The major factor responsible for controlling stroke volume is inotropicity or contractility.
In Figure 2.04, a shift from A -> B indicates increased end-diastolic volume (Frank-Starling mechanism). A shift from A -> C indicates a positive inotropic mechanism (increased available Ca++?). A shift from A -> D also indicates a positive inotropic mechanism.
Actually, a whole family of ventricular function curves are possible (ie. a whole spectrum of inotropic states) and undoubtedly exist in a given individual under different physiological conditions. Increased inotropicity produces greater performance at the same or smaller preload. There is of course an increased cost for this in terms of ATP, oxygen utilization, etc., and efficiency, in its turn, is decreased.
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