Special Considerations

Coagulation results from a sequence (cascade) of reactions involving coagulation factors, for example, factor I, factor II. Some of these factors have other names, for example, Factor I (fibrinogen), Factor II (prothrombin), and Factor XII (Hageman factor). These proteins are produced in the liver and secreted into the blood. Some of the Factors (that is, II, VII, IX, and X) require Vitamin K for their synthesis. Warfarin (Coumadin) is a commonly-used anticoagulant drug. It acts by inhibiting the Vitamin K-requiring enzyme in the liver, which prevents formation of some of the clotting factors, thereby inhibiting coagulation.

When clotting factors contact damaged tissue, the coagulation sequence is initiated. Each clotting factor reaction triggers the next reaction. The final product of the cascade is the fibrin clot (blood clot).

Factor X can be activated by 2 separate sequences of chemical reactions. The factors involved in the 2 sequences are referred to as the intrinsic system and the extrinsic system. The intrinsic system involves activation of Hageman factors by tissue not normally in contact with blood, followed by sequential activation of factors XI, IX, and X, in the presence of factor VIII. The extrinsic system involves activation of factor VII by thromboplastin (also called tissue factor), which is a protein released from the membranes of damaged tissues.

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