Under the Frank-Starling mechanism, which variable primarily increases as venous return rises when contractility is constant?

• A. Afterload
• B. Contractility
• C. Preload
• D. Heart rate

Answer: (C)

The main idea here is preload. Venous return increases the amount of blood filling the ventricle by the end of diastole, which stretches the ventricular fibers more. That increased stretch (preload), when contractility is held constant, makes the heart contract more forcefully and raise the stroke volume. So preload is the variable that rises with greater venous return. Afterload is the pressure the heart has to work against and isn’t driven by venous return in this scenario. Heart rate is governed by other factors like autonomic input, not the Frank-Starling filling changes.