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Coagulation (blood clotting) results from a sequence of reactions involving several proteins known as coagulation factors. Some of these factors have other names. For example, Factor I is also called fibrinogen, Factor II is prothrombin, and Factor XII is Hageman factor. The liver produces these proteins and secretes them into the blood. In addition, vitamin K is important to blood clotting because your body converts it into prothrombin.
Some people take warfarin to keep their blood from clotting. Warfarin inhibits prothrombin, thus interrupting the clotting cascade. Because of the link between vitamin K and prothrombin, people who take warfarin need to have consistent levels of vitamin K in their diet, as instructed by their doctor.
Coagulation begins when some of the coagulation factors contact damaged tissue. Each factor reaction triggers the next reaction, in a cascade. The final product of the coagulation cascade is the blood clot.
Factor X can be activated by two separate sequences of chemical reactions. The factors involved in the two sequences are referred to as the intrinsic system and the extrinsic system.
The intrinsic system involves activation of Hageman factor by tissue not normally in contact with blood, followed by sequential activation of factors XI, IX, and X, in the presence of factor VIII.
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