The difference between the rate at which the heart pumps blood at rest and its maximum capacity is called the:

• A. Cardiac reserve
• B. Cardiac output
• C. Stroke volume
• D. End systolic volume

Answer: (A)

Cardiac reserve is the heart’s extra pumping ability beyond resting output. It’s defined as the difference between the maximum cardiac output the heart can achieve during exercise and the cardiac output at rest. Cardiac output equals heart rate multiplied by stroke volume, so the reserve reflects how much you can boost either rate or the amount ejected per beat (or both) to meet greater demands. For context, resting output is about 5 L/min for many people, while peak output during intense activity can reach around 15–25 L/min, giving a substantial reserve. End systolic volume, the amount of blood left in the ventricle after contraction, and stroke volume (per beat) describe other aspects of pumping but not the heart’s extra capacity above rest.