Coagulation begins almost instantly after an injury to the blood vessel has damaged the endothelium lining the vessel. When blood is exposed to proteins such as tissue factor it starts changes to blood platelets and the plasma protein fibrinogen, which is a clotting factor. Platelets immediately form a plug at the site of injury. Proteins in the blood plasma, called "coagulation factors" or "clotting factors", respond in a complex cascade to form fibrin strands. These strengthen the platelet plug.