What defines preload in relation to the heart?

Preload is defined as the degree of stretch of the heart muscle fibers (myocardium) at the end of diastole, just before the heart contracts. This phase corresponds to the filling of the ventricles with blood. Preload is primarily determined by the volume of blood returning to the heart via the venous system. When the preload is increased, it results in greater stretch of the myocardium, which can lead to a more forceful contraction during systole due to the length-tension relationship of cardiac muscle. This relationship is often described by the Frank-Starling law of the heart, which states that an increase in the volume of blood in the ventricles leads to an increase in the force of contraction.

Understanding the distinction between preload and other related concepts is crucial. For example, contractility refers to the strength of the heart's contraction, afterload describes the pressure the heart must work against to eject blood (like the pressure in the aorta), and heart rate variability pertains to changes in the time interval between heartbeats, which is unrelated to the physical state of the filling of the heart. Thus, recognizing that preload specifically applies to the stretch of the ventricles at the end of diastole clarifies its important role in cardiovascular