This is a description of basic haemodynamics, somewhat simplified to the eyes of a physiologist, but which I hope will satisfy the needs of an imager of the heart. If you can detect where I have glossed over the mechanisms of blood pressure control, then you did not need to read it in the first place!
The ventricles of the heart are scavenge pumps, which is to say that their aim is to eject blood at the same rate as it delivered to them by their atrium. In this way, the two sides of the heart avoid having differing outputs, which would be an untenable situation over more than a few beats. Also, the heart will immediately increase its output when venous return is increased by exercise. The ventricles match their output to venous return in three ways:
Firstly, an increase in venous return distends the diastolic volume of the receiving ventricle: the ventricles actually commit energy into mechanisms which ensure that in diastole they are as flaccid as possible, so that they may be distended by venous return without generating high filling pressures. Increased distension results in an enhanced systolic contraction of the ventricle, and as a consequence, an increased stroke volume (Starling's rule). The end diastolic pressure (EDP) of the ventricle is the usual measurement of the effort required to fill it. In the healthy heart, the increase in EDP needed to produce a large increase in stroke volume is remarkably slight, but in the diseased heart the higher EDP needed can lead to the separate or combined syndromes of pulmonary and systemic venous congestion. This mechanism satisfies the small variations in venous return that occur at rest.