The extrinsic and intrinsic pathways of coagulation involve reaction cascades of clotting factors. They merge into the common pathway, eventually forming a fibrin clot. Any trauma to the blood vessel wall, which causes blood to exit the vascular system into peripheral tissue, initiates the extrinsic pathway. As the blood oozes into the injured tissue, it is exposed to tissue factor or thromboplastin, a mixture of lipoproteins and phospholipids released from the damaged cells. The tissue factor combines calcium and circulating Factor VII to form the tissue factor-Factor VII complex, which activates Factor X. On the other hand, internal trauma, such as the blood vessel's damaged endothelial lining, activates the intrinsic pathway in the bloodstream. This pathway involves a series of linked reactions assisted by platelet factor 3 and calcium to activate Factors VIII and IX, which combine to form an enzyme complex to activate Factor X. The activated Factor X from either or both pathways activates the prothrombin activator, initiating the common pathway to form insoluble fibrin, creating a solid blood clot