End-diastolic volume refers to:

End-diastolic volume is the term used to describe the volume of blood present in the ventricles of the heart right before they contract, at the end of the diastolic phase of the cardiac cycle. This phase is when the heart is at rest, and the chambers fill with blood. The correct answer highlights that this volume reflects the amount of blood in each ventricle after it has finished filling, just before the next contraction begins.

Understanding this volume is crucial because it is a significant determinant of stroke volume, the amount of blood the heart pumps with each beat. The more blood that fills the ventricles during diastole, the greater the stretch of the ventricular walls, which can lead to a more powerful contraction, according to the Frank-Starling mechanism.

The other options describe different phases or aspects of heart function that do not accurately characterize end-diastolic volume, like the timing of systolic contraction or the blood expelled during exercise.