Cardiac muscle. Coloured transmission electron micrograph (TEM) of cardiac muscle fibrils (green) from a healthy heart. Mitochondria (purple) supply the muscle cells with energy. The muscle fibrils, or myofibrils, are crossed by transverse tubules (dark green lines). These tubules mark the division of the myofibrils into contractile units (sarcomeres). The central structure is a capillary (orange) seen in cross-section, with a bi-concave red blood cell (red) passing along it. Cardiac muscle is under subconscious control and continuously contracts to pump blood around the body without tiring. Magnification: 3600x when printed 10 centimetres across.