The Frank-Starling relation describes how increasing preload affects stroke volume. What is the primary mechanism for the increase in stroke volume with increased preload?

• A. Greater end-diastolic fiber length increases actin-myosin overlap and contractility up to a limit
• B. Decreased heart rate
• C. Increased afterload
• D. Decreased preload

Answer: (C)

Stretching the cardiac muscle fibers by increasing preload moves sarcomeres toward a length where actin and myosin have a more optimal overlap. This yields more cross-bridge interactions during systole and, for a given calcium, a stronger contraction. The result is a greater stroke volume as end-diastolic volume rises. Increasing afterload would oppose stroke volume, and decreasing preload would reduce it, so the key idea is the length-tension relationship of cardiac muscle—more stretch up to an optimal length boosts the force of contraction and thus the amount of blood pumped each beat.