Category C - full version without watermark 720x576 (check terms and conditions at www.teledesign.de) The embed version with watermark is free of charge. - Description: A heart valve does not close tightly and the resulting changes in blood stream activate the coagulation cascade according to Virchow's triad. Due to an initial trigger like that, factor X is intrinsically and extrinsically activated to factor Xa. Factor Xa for its part then activates factor II -- the prothrombin. As a next step the activated factor IIa -- the thrombin - develops. Thrombin can now bind to fibrin fibers and activate them, too. The activated fibrinogen is now able to crosslink with other fibrinogen molecules. At first fibrin develops, then big fibrinopeptides. Fibrinopeptides crosslink thrombocytes and erythrocytes at the vessel wall to a small thrombus. Over time more and more thrombocytes stick until a big thrombus develops. Then rapid action is required. Because the thrombus may detach and block a vessel somewhere else -- for instance in the brain.