The shortest, and least complex of the three pathways, the extrinsic pathway, primarily focuses on the interaction of tissue factor (sometimes referred to as factor III) with factor VII, leading to the activation of factor VII.
Tissue factor, a substance expressed on the surface of cells such as fibroblasts and macrophages found outside the vasculature, initiates coagulation when plasma contained within the vessel walls leaks outside the broken vessel and comes into contact with these cells. The name extrinsic pathway comes from the fact that tissue factor is external to the vasculature.
Once a vessel has been breached, tissue factor is exposed to circulating factor VII, and the two substances bind to form a complex. The newly formed tissue factor/factor VII complex is considered the primary physiological stimulus for blood coagulation. In other words, more hemostatic activities are initiated by the extrinsic pathway than the intrinsic. This complex leads to the activation of factor VII (factor VIIa), which is now ready to catalyze the conversion of factor X to factor Xa as part of the common pathway.
This pathway is sometimes referred to as the Tissue Factor Pathway.
