Cardiac muscle. Coloured scanning electron micrograph (SEM) of healthy heart (cardiac) muscle fibrils (blue). Mitochondria (pink) supply the muscle cells with energy. The muscle fibrils, or myofibrils, are crossed by transverse tubules (running vertically). These tubules mark the division of the myofibrils into contractile units known as sarcomeres. Cardiac muscle is under subconscious control and continuously contracts to pumps blood around the body without tiring. Muscle fibrils are around 1-2 micrometres (millionths of a metre) in diameter.