What is defined as the volume of blood in the ventricles at the end of diastole?

The volume of blood in the ventricles at the end of diastole is defined as preload. Preload refers specifically to the initial stretching of the cardiac muscle fibers prior to contraction, which is influenced by the volume of blood returning to the heart. In clinical terms, understanding preload is essential as it relates to the ability of the heart to generate forceful contractions. The greater the volume of blood (preload), the more the heart muscle is stretched, leading to a more forceful contraction according to the Frank-Starling law of the heart.

Other terms in the context of cardiac function do not accurately describe this specific volume measurement. Afterload refers to the pressure that the heart must work against to eject blood during systole. Stroke volume is the amount of blood ejected from the ventricle with each heartbeat, while cardiac output is the total volume of blood pumped by the heart per minute, which is a product of stroke volume and heart rate. Thus, preload specifically addresses the volume of blood present in the ventricles at the end of diastole, making it the correct answer.