Hemostasis blood clotting system

Platelets play a role in each of the mechanisms of normal hemostasis vasoconstriction, formation of the platelet plug, and blood coagulation. However, they are also involved in pathological processes that lead to atherosclerosis and thrombosis (formation of a blood clot within the vascular system). Antiplatelet drugs interfere with platelet function and are used to prevent the development of atherosclerosis and formation of arterial thrombi. 

Self-protection. To prevent blood loss when a vessel is injured, the blood has systems for stanching blood flow and coagulating the blood (hemostasis see p. 290). The dissolution of blood clots (fibrinolysis) is also managed by the blood itself (see p. 292). 

Little intravascular coagulation of blood occurs in normal physiological conditions. Hemostasis involves the interplay of three procoagulant phases vascular, platelet, and coagulation) that promote blood clotting to prevent blood loss (Fig. 22.1). The fibrinolytic system prevents propagation of clotting beyond the site of vascular injury and is involved in clot dissolution, or lysis (Fig. 22.2). 

Normal hemostasis is a balance between excessive and inadequate blood clotting. Overactive blood clotting is harmful because of the tendency for thrombus formation and occlusion of arteries and veins. Vessels may become directly blocked by the thrombus, or a portion of the thrombus may break off and create an embolism that lodges elsewhere in the vascular system. The tendency for excessive thrombus formation in the venous system is usually treated with anticoagulant drugs such as heparin and warfarin. Platelet inhibitors such as aspirin help prevent arterial thrombogenesis. Thrombolytic drugs (streptokinase, t-PA) that facilitate the dissolution of harmful clots may successfully reopen... 

Laboratory tests can identify a faulty component of clotting or whether an elderly patient is required to alter coumarin drug or vitamin K dosages before a tooth is extracted. Samples of the patient s blood are taken and treated in various ways to allow the observation of the appropriate steps in the hemostasis pathways. For example, tissue factor is added to the blood sample for the prothrombin time (PT) test. Alternatively, certain clotting system components in the blood sample are removed or inactivated. These tests can be run quickly on small samples of blood. 

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