Hemostasis tissue factor pathway

Blood vessel walls and activated platelets play central roles in primary hemostasis. Damage to a vessel wall initiates vasoconstriction and the exposure of collagen and tissue factor to blood. This exposure initiates coagulation via the tissue factor pathway. 

FIGURE 108-2. Cell-based model of hemostasis. TF, tissue factor TFPI, tissue factor pathway inhibitor vWF, von Willebrand factor. (Reproduced from Roberts HR etal. Molecular biology and biochemistry of the coagulation factors and pathways of hemostasis. In Beutler et al. Williams Hematology, 6th ed. New York, McGraw-Hill, 2001, pp 1409-1434.)... 

Hemostasis begins with the formation of the platelet plug, followed by activation of the clotting cascade, and propagation of the clot. One of the major multicomponent complexes in the coagulation cascade consists of activated factor IX (factor IXa) as the protease, activated factor VIII (factor Villa), calcium, and phospholipids as the cofactors, and factor X as the substrate. Factor IXa can be generated by either factor Xa activation of the intrinsic pathway or by the tissue factor/factor Vila complex. 

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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