What happens when preload is excessive and surpasses the heart’s optimal stretch point?

What happens when preload is excessive and surpasses the heart’s optimal stretch point?

• A. Stroke volume increases with preload indefinitely
• B. Stroke volume decreases due to overdistension
• C. Preload has no effect on stroke volume
• D. Afterload is the only determinant of stroke volume

Answer: (B)

The main idea is the preload–stroke volume relationship described by the length–tension (Frank-Starling) mechanism. As the ventricle fills from diastole, muscle fibers stretch and contract more forcefully up to an optimal length. When preload becomes excessive and stretches the fibers beyond that point, contractility falls because the actin–myosin overlap is no longer ideal. The result is a weaker contraction and a smaller stroke volume. So, although increasing preload usually boosts stroke volume, pushing preload past the optimal stretch causes overdistension and a decrease in the amount ejected with each beat. This explains why simply increasing preload indefinitely isn’t beneficial, and why preload does matter for stroke volume rather than being irrelevant or being determined solely by afterload.

The main idea is the preload–stroke volume relationship described by the length–tension (Frank-Starling) mechanism. As the ventricle fills from diastole, muscle fibers stretch and contract more forcefully up to an optimal length. When preload becomes excessive and stretches the fibers beyond that point, contractility falls because the actin–myosin overlap is no longer ideal. The result is a weaker contraction and a smaller stroke volume. So, although increasing preload usually boosts stroke volume, pushing preload past the optimal stretch causes overdistension and a decrease in the amount ejected with each beat.

This explains why simply increasing preload indefinitely isn’t beneficial, and why preload does matter for stroke volume rather than being irrelevant or being determined solely by afterload.