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see blood clottingblood 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.
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a protein of high molecular weight formed by the action of the enzyme thrombin on blood plasma fibrogen; it is composed of smooth or transversely striated fibers, agglomerations of which form the basis of the thrombus occurring when blood coagulates.

Fibrin is formed in three stages. In the first stage, two peptides A (molecular weight approximately 2,000) and two peptides B (molecular weight approximately 2,500) split off from the fibrinogen molecule by the action of thrombin, forming the fibrin monomer, which is constructed of two identical subunits joined by disulfide bonds. Each of the subunits consists of three dissimilar polypeptide chains, which are designated α, β, and γ.

In the second stage, the fibrin monomer spontaneously converts to a clot, which is called a fibrin aggregate, or nonstabilized fibrin. The aggregation of the fibrin monomer (the self-assembly of fibrin fibers) includes the transition of the molecule from the globular state to the fibrillary state. Hydrogen and electrostatic bonds and forces of hydrophobic interaction, which may be weakened in concentrated solutions of urea and other denaturing agents, participate in the formation of the fibrin aggregate. This process leads to the reconstitution of the fibrin monomer. The formation of the fibrin aggregate is accelerated by substances that carry a positive charge, such as calcium ions and protamine sulfate, and is inhibited by negatively charged compounds, such as heparin.

In the third stage the fibrin aggregate undergoes changes caused by the enzymatic action of the fibrin-stabilizing factor XIIIa, or fibrinase. Through the action of this factor, stable covalent bonds are formed between the γ- and α-polypeptide chains of fibrin-aggregate molecules, as a result of which the fibrin aggregate is stabilized as a fibrin polymer that is insoluble in concentrated solutions of urea. In cases of congenital or acquired insufficiency of factor XIIIa in the body and in certain diseases, the fibrin aggregate is not stabilized as a fibrin polymer, resulting in hemorrhagic diatheses.

Fibrin is obtained by rinsing and drying a blood clot. It is used to prepare sterile sponges and films for stopping hemorrhages from small vessels during various surgical operations.


Belitser, V. A., and T. V. Varetskaia. “Fibrinogen i fibrin: stroenie molekul, samosborka volokon.” Uspekhi sovremennoi biologii, 1975, vol. 80, issue 1(4).



The fibrous, insoluble protein that forms the structure of a blood clot; formed by the action of thrombin.


a white insoluble elastic protein formed from fibrinogen when blood clots: forms a network that traps red cells and platelets
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