Mechanisms of thrombus formation and lysis.

Maintenance of a patent vasculature is critical to provide nutrient blood flow to dependent tissues. This is normally facilitated by vessels composed of actively nonthrombogenic endothelium and blood that contains both nonactivated platelets and inactive coagulation proenzymes. Following vessel injury, active hemostasis results from vasoconstriction, adherence and aggregation of activated platelets, and coagulation enzyme activation. The resulting thrombus contains a mixture of blood cells and the insoluble product of coagulation, fibrin, which further stimulates the activation of another blood enzyme system known as the fibrinolytic system. This results in the conversion by plasminogen activators of the circulating plasma proenzyme plasminogen into the active fibrinolytic enzyme plasmin, which dissolves the clot into degradation products. Vascular patency ultimately is restored and healing is thus facilitated. The hemostatic system is highly regulated by a variety of processes. Derangement of regulation can lead to disease manifesting either as thrombosis or hemorrhage. Furthermore, improved understanding of the molecular interactions involved in these processes has led to design of newer therapeutic interventions targeted toward amelioration of the sequelae of thromboembolic disease.