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The normal vascular system maintains a delicate balance between clotting mechanisms and clot lysis, with the result that internal or external bleeding is controlled and pathologic thrombosis prevented (see Chapter 9: Abnormalities of Blood Supply).

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Several factors acting in concert maintain this equilibrium:

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  1.  Blood vessels maintain anatomic integrity and a smooth endothelium. Following minor injuries, the blood vessels undergo vasoconstriction, thereby preventing bleeding from the injured site and permitting repair. Vasoconstriction is an effective method of hemostasis in small vessel injuries but is not adequate when large vessels are damaged.

  2.  Platelets form the initial hemostatic plug that seals a site of vascular injury. Platelets also play a major role in forming the permanent thrombus that seals the injury.

  3.  Blood coagulation is the formation of fibrin from plasma precursors. Fibrin and platelets constitute the permanent hemostatic plug.

  4.  Fibrinolysis is the production of factors such as plasmin from plasma precursors that lyse and remove fibrin thrombi which have formed in the circulation.

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Disturbance of any one of these mechanisms may produce abnormal bleeding on the one hand (Table 27-1) or abnormal thrombosis on the other. Pathologic thrombosis is discussed in Chapter 9: Abnormalities of Blood Supply.

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Table 27–1. Hemorrhagic Disorders: Principal Causes.
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The clinical effects of abnormal bleeding disorders (Table 27-2) are similar regardless of the mechanisms. Laboratory testing is generally required to reach a precise clinical diagnosis, following which appropriate therapy may be selected.

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Table 27–2. Common Clinical Manifestations of Hemorrhagic Disorders.
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Along with thrombocytopenia, vascular defects represent the most common cause of bleeding diathesis. In certain vascular disorders, the underlying defect relates to production of abnormal collagen or elastin (Table 27-1); in vasculitis, inflammation ...

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