Coagulation involves a cascade of enzymatic reactions involving coagulation factors, several of which are serine-active proteases at the active site and subjected to activations and inhibitions. The final step is the transformation of soluble fibrinogen into fibrin filaments that encircle circulating cells in their meshes. The factors of coagulation are designated by numbers from I to XIII. With the exception of the factor XIII which intervenes in the last stage of the coagulation.
Coagulation involves two pathways, one intrinsic, the other extrinsic, leading to a common final pathway.
The intrinsic pathway involves the factors present in the circulation (which are factor XII, IX, and XI, they are triggered in that order. and also cofactor VIII which helps to activate the factor IX).
The extrinsic pathway involves the tissue factors not normally present in the circulation but which are released during a vascular lesion (thromboplastin (factor III) and factor VII).
Factor Xa is the meeting point of the intrinsic and extrinsic pathways. The Xa, Va, Ca2 + set and a platelet phospholipid are sometimes called prothrombinase (or prothrombin activator).