blood clotting

blood clotting process by which the blood coagulates to form solid masses, or clots. In minor injuries, small oval bodies called platelets, or thrombocytes, tend to collect and form plugs in blood vessel openings. To control bleeding from vessels larger than capillaries a clot must form at the point of injury. The coagulation of the blood is also initiated by the blood platelets. The platelets produce a substance that combines with calcium ions in the blood to form thromboplastin, which in turn converts the protein prothrombin into thrombin in a complex series of reactions. Thrombin, a proteolytic enzyme, converts fibrinogen, a protein substance, into fibrin, an insoluble protein that forms an intricate network of minute threadlike structures called fibrils and causes the blood plasma to gel. The blood cells and plasma are enmeshed in the network of fibrils to form the clot. Blood clotting can be initiated by the extrinsic mechanism, in which substances from damaged tissues are mixed with the blood, or by the intrinsic mechanism, in which the blood itself is traumatized. More than 30 substances in blood have been found to affect clotting; whether or not blood will coagulate depends on a balance between those substances that promote coagulation (procoagulants) and those that inhibit it ( anticoagulants ). Prothrombin, a substance essential to the clotting mechanism, is produced by the liver in the presence of vitamin K. When the body is deficient in this vitamin, bleeding is more difficult to control. In hemophiliacs, or "bleeders," the blood's coagulation time is greatly prolonged (see hemophilia ). The coagulation of blood within blood vessels in the absence of injury can cause serious illness or death, especially when a clot forms in the coronary arteries ( thrombosis ) or cerebral arteries ( stroke or apoplexy). To prevent coagulation of the blood in persons with known tendency to clot formation, and also as prophylaxis before performing surgery or blood transfusion, the blood's natural anticlotting substance, heparin, is reinforced by an additional amount of an anticoagulant such as Dicumarol injected into the body.

blood clotting The process by which the soluble protein fibrinogen in blood plasma is converted to insoluble fibrin, thus preventing blood loss through cuts, etc. Vitamin K is required for synthesis of the clotting proteins, and deficiency is characterized by excessive bleeding.

Thrombosis is the inappropriate formation of blood clots in the blood vessels, and can be a cause of serious illness and death when blood vessels are blocked. Antagonists of vitamin K, including Warfarin, are commonly used as anticoagulants to reduce clotting in patients at risk of thrombosis.

blood clotting Protective mechanism that prevents excessive blood loss after injury. A mesh of tight fibres (of insoluble fibrin) coagulates at the site of injury through a complex series of chemical reactions. This mesh traps blood cells to form a clot which dries to form a scab. This prevents further loss of blood, and also prevents bacteria getting into the wound. Normal clotting takes place within five minutes. The clotting mechanism is impaired in some diseases, such as haemophilia.