Question

Which of the following substances, if introduced into the bloodstream, would cause coagulation of blood at the site of its introduction?

• A. Thromboplastin
• B. Fibrinogen
• C. Heparin
• D. Prothrombin

Answer 1

The correct option is A Thromboplastin

• Thromboplastin is released by the injured tissue. It reacts with Ca${}^{+}$${}^2$ ions present in blood and forms a proteinaceous enzyme called prothrombinase.
  
• Later in the presence of 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) to 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 stops bleeding and haemostasis is achieved.

• In intact blood vessels, thromboplastin is not released due to which blood does not clot inside them. But external thromboplastin to blood will cause blood clotting at the site of its introduction due to the formation of Prothrombokinase.
  

So, the correct answer is 'Thromboplastin'.