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Factors Controlling Stroke Volume
Ventricular Function Plot
Preload is the filling pressure of the heart at the end of diastole. The left atrial pressure (LAP) at the end of diastole will determine the preload.
The greater the preload, the greater will be the volume of blood in the heart at the end of diastole. (Like blowing up a balloon, the more pressure that is applied, the bigger is will get.)
Starling's Law states that the heart will eject a greater stroke volume if it is filled to a greater volume at the end of diastole. The volume of the heart at end diastole is related to the filling pressure of the heart (preload) which is determined by the left atrial pressure (LAP). Starlings' Law is therefore usually plotted as the relationship of stroke volume index to LAP.
This relationship is modified by contractility and the afterload.