Final answer:
The international normalized ratio (INR) is a measure used to evaluate how quickly blood clots by comparing the coagulation speed to a normal value. An increased INR indicates slower clotting, commonly due to anticoagulant medication. Anticoagulants like antithrombin and heparin are critical for preventing clot formation outside of injured regions.
Step-by-step explanation:
The international normalized ratio (INR) is a standardized number that's calculated in the lab after an individual has a Prothrombin Time Test (PT/INR). It is used to assess how quickly blood clots by measuring the speed of the coagulation pathway compared to normal. An increased INR indicates that blood is taking longer to clot than usual, which is often an intended effect of medications like anticoagulants, used to prevent harmful clots.
Circulating plasma anticoagulants, such as antithrombin and heparin, play crucial roles in limiting clot formation to regions of injury and maintaining blood in a fluid state elsewhere. For example, antithrombin inactivates factor X and opposes the conversion of prothrombin (factor II) into thrombin, which is an essential step in the common pathway of the coagulation process.
The intrinsic pathway, initiated by factors within the bloodstream such as factor XII, and the extrinsic pathway, typically initiated by external trauma, converge at the common pathway, where factor X is activated. This step is crucial for the formation of a blood clot and is a focus of coagulation tests like the PT/INR.