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Blood clot. Coloured scanning electron micrograph (SEM) of a blood clot. Red blood cells (erythrocytes, red) are seen trapped in filaments of fibrin protein (brown). Blood clotting is the solidification of blood that occurs when blood vessels are damaged, limiting blood loss. Blood vessel damage activates blood cells called platelets, which in turn help to stimulate the formation of fibrin filaments at the site of injury. These filaments enmesh platelets and red and white blood cells, contracting around them to form a solid clot. Inappropriate clotting is a major cause of strokes and heart attacks. Magnification: x1000 at 6x7cm size. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)