Which of the following substances, if introduce in the blood stream, would cause coagulation, at the site of its introduction?

• Fibrinogen

• Prothrombin

• Heparin

• Thromboplastin

D.

Thromboplastin

Lipoproteinaceous, thromboplastin is released by the injured tissue. It reacts with Ca²⁺ ions present in blood and forms prothrombinase enzyme. Later, in the presence of Ca²⁺ ions, it inactivates heparin (anticoagulant) and catalyses prothrombin (inactive plasma protein) into an active thrombin protein. 

Thrombin acts as an enzyme and catalyses fibrinogen (soluble plasma protein) into an insoluble  fibre like polymer, fibrin. These form a dense network upon the wound and trap blood corpuscles and thus form a clot. This further seals the wound and stops bleeding

In blood vessels, thromboplastin does not release due to which blood does not clot. However, external thromboplastin causes blood clotting at the site of its introduction due to formation of prothrombinase enzyme.