Which term describes the amount of blood returning to the ventricles?

• A. Stroke Volume
• B. Preload
• C. Afterload
• D. Cardiac Output

Answer: (B)

Preload is the amount of blood returning to the ventricles at the end of diastole, which stretches the ventricular walls just before contraction. The more venous return a ventricle receives, the more it is stretched, and up to its limit this increased stretch leads to a stronger contraction (Frank-Starling mechanism). This is why preload specifically describes the filling, or incoming blood, rather than the amount ejected or the pressure against which the heart must pump. Stroke volume is the amount ejected with each beat, afterload is the pressure the ventricle must overcome to eject blood, and cardiac output is the total blood pumped per minute (stroke volume times heart rate). Clinically, low preload (as in dehydration or hemorrhage) reduces stroke volume and can cause hypotension, while high preload can occur with fluid overload and may contribute to congestion.