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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.
Substances that inhibit or turn off the action of the coagulation factors are activated along with the coagulation factors. These substances function more slowly, over a longer period of time, than the coagulation factors. This allows a clot to form, stopping the bleeding, then, after enough time has passed for the tissue to heal, the clot is dissolved and blood flow restored. The most important coagulation inhibitor is antithrombin III.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
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