Historical models of coagulation. (A) Morawitz classical theory demonstrating prothrombin (FII) activation to thrombin (FIIa) by thrombokinase (TK) in the presence of calcium. Thrombin converts fibrinogen (FGN) into a fibrin (FIB) clot. (B) The cascade or waterfall model, illustrating sequential enzyme activation initiated by extrinsic and intrinsic triggers, which merge into the common pathway at FX to generate thrombin. (C) The cell-based model. The TFase and phosphatidylserine (PS) surfaces coordinate enzymology in 3 overlapping stages to generate a thrombin surge required for clot formation. The figure highlights how each concept builds on preexisting models to advance coagulation understanding.