Stroke Volume is determined by which factors?

• A. Preload, Contractility, Afterload
• B. Heart rate and blood pressure
• C. Oxygen content of blood
• D. Preload only

Answer: (D)

Stroke volume is governed by three interacting factors: preload, afterload, and contractility. Preload—the filling of the ventricle at the end of diastole—sets the initial stretch of the cardiac muscle; increased preload (within physiological limits) leads to a stronger contraction and a larger stroke volume via the Frank-Starling mechanism. Afterload is the pressure the ventricle must overcome to eject blood; higher afterload makes it harder to pump blood out, reducing stroke volume. Contractility is the intrinsic strength of the heart’s contraction; greater contractility increases the amount of blood ejected for a given preload, increasing stroke volume.

Heart rate and oxygen content of blood don’t directly determine how much blood is ejected with each beat. Heart rate changes cardiac output (CO = SV × HR), and oxygen content affects how much oxygen is delivered, not the volume pumped per beat. Therefore, the determinants of stroke volume are preload, afterload, and contractility.