Heart muscle. Coloured scanning electron micrograph (SEM) of healthy heart (cardiac) muscle fibres (green). Mitochondria (red) supply the muscle cells with energy. The muscle fibres, 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, which is mostly under subconscious control, continuously pumps blood around the body without tiring. Magnification unknown.
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