Coagulation is initiated immediately following an injury to a blood vessel that has suffered damage in the endothelium lining. The blood is exposed to the subendothelial space. It gives way to two different processes: the platelets undergo change, and the subendothelial tissue factor is exposed to plasma factor VII, resulting in cross-linked fibrin formation. A plug is formed at the site of injury resulting in primary hemostasis. Secondary hemostasis occurs at the same time. Blood coagulation occurs not only in humans but all other mammals and animals as well. In mammals, the blood clotting mechanism comprises both protein and cellular components.