There are the intrinsic and extrinsic pathways in clotting. The intrinsic pathway is initiated when blood comes in contact with damaged endothelium or collagen, and involves clotting factors XII, XI, IX, and VIII. The extrinsic pathway is activated when being exposed to tissue factor from tissue injury or the addition of thromboplastin to blood, and involves clotting factor VII. The two pathways meet at the point of clotting factor X activation to lead the final common pathway. From here, factor X is converted to prothrombin, prothrombin to thrombin, thrombin to fibrinogen, fibrinogen to fibrin, and finally fibrin to fibrin clot. Platelets, activated by thrombin, adhere to the damaged endothelium wall or collagen to form a plug. At the same time, they activate clotting factors VII and X. More platelets are stimulated by fibrin clots, resulting in reinforcing the formed clots.