Thrombus. Illustration of a thrombus, an abnormal blood clot formed inside a vein or artery. Red blood cells appear trapped inside a network of fibres of the insoluble protein fibrin. Also visible are platelets (lower left and upper right), small blood cells that trigger the clotting process. Thrombi form inside intact blood vessels due to an abnormal increase in clotting tendency of the blood. This is most often caused by atherosclerosis, where fatty deposits build up on the inside walls of blood vessels. Thrombi can grow to block blood vessels or break off and be carried through the circulation to the brain or lungs, with potentially fatal results.