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Ulcerative colitis. Coloured scanning electron micrograph (SEM) of clotting blood on the bowel surface of a patient suffering from ulcerative colitis. Red blood cells (erythrocytes, red) are seen, trapped in a mesh formed from strands of the protein fibrin (light brown). Ulcerative colitis is the inflammation and ulceration of the colon and rectum, which may cause bleeding on the lining of the digestive tract, as here. It may be treated with anti-inflammatory drugs. The blood clot seals off the damaged tissue, and stops further bleeding. Magnification: x1000 at 6x7cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)