Describe the Frank-Starling law in the heart.

Describe the Frank-Starling law in the heart.

• A. Stroke volume increases when arterial pressure rises
• B. Stroke volume increases in response to an increase in filling of the heart (venous return)
• C. Cardiac output decreases as preload increases
• D. Heart rate determines stroke volume directly

Answer: (B)

The idea being tested is how the amount the heart fills with blood affects how much it can pump out with each beat. When the ventricle fills more during diastole (more venous return), the muscle fibers are stretched to a greater length. Within the normal range, this stretch increases the force of contraction due to the length-dependent activation of the cardiac muscle, so the stroke volume rises. This relationship—more filling leading to a stronger, bigger contraction and thus more blood ejected per beat—is the Frank-Starling mechanism. It also helps balance the output of the two sides of the circulation, so the heart pumps out roughly what it receives. That makes the correct statement the best choice: stroke volume increases in response to an increase in filling of the heart (venous return). Increases in arterial pressure (afterload) generally make it harder to eject blood, so stroke volume does not rise with higher afterload. Increasing preload does not cause cardiac output to decrease; rather, in the normal range, increasing preload increases stroke volume and thus cardiac output. Heart rate does not directly set stroke volume; stroke volume is determined by preload, contractility, and afterload, while heart rate mainly affects cardiac output through the product of heart rate and stroke volume.

The idea being tested is how the amount the heart fills with blood affects how much it can pump out with each beat. When the ventricle fills more during diastole (more venous return), the muscle fibers are stretched to a greater length. Within the normal range, this stretch increases the force of contraction due to the length-dependent activation of the cardiac muscle, so the stroke volume rises. This relationship—more filling leading to a stronger, bigger contraction and thus more blood ejected per beat—is the Frank-Starling mechanism. It also helps balance the output of the two sides of the circulation, so the heart pumps out roughly what it receives.

That makes the correct statement the best choice: stroke volume increases in response to an increase in filling of the heart (venous return). Increases in arterial pressure (afterload) generally make it harder to eject blood, so stroke volume does not rise with higher afterload. Increasing preload does not cause cardiac output to decrease; rather, in the normal range, increasing preload increases stroke volume and thus cardiac output. Heart rate does not directly set stroke volume; stroke volume is determined by preload, contractility, and afterload, while heart rate mainly affects cardiac output through the product of heart rate and stroke volume.