Introduction to Hemostasis

Hemostasis is a complex physiologic process that keeps circulating blood in a fluid state and then, when an injury occurs, produces a clot to stop the bleeding, confines the clot to the site of injury, and finally dissolves the clot as the wound heals. When hemostasis systems are out of balance, hemorrhage (bleeding) or thrombosis (pathological clotting) can be life-threatening. Hemostasis involves the interaction of vasoconstriction, platelet adhesion, and aggregation, and coagulation enzyme activation to stop bleeding. Primary hemostasis refers to the role of blood vessels and platelets in response to a vascular injury. Platelets become activated, adhere to the site of injury, secrete the contents of their granules, and aggregate with other platelets to form a platelet plug. Vasoconstriction and platelet plug formation comprises the initial, rapid, short-lived response to vessel damage, but to control major bleeding in the long term, the plug must be reinforced by fibrin. Secondary hemostasis describes the activation of a series of coagulation proteins in the plasma.