What is the Frank-Starling mechanism?

• A. Increased afterload increases SV
• B. Increased heart rate increases SV
• C. Increased preload increases cross-bridge formation, increasing contractility and SV
• D. Increased afterload decreases contractility

Answer: (C)

The main concept being tested is how filling of the ventricle (preload) changes the strength of contraction and thus the amount of blood pumped with each beat. When the ventricle fills more, the muscle fibers are stretched toward their optimal length, which increases the number of cross-bridges that can form between actin and myosin during systole. More cross-bridge formation leads to a stronger contraction and a higher stroke volume. This is the Frank-Starling relationship: under normal conditions, greater end-diastolic volume gives a stronger heartbeat and a larger volume ejected per beat.

The other ideas don’t reflect this mechanism. Increasing afterload makes it harder to eject blood, so stroke volume usually decreases rather than increases. Raising heart rate changes cardiac output mainly by changing how often the heart beats, and can actually reduce stroke volume if filling time is shortened. And afterload doesn’t directly dictate contractility in the short term; contractility is intrinsic to the myocardium and is modulated by preload via the length-tension relationship rather than by afterload.