The same kinds of extrinsic mechanisms affect stroke volume as heart rate, i.e. the cardiopulmonary receptors acting through sympathetic and parasympathetic efferents of the autonomic nervous system.
Norepinephrine released on the ventricular myocardium and epinephrine from the adrenal medulla reaching the heart through the blood, exert positive inotropic effects, enhancing contractility. These catecholamines also act on the atrial myocardium, but the effect is unimportant. Acetylcholine is a negative inotrope, but there are few parasympathetic endings on the ventricles.
Parasympathetic innervation may play a minor role in modifying atrial contractility.
Increase in stroke volume is gained mainly through increases in inotropicity (especially in younger people), while Frank-Starling contributes little. See Misconception 8.
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