The production of a mass of semisolid material at the site of an injury that closes the wound, helping to prevent further blood loss and bacterial invasion. The clot is formed by the action of clotting factors and platelets. The cascade of reactions that culminate in the formation of a blood clot is initiated by thromboplastin (Factor III), a membrane glycoprotein found in tissue cells. When tissue is damaged, this forms a cell-surface complex that, with phospholipid and calcium ions, converts a plasma glycoprotein, Factor X, to Factor Xa, which in turn converts prothrombin in the blood to its enzymically active form thrombin. Thrombin catalyses the formation of the insoluble protein fibrin from soluble fibrinogen; the fibrin forms a fibrous network in which blood cells become enmeshed, producing a clot.
http://web.indstate.edu/thcme/mwking/blood-coagulation.html Overview of blood coagulation and its disorders from the Indiana University School of Medicine