Final answer:
An anticoagulant is a substance that opposes coagulation. One example is heparin, which inactivates factor X and opposes the conversion of prothrombin to thrombin in the common pathway.
Step-by-step explanation:
An anticoagulant is any substance that opposes coagulation. There are several circulating plasma anticoagulants that play a role in limiting the coagulation process and maintaining a normal, clot-free condition of blood. One example is heparin, which is a short-acting anticoagulant that inactivates factor X and opposes the conversion of prothrombin to thrombin in the common pathway.