STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY STEVE GSCHMEISSNER / SCIENCE PHOTO LIBRARY
Heart muscle. Coloured scanning electron micrograph (SEM) of healthy human heart (cardiac) muscle fibres (blue). Mitochondria (mauve) supply the muscle cells with energy. The muscle fibres, or myofibrils, are crossed by transverse tubules (orange). These tubules mark the division of the myofibril into contractile units known as sarcomeres. Cardiac muscle, which normally works subconsciously, pumps blood continuously around the body without tiring. Magnification unknown.
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