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Hierarchical structure of heart. The myocardium, which is one of the walls of the heart, is an assembly of the cardiomyocytes. There are muscle fibers in the cardiomyocyte, and the muscle fiber is a bundle of myofibrils. A myofibril is constituted of sarcomeres that are connected in series. Shortening and lengthening of sarcomeres cause contraction and expansion of the heart
