Hemostasis is a series of rapid, defensive reactions triggered to stop excessive bleeding and promote healing after a vascular injury. It is a highly localized response governed by clotting factors and platelets that commences only at the injury site. Injury or trauma sets in the vascular phase, where the blood vessels quickly constrict to reduce blood flow. The platelet phase follows vasoconstriction, where the collagen from the damaged endothelial cell lining initiates platelet adhesion. These localized platelets are activated and release several compounds to attract more platelets, forming a loose platelet plug at the injury site. This process is termed platelet aggregation. Additionally, platelets also release clotting factors involved in the reactions of the final coagulation phase. The clotting factors convert the plasma protein fibrinogen into insoluble fibrin strands, forming a meshwork that solidifies the platelet plug into a stable clot.