Basic mechanisms of coagulation. Coagulation factor VII searches for sites of vascular damage where subendothelial tissue factor is exposed. Tissue factor is expressed on the surface of fibroblasts and pericytes in the subendothelial space. Binding of coagulation factor VIIa to tissue factor results in a cascade of blood-clotting reactions, leading to thrombin generation (the initiation pathway). Once small amounts of thrombin are generated in this pathway, thrombin plays a crucial role in the amplification and propagation phases of coagulation by activating coagulation factors V, VIII, and XI (the amplification pathway). This leads to a burst of additional thrombin generation, which is essential for forming sufficient fibrin and sealing the sites of vascular damage. Coagulation factor XIII then crosslinks fibrin fibers, a fundamental process for stabilizing fibrin clots. Contact activation of coagulation factor XII, another important trigger of coagulation in laboratory tests, is not considered essential for hemostasis.