What does the term "preload" refer to in cardiac physiology?

In cardiac physiology, "preload" refers to the degree of stretch of the cardiac muscle fibers at the end of diastole, just before the heart contracts. This stretch is crucial because it directly affects the force of the subsequent contraction—this relationship is described by the Frank-Starling law of the heart. Essentially, when the heart muscle is stretched more due to an increased volume of blood (venous return), it can contract more forcefully. This means that the preload is significant in determining stroke volume and overall cardiac output.

In contrast, the amount of blood ejected from the heart is related to stroke volume and does not define preload itself. The pressure in the aorta during systole pertains more to afterload, which is the resistance the heart must overcome to pump blood out. The volume of blood in the coronary arteries is not relevant to preload, as it concerns myocardial perfusion rather than the stretching of the heart muscle prior to contraction. Understanding preload is essential for grasping how the heart's performance is influenced by filling pressures and volumes.