Stroke volume is the amount ejected by the ventricle with each heartbeat. Its primary determinants are:

• A. Preload, afterload, and contractility
• B. Heart rate, preload, and afterload
• C. Heart rate, contractility, and afterload
• D. Aortic pressure, venous return, and EDV

Answer: (A)

Stroke volume—the amount ejected by the ventricle with each beat—depends on three interacting factors: preload, afterload, and contractility. Preload is the initial stretch of the cardiac muscle before contraction, linked to venous return and end-diastolic volume. When preload is higher, the muscle fibers are more optimally stretched, and the ventricle can pump out more blood (the Frank-Starling mechanism), increasing stroke volume. Afterload is the pressure the ventricle must overcome to open the aortic valve and eject blood; higher afterload makes it harder to eject blood, so more remains in the ventricle after systole and stroke volume falls. Contractility is the intrinsic strength of the ventricular contraction; greater contractility pumps out more blood for a given preload and afterload, increasing stroke volume, while reduced contractility lowers it.

Heart rate, while it affects overall cardiac output (CO = SV × HR), does not directly determine how much blood is ejected with each heartbeat. Therefore, the primary determinants of stroke volume are preload, afterload, and contractility. That’s why the correct choice lists those three factors.