Identify a factor that can increase stroke volume in response to endurance training.

• A. A decrease in heart rate
• B. An increase in afterload
• C. An increase in cardiac contractility
• D. An increase in venous return

Answer: (C)

Endurance training boosts the heart’s ability to pump blood by increasing its contractile strength. When the ventricle contracts more forcefully, it ejects a larger portion of the blood it fills, raising stroke volume. This comes from adaptations like improved calcium handling and excitation–contraction coupling in cardiac muscle, plus structural changes that support stronger contractions. While factors like venous return (preload) and autonomic changes also contribute to higher stroke volume with training, the most direct driver in this context is the increase in cardiac contractility, which makes each beat more forceful and hence the heart pumps more blood per beat. An increase in afterload would tend to reduce stroke volume, and a decrease in heart rate doesn’t inherently increase the amount ejected per beat, though it can allow more filling time.