**5. The common pathway**

The intrinsic and extrinsic pathways both lead to activation of FX. FXa, FVa and Ca2+ activate prothrombin (FII) to form thrombin (FIIa). Thrombin has several actions: it activates fibrinogen (FI) to yield fibrin monomers; it amplifies the coagulation process by activating further prothrombin, FV and FVIII; it activates FXIII whose active form promotes cross-linking of fibrin monomers to form stable polymers; (as discussed later in the cell-based model, it also participates in processes that temper coagulation to avoid overwhelming pathological thrombosis.) The common pathway thus yields a mesh of covalently cross-linked fibrin strands and entrapped red blood cells, which form a stable blood clot. Disorders of the common pathway will affect tests of the intrinsic and extrinsic pathways.
