What is the relationship between heart size and stroke volume?

• A. Larger hearts have smaller stroke volumes.
• B. Smaller hearts have smaller stroke volumes, necessitating a faster heart rate to maintain cardiac output.
• C. Stroke volume is independent of heart size.
• D. Larger hearts pump blood faster but with the same stroke volume.

Answer: (B)

Stroke volume is the amount of blood the heart ejects with each beat, and it depends on how much blood the chamber can hold (preload) and how forcefully the heart contracts (contractility) as well as afterload. Cardiac output, the total blood pumped per minute, is the product of heart rate and stroke volume. If the heart is smaller, the chambers hold less blood and the muscle mass is typically less, so the amount ejected per beat tends to be lower. To keep cardiac output from dropping, the system increases heart rate, compensating for the smaller stroke volume. That’s why a smaller heart often pairs with a higher resting or active heart rate to maintain the same overall blood flow. In contrast, a larger heart can eject more blood per beat, so it doesn’t need to beat as fast to achieve the same cardiac output.