How do heart rate and stroke volume relate to cardiac output?

• A. Cardiac output varies directly with changes in heart rate and stroke volume.
• B. Cardiac output is independent of heart rate.
• C. Cardiac output decreases as heart rate increases.
• D. Cardiac output depends only on stroke volume, not heart rate.

Answer: (A)

Cardiac output is the product of heart rate and stroke volume. This means CO changes directly when either parameter changes: if heart rate goes up while stroke volume stays the same, CO rises; if stroke volume goes up with a constant heart rate, CO also rises. In normal physiology the body can increase both to meet higher tissue demand, yielding a higher CO (for example, at rest a heart rate of 70 beats per minute with a stroke volume around 70 mL per beat gives about 4.9 L/min; increasing the rate to 100 bpm with the same stroke volume raises CO to about 7 L/min). However, at very high heart rates, filling time shortens, which can reduce stroke volume and limit the CO increase. So the best description is that cardiac output varies directly with changes in heart rate and stroke volume.