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The regulatory systems that control the pump function of the heart are the mechanisms that modulate the level of myocardial contractility and the lenght-tension relationship. The Frank-Starling relationship is important in balancing -- beat-to-beat -- the outputs of both ventricles. More drastic changes in circulatory dynamics, as during exercise and in old age, are accompanied by changes in myocardial contractility. The young heart normally becomes smaller during exercise, even though the cardiac output is markedly increased. The increased myocardial contractility and the normal distensibility make it possible that the length-tension curve moves to the left and upward. The old heart and the failing heart tend to a relative dilation, even though the cardiac output is markedly decreased. Because of the diminished myocardial contractility in combination with the decreased distensibility, the length-tension curve is moved to the right and downward.