What is the main factor influencing stroke volume?

The primary factor influencing stroke volume is venous return. Stroke volume, which is the amount of blood ejected by the heart during each contraction, is highly dependent on the volume of blood returning to the right atrium of the heart. Venous return encompasses factors such as blood volume, the pressure gradient that promotes blood flow back to the heart, and the role of the skeletal muscle pump and respiratory pump in aiding venous flow. When venous return increases, the heart fills more completely during diastole, leading to greater stretch of the myocardial fibers, which in turn enhances contraction strength according to the Frank-Starling law.

While factors like heart rate, blood pressure, and contractility are important, they do not directly affect stroke volume to the same extent as venous return. Heart rate can influence cardiac output but does not alter stroke volume on its own. Blood pressure relates more to the systemic vascular resistance and afterload, affecting how hard the heart must work rather than how much it ejects per beat. Contractility refers to the strength of the heart's contractions but is often influenced by how much blood returns to the heart. Therefore, the venous return is the most critical factor for determining stroke volume.