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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.

The fetal autonomic nervous system, the fetal heart rate and the beat-to-beat irregularity.

In the exteriorized fetal lamb an experimental study on the influence of the fetal autonomic nervous system upon the heart rate pattern is described by means of blockage of the cholinergic, alpha-adrenergic and beta-adrenergic system. The alpha-adrenergic system proved to have no effect upon heart rate. The beta-adrenergic system had a positive chronotropic effect and the parasympathetic system had a negative chronotropic effect. Beat-to-beat irregularity was independent of a functional alpha-adrenergic system, but at a high basic heart rate disappeared completely after cholinergic blockage and at a low rate after beta-adrenergic blockage. It is concluded that most probably the beat-to-beat irregularity is the result of the competition of the enhancing influence of the beta-adrenergic system and the inhibiting influence of the cholinergic system on fetal heart rate.