NEET Biology Body Fluids And Circulation Based Practice Questions & Answers with Viewers Discussion and Detailed Explanation

Answer: (d)

When an injury is caused to a blood vessel, bleeding starts which is stopped by blood clotting. At the site of injury blood platelets release platelet factor - 3 and injured tissues release thromboplastin. The two combine to form prothrombinase enzyme which converts prothrombin to thrombin. The latter stimulates formation of fibrin thread or clot. Blood contains an anticoagulant heparin which prevents blood clotting in uninjured vessels.