False-color SEM of a blood clot. Red blood cells (colored orange) have been trapped behind a web of thin green strands. These threads are composed of a substance called fibrin, formed from an insoluble protein. Fibrin is produced when a blood vessel is injured. Substances released into the blood cause these strands to seal the site from excessive blood loss and create a blood clot, occuring either on the skin surface, in cases of injuries, or inside blood vessels where they are known as a coagulation thrombus, which may lead to a heart attack if it blocks a heart artery. Magnification: x900 at 35mm, x1800 at 6x7cm size.