Which of the following substances, if introduced into the blood...

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. Proteolytic cleavage of prothrombin generates thrombin, which converts fibrinogen into fibrin for clot formation. Fibrinogen is a protein produced by the liver which helps stop bleeding by helping blood clots to form. Heparin is used to prevent blood clots from forming. Related Theory Thromboplastin is released by the injured tissue. This reacts with $\mathrm{Ca}^{++}$ions present in blood and forms a proteinaceous enzyme called prothrombinase. Later in the presence of $\mathrm{Ca}^{++}$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. Caution All the options, except heparin are required during clotting of blood. However, thromboplastin is an enzyme, which is released only when stimulated under suitable condition like at the site of injury; otherwise, are not present in the blood. Prothrombin and fibrinogen are already present in blood. Thus, if the enzyme thromboplastin is introduced into the blood, all the clotting component become active and form a cascade reaction at the site of introduction.