What does 'preload' refer to in cardiac function?

Preload refers specifically to the degree of stretch of the cardiac muscle fibers at the end of diastole, which is the phase of the heart cycle when the heart chambers fill with blood. This concept is crucial in understanding how the heart's ability to pump blood is influenced by the volume of blood returning to it. As more blood fills the ventricles during diastole, the muscle fibers stretch more, which allows for a greater force of contraction during systole, the phase of contraction.

This relationship is best described by the Frank-Starling law of the heart, which states that the strength of the heart's contraction is directly related to its initial muscle fiber length. When preload increases, it results in a more forceful contraction, leading to a more effective heart function.

The other choices touch on different aspects of cardiac function but do not accurately define preload. Resistance to ejection of blood is related to afterload. The rate of heart contractions pertains to heart rate and does not factor in the stretch of muscle fibers. Blood volume ejected during contraction describes stroke volume, which is influenced by preload but does not define it.