![Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. The glycoprotein prothrombin, …[Credits : Eye of Science / Photo Researchers, Inc.]](http://media-2.web.britannica.com/eb-media/28/98328-004-5514AFAC.jpg)
Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. The glycoprotein prothrombin, which occurs in blood plasma, is transformed into thrombin by a clotting factor known as factor X or prothrombinase; thrombin then acts to transform fibrinogen, also present in plasma, into fibrin, which, in combination with platelets from the blood, forms a blood clot.
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