The extrinsic system.

Factor VII appears to be the key regulatory protein in the initiation of both the intrinsic and extrinsic systems of coagulation. The single chain, or zymogen form, of factor VII possesses enzymatic activity which makes it an ideal candidate for the initiation of coagulation following vascular injury. A number of interactions between the intrinsic and extrinsic systems of coagulation have been identified. It appears that factor VII is capable of directly activating factor IX and vice versa. The study of factor VII variants with associated thromboembolic complications may provide a number of answers regarding the initiation and regulation of the blood coagulation process.