Lipoproteinaceous, thromboplastin is released by the injured tissue. This reacts with $\mathrm{Ca}^{2+}$ ions present in blood and forms a proteinaceous enzyme called prothrombinase. Later in the presence of $\mathrm{Ca}^{2+}$ inactivates heparin (anticoagulant) and catalyses prothrombin (inactive plasma protein) into an active thrombin protein. Thrombin acts as an enzyme and catalyses fibrinogen (a soluble plasma protein) into an insoluble fibre like polymer, fibrin. These fibres form a dense network upon the wound and trap blood corpuscles (WBCs, RBCs and platelets) and thus form a clot. This clot seals the wound and stop bleeding. In blood vessels, thromboplastin do not release due to which blood does not clot. But external thromboplastin to blood will cause blood clotting at the site of its introduction due to formation of prothrombinase.