Three different pathways (extrinsic, intrinsic, and common) are involved. The intrinsic pathway is activated by factors already circulating in the blood, while the extrinsic pathway is activated by tissue factor.
The main difference between intrinsic and extrinsic pathways in blood clotting is that intrinsic pathway is activated by a trauma inside the vascular system whereas extrinsic pathway is activated by external trauma.
The common pathway begins with activation of factor X (to factor Xa) via either the extrinsic pathway or the intrinsic pathway. It is the final stage of the coagulation cascade and leads to the formation of thrombin and fibrin.
This process occurs via two pathways which unite downstream to form the common pathway. These are: The extrinsic pathway: This is triggered by external trauma which causes blood to escape the circulation The intrinsic pathway: This is triggered by internal damage to the vessel wall.
The intrinsic and extrinsic pathways, triggered by internal and external damage respectively, converge in the common pathway, culminating in fibrin clot formation.
Intrinsic vs. extrinsic pathways One of the most confusing things about coagulation (until someone explains it to you) is the idea of the intrinsic and extrinsic arms of the coagulation cascade, and the way they interact during coagulation in the body.
Two pathways lead to the formation of a fibrin clot: the intrinsic and extrinsic pathway. Although they are initiated by distinct mechanisms, the two converge on a common pathway that leads to clot formation. Both pathways are complex and involve numerous different proteins termed clotting factors.
The extrinsic pathway is turned off; the measured PT will be high, but blood can still clot via the intrinsic pathway. If there is no protein C, the normal inhibition mechanism of the intrinsic pathway is turned off. This means there is an increased tendency to clot via the intrinsic pathway.