Final answer:
Antithrombin III is an essential plasma anticoagulant that inactivates factor X and opposes the conversion of prothrombin (factor II) into thrombin in the common pathway.
Step-by-step explanation:
Antithrombin III is an essential plasma anticoagulant that inactivates factor X and opposes the conversion of prothrombin (factor II) into thrombin in the common pathway. Antithrombin III plays a crucial role in preventing excessive clot formation by inhibiting the formation of thrombin, which is necessary for the final steps in the formation of a fibrin clot. It is produced by the liver and is one of the several circulating plasma anticoagulants that play a role in limiting the coagulation process to the region of injury and restoring a normal, clot-free condition of blood.