What is the primary determinant of preload?

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Preload is the initial stretch of the ventricular muscle at end-diastole, set mainly by how much blood returns to the heart during diastole—the venous return. When more blood fills the ventricles, the ventricular end-diastolic volume increases, stretching the cardiac fibers more and raising preload. This relationship, the Frank-Starling mechanism, explains why greater preload leads to a stronger subsequent contraction and larger stroke volume up to a limit.

Heart rate can influence how long the heart has to fill, but it doesn’t determine preload itself. Afterload is the pressure the ventricle must overcome to eject blood and affects how much blood is pumped out, not how much fills the ventricle. Contractility changes the force of contraction for a given preload, not the amount of filling. Venous return, influenced by blood volume, venous tone, respiratory changes, and posture, is what mainly sets preload.