receptors once this step happens the bleeding stops and is the end of the "bleeding time" test although clot has formed, it is unstable and easily disrupted Maturation fibrinogen is converted to fibrin with cross-linking to strengthen plug Ca2+ also strengthens platelet plug