Stroke Volume is determined by which factors?

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 determined by three interacting factors: preload, afterload, and contractility. Preload reflects venous return and end-diastolic stretch of the ventricle; more preload stretches the myocardium more and, via the Frank-Starling mechanism, increases the amount of blood ejected with each beat. Afterload is the resistance the ventricle must overcome to open the aortic/pulmonary valve and eject blood; higher afterload makes it harder to eject blood, reducing stroke volume. Contractility is the intrinsic strength of the cardiac muscle; greater contractility ejects more blood at the same preload, increasing stroke volume. Heart rate affects cardiac output, not the amount ejected per beat, and oxygen content of blood doesn’t directly set stroke volume. So focusing only on preload misses the important roles of afterload and contractility in shaping how much blood the ventricle pumps with each beat.