Assertion: Blood does not coagulate in unijured blood vessels.
Reason: Uninjured blood vessels release an anticoagulant heparin.

Options

A. If both assertion and reason are true reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Answer

D

Explanation

Both the assertion and reason are false .
When a blood vessel gets injured, blood platelets get clumped at the injured spot and release certain chemicals called platelet factors , which promote blood coagulatio.Injured tissues or platelets also release coagulation promoting substances called thromboplastins which help in the formation of an enezyme prothrombinase. This enzyme activates inactive protein prothrombin to active thrombin which further promotes the coagulation process. Blood normally contains an anticoagulant heparing which is released from mast cell granules that prevents the activation of prothrombin in uninjured blood vessles. Blood also contains antithrombin which inhibits any thrombin formed accidentally.Morover, uninjured tissues do not release thromboplastins, hence blood does not coagulated in uninjured blood vessels.