The Complex Chain of Coagulation Reactions A balance normally exists between the factors that stimulate formation of thrombin and forces acting to delay thrombin formation. This balance maintains circulating blood as a fluid. When injury occurs or blood is removed from a vessel, this balance is upset, and coagulation occurs. Blood clotting involves four progressive stages. The Roman numerals assigned to the coagulation factors identify their order of discovery rather than their involvement in the stages of clot formation.
| Stage | Components of Stages |
|---|---|
| Stage I (3–5 min) | |
| Phase I—platelet activity; platelets serve as a source of thromboplastin. | 90% of all coagulation disorders are caused by defects in phase I. |
| Phase II—thromboplastin; factor III, an enzyme thought to be liberated by damaged cells, is formed by five different factors plus calcium. | Components involved in the formation of tissue thromboplastin (intrinsic prothrombin activation) include calcium and Factors V, VII, IX, X and XI. |
| Stage II (8–15 s) | |
| Prothrombin factor II is converted to thrombin in the presence of calcium. | Factors II, X, VII, and V are involved in the conversion of fibrinogen to fibrin. |
| Stage III (1 s) | |
| Thrombin interacts with fibrinogen (factor I) to form the framework of the clot. | At the end of stage III, factor XIII functions in the stabilization of the clot. |
| Stage IV | |
| Fibrinolytic system (antagonistic check-and balance to the clotting mechanism) is activated. | Removal of fibrin clot through fibrinolysis. Plasminogen is converted to plasmin, which breaks clot into fibrin split products. |