What 3 things contribute to stroke volume?

• A. Preload, myocardial contractility, afterload
• B. Heart rate, preload, afterload
• C. Preload, heart rate, contractility
• D. Afterload, heart rate, contractility

Answer: (A)

Stroke volume is the amount of blood ejected from the ventricle with each heartbeat. It is determined by three main factors: preload, myocardial contractility, and afterload. Preload is the filling of the ventricle at the end of diastole—the greater the venous return, the more the heart muscle fibers are stretched, and the stronger the subsequent contraction tends to be, boosting stroke volume up to a limit. Contractility is the inherent strength of the heart’s contraction; when contractility is higher, the ventricle can pump more of its filled volume, increasing stroke volume. Afterload is the pressure the ventricle must overcome to eject blood; higher afterload makes ejection harder and lowers stroke volume, while lower afterload facilitates ejection and raises stroke volume. Heart rate affects cardiac output (CO = SV × HR) but does not determine the amount of blood pumped per beat, so it isn’t a determinant of stroke volume.