Heart blood vessels. Coloured scanning electron micrograph (SEM) of a freeze-fractured section through cardiac muscle, exposing the blood vessels of the heart. Large blood vessels are seen lined with endothelium (the single-celled layer coating the inside of blood vessels, brown). The smaller blood vessels (capillaries, small holes) supply blood to all parts of the heart. The blood vessels have been emptied of blood. Cardiac (heart) muscle continually contracts (under subconscious control) to pump blood round the body. As such, it requires a rich supply of oxygenated blood, explaining the vascularity (presence of many blood vessels) of this tissue. Magnification: x300 at 6x7cm size.