Blood clot. Coloured scanning electron micrograph (SEM) of a blood clot. Red blood cells, also known as erythrocytes (red), are seen enmeshed in filaments of fibrin protein (purple/pink). Blood clotting is the solidification of blood that rapidly 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 plate- lets 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: x3800 at 6x7cm size.