The FrankStarling law of the heart states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end diastolic volume) when all other factors remain constant. The strength of the heart's systolic contraction is directly proportional to its diastolic expansion with the result that under normal physiological conditions the heart pumps out of the right atrium all the blood returned to it without letting any back up in the veins.

When the stroke volume was increased, pump rate decreased. This happened because as stroke volume increases, more time is required to fill the pump and hence pump rate becomes slower. This occurs because the stroke volume must be greater when heart rate is low in order to provide a proper blood supply. Increasing the stroke volume would increase the cardiac output if pump rate remained unchanged. The results support my predictions.

Stroke volume is determined by three factors, altering any of them can change the stroke volume. These factors are preload, afterload, and contractility. Preload is the end diastolic volume that stretches the right or left ventricle of the heart to its greatest dimensions under variable physiologic demand. Afterload is the pressure in the wall of the left ventricle during ejection. In other words, it is the end load against which the heart contracts to eject blood. The contractility is the force that the muscle can create at the given length.Preload and contractility are directly proportional to the stroke volume and afterload is inversely proportional to stroke volume. If you increase preload (within certain limits), stroke volume will increase according to the Starling curve. Increasing contractility (many things can increase this), makes the heart pump harder and increases stroke volume. Increasing afterload decreases stroke volume. All of these can be reversed (decreasing preload and contractility = decreased stroke volume, etc).