Four substances are necessary for coagulation of blood; prothrombin, thromboplastin, calcium and fibrinogen. Prothrombin gives rise to thrombin, an enzyme. Fibrinogen, prothrombin, and calcium are present in circulating blood. Thromboplastin (a lipid or fat-like compound containing phosphorus) is widely distributed throughout the tissues, the lung and brain being especially rich in this factor. It is absent or present in only small quantities in blood plasma. When blood is shed, thromboplastin is liberated from injured tissue and probably also from the leucocytes of the blood itself. The thromboplastin, acting upon the prothrombin in the presence of calcium in an ionised form, converts, it to active thrombin. Thrombin acts in turn upon the inactive fibrinogen, converting it into insoluble fibrin which is deposited as fine threads to form the framework of the clot. In simplest possible terms the chief factors are summarised in the following scheme:
Blood does not normally clot in the blood vessels because there is not sufficient free thromboplastin to convert the inactive prothrombin into the active thrombin.