What is the Frank-Starling law?

• A. Contractility increases as afterload increases.
• B. It states that the stroke volume increases in response to increased venous return.
• C. It describes the relationship between heart rate and blood pressure.
• D. It dictates afterload determines preload.

Answer: (B)

The main idea is that the heart’s pumping strength adjusts to how much it is filled at the end of diastole. When more blood returns to the heart, the ventricles fill more (greater preload), stretching the cardiac muscle fibers. This stretch moves the fibers toward an optimal length for contraction, so the next contraction is stronger and ejects more blood. That automatic adjustment—stroke volume increasing with increased venous return—is the essence of the Frank-Starling mechanism.

So, the correct statement is that stroke volume increases in response to increased venous return, because preload (the filling), not afterload or heart rate, governs the force of the next beat.

The other ideas aren’t the Frank-Starling relation: contractility isn’t defined by how much afterload rises; heart rate and blood pressure relate to separate regulatory mechanisms; and preload is determined mainly by venous return, not by afterload.