Plasma coagulation protease reactions. Scheme for the cascade/waterfall model of thrombin generation triggered by activation of FXII. Conversion of FXI to FXIa is shown in red. A mechanism for initiation of coagulation by the FVIIa/TF complex through FX activation is also shown. Roman numerals indicate unactivated coagulation factors, and activated factors are indicated by “a.” Nonenzymatic cofactors are indicated by numerals in black ovals. The dotted line designated “1” indicates feedback activation of FXI by thrombin, whereas the dotted line indicated by “2” represents activation of FIX by FVIIa/TF.