What is preload primarily determined by?

Preload refers to the degree of stretching of the ventricular walls at the end of diastole, just before the heart contracts. It is primarily influenced by the volume of blood returning to the heart, known as venous return. As more blood flows back to the heart, the volume in the ventricles increases, leading to greater stretching of the ventricular walls, which enhances the force of contraction in accordance with the Frank-Starling law of the heart.

Thus, venous return is the key determinant of preload. Factors that affect venous return, such as blood volume, body position, and the capacity of the venous system, ultimately influence the preload. This principle is fundamental in understanding how the heart adjusts its output based on the amount of blood it receives, ensuring adequate circulation throughout the body.

Atrial contraction, while important for the complete filling of the ventricles, plays a secondary role in determining preload. Ventricular contraction is a result of preload rather than a contributor to it, as it refers to the heart's ability to pump blood once diastole concludes. Systemic vascular resistance affects afterload, not preload, by influencing the pressure against which the heart must work during systole. Thus, seeing preload as primarily driven by