 Normal coagulation is initiated by the release of tissue factor from damaged tissue . Tissue factor binds to circulating factor VIIa . The combination activates factor X to factor Xa and factor IX to factor IXa . Factor Xa ( in the presence of factor V ) activates prothrombin into thrombin . Thrombin is a central enzyme in the coagulation process : it generates fibrin from fibrinogen , and activates a number of other enzymes and cofactors ( factor XIII , factor XI , factor V and factor VIII , TAFI ) that enhance the fibrin clot . The process is inhibited by TFPI ( which inactivates the first step catalyzed by factor VIIa / tissue factor ) , antithrombin ( which inactivates thrombin , factor IXa , Xa and XIa ) , protein C ( which inhibits factors Va and VIIIa in the presence of protein S ) , and protein Z ( which inhibits factor Xa ) . 
