Reduced to its simplest form, the vascular system is a series of interconnected biologic conduits filled with fluid moving under pressure. This system is subject to injury and must repair itself in order to avoid exsanguination. This latter function is provided by the hemostatic system. Thus, normal hemostasis is a reparative process and consists of three major mechanisms: (1) vasospasm, which is the main mechanism for controlling bleeding after transection/avulsion injuries of large arteries, arterioles, and veins; (2) platelet plug formation; and (3) the procoagulant system. The procoagulant system consists of the mechanisms by which fluid blood is converted into an insoluble hemostatic thrombus. While inherited or acquired disorders of all three mechanisms can cause clinically abnormal bleeding, this chapter reviews the molecular basis of inherited disorders of the procoagulant system (e. g., hemophilia A and B, von Willebrand disease). Inherited vascular and platelet disorders associated with abnormal bleeding are not discussed here.
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