Assertion : Blood coagulate in uninjured blood vessels. Reason : Uninjured blood vessels release an anticoagulant heparin.

Step-by-step solution

When a blood vessel gets injured, blood platelets get clumped at the injured spot and release certain chemicals called platelet factors, which promote blood coagulation. Injured tissues or platelets also release coagulation promoting substances called thromboplastins which help in the formation of an enzyme prothrombinase. This enzyme activates inactive protein prothrombin to active thrombin which further promotes the coagulation process. Blood normally contains an anticoagulant heparin which is released from mast