factor,blood clotting
Any of a group of protein factors in the blood serum that act according to a defi ned pathway to produce a blood clot. In blood clotting, the breakdown of platelets at the wound site is the fi rst step. Factors VII, VIII, IX, and XI become activated by tissue factor and, in the presence of calcium, convert factor X to activated thromboplastin. Thromboplastin then converts prothrombin into thrombin, the clotting enzyme. Thrombin catalyzes the conversion of soluble fi brinogen in the serum into the insoluble protein fi brin. Fibers made of fi brin are the basic structure of the fi nal clot and are made fi rm by factor XIII, the fi brin-stabilizing factor. The genetic disease, hemophilia, is the result of a defect in the gene that codes for one of these factors (factor VIII).