The key idea is that the heart adjusts its pumping strength based on how much it is filled during diastole. When venous return increases, the end-diastolic volume rises, stretching the cardiac muscle fibers to a more optimal length. This length-tension relationship makes the heart contract more forcefully, so the stroke volume increases. In other words, stroke volume goes up as filling (preload) increases, helping match the output to the amount of blood returning to the heart. This explanation fits the scenario because the Frank-Starling mechanism describes how SV is directly related to preload due to venous return, up to physiological limits. It does not say that heart rate alone governs the pump effect, it does not claim that cardiac output falls with more filling, and it does not state that afterload primarily determines heart rate.