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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.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



an enzyme of the hydrolase class, a major element in the blood-clotting system of man and animals. It is present in blood in the form of the inactive precursor prothrombin and is activated by prothrombinase, or active thromboplastin. Chemically, thrombin is a glycoprotein with a molecular weight of about 40,000; it contains about 5 percent carbohydrate.

Thrombin was isolated in crystalline form in 1972 by the American physiologist W. Seegers and his associates. It is similar in primary and tertiary structure to the serine proteolytic enzymes such as trypsin. A thrombin molecule consists of two polypeptide chains linked by a disulfide bond. The A chain of bovine thrombin contains 49 amino acid radicals, and the B chain, 265 radicals. The active center of the enzyme, as well as the carbohydrate constituent, are located in the B chain. Thrombin exists in several active forms, which differ in the structure of the B chain.

The main function of thrombin is to convert fibrinogen into fibrin. Thrombin hydrolyzes four arginyl-glycine bonds in the fibrinogen molecule. Four peptides then split off to form a fibrin monomer, which becomes polymerized into the fibrin clot that is the basis of a thrombus. The reactions of limited proteolysis with the participation of thrombin are accompanied by the activation of Factor XIII (fibrin stabilizing factor) and of Factors V and VIII, which take part in the reactions of the internal blood-clotting mechanism. Thrombin aids in platelet aggregation and in clot retraction. A relative excess of thrombin in the body has been shown to automatically initiate the activity of the anticoagu-lating system. This is accompanied by the introduction of heparin and the activator of plasminogen into the bloodstream, substances that help maintain the blood’s liquid state.

Thrombin is inactivated by diisopropyl fluorophosphate, which blocks the hydroxyl group of serine that enters the active center, and by other inhibitors characteristic of the serine proteinase group. Thrombin is inactivated in blood by the plasma antithrombins α2-macroglobulin, antithrombin III, and/or heparin. A specific nonplasma thrombin inhibitor is the polypeptide hirudin, found in the buccal glands of leeches.

Thrombin is used to arrest capillary bleeding by the application of fibrin foam soaked in thrombin to the affected surface.


Magnusson, S. “Thrombin and Prothrombin.” In The Enzymes, 3rd ed., vol. 3. New York–London, 1971.
“A New Thrombin: Purification, Aminoacid Composition, and Crystallization.” Thrombosis Research, 1972, vol. 1, p. 533.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


An enzyme elaborated from prothrombin in shed blood which induces clotting by converting fibrinogen to fibrin.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.