fibrinogen

n.
A protein in the blood plasma that is essential for the coagulation of blood and is converted to fibrin by the action of thrombin in the presence of ionized calcium.

fibrinogenous fi'bri·nog'e·nous (fī'brə-nŏj'ə-nəs) adj.

The major clot-forming substrate in the blood plasma of vertebrates. Though fibrinogen represents a small fraction of plasma proteins (normal human plasma has a fibrinogen content of 2–4 mg/ml of a total of 70 mg protein/ml), its conversion to fibrin causes a gelation which blocks the flow of blood. Upon injury, sufficient amounts of the clotting enzyme, thrombin, are generated in about 5 min clotting time to produce a gel. Although clotting in the circulation (thrombosis) can be extremely dangerous, clotting is an essential and normal response for preventing the loss of blood. Individuals born with the hereditary absence of fibrinogen (afibrinogenemia) suffer from severe bleeding, which can be counteracted by transfusing normal plasma or purified fibrinogen. See also Hemorrhage.

Fibrinogen is synthesized by the hepatocytes in the liver, and the synthetic rate can be stimulated by hormones. Significant amounts of carbohydrates become attached to the protein before it is secreted into the circulation; alterations in its carbohydrate composition as found in some liver diseases can give rise to abnormal fibrinogens with defective clotting properties.

Clotting is regulated by two enzymes, thrombin and factor XIII_a (fibrinoligase, activated fibrin-stabilizing factor, transglutaminase). Thrombin exerts a dual control by regulating the rate of fibrin formation as well as producing factor XIII_a. In the plasma milieu, the fibrin molecules readily aggregate into a clot. In order to obtain a clot structure of a strength sufficient to stem bleeding, however, it is necessary for the thrombin-modified factor XIII to be activated to XIII_a. Factor XIII_a acts as a transamidating enzyme which strengthens the fibrin clot by creating cross-links between the molecules. Without such cross-links, a clot structure would be like a brick wall without mortar. Individuals with the hereditary absence of factor XIII often suffer from severe bleeding, even though their clotting times are in the normal range. See also Blood; Hemophilia; Immunoglobulin.

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One of the proteins of the blood plasma which is responsible for the clotting of blood. When prothrombin is activated to thrombin in response to injury, it converts fibrinogen to fibrin, which is deposited as strands that form the clot.

(fībrin′əjən)
n
factor I, profibrin

A soluble plasma protein (globulin) that is acted on by thrombin to form fibrin. The normal level is 200 to 400 mg/100 ml in plasma. Coagulation is impaired if the concentration is less than 100 mg/100 ml. Another form of fibrinogen called tissue fibrinogen, which has the power of clotting the blood without the presence of thrombin, occurs in body tissues.

A relatively soluble blood protein. It is converted to fibrin during blood clotting.

A high-molecular-weight protein in the blood plasma that by the action of thrombin is converted into fibrin; called also clotting factor I. In the clotting mechanism, fibrin threads form a meshwork for the basis of a blood clot. Most of the fibrinogen in the circulating blood is formed in the liver. Elevations in the blood are nonspecific indicators of inflammatory disease.

• f. deficiency — may be due to afibrinogenemia, hypofibrinogenemia or dysfibrinogenemia.
• f. degradation products (FDPs) — see fibrin–fibrinogen degradation products.
• f. split products — see fibrin–fibrinogen degradation products.