Which factors determine cardiac output?

• A. Both heart rate and stroke volume
• B. Heart rate
• C. Stroke volume
• D. Neither

Answer: (A)

Cardiac output is the blood pumped by the heart per minute, and it depends on two things: how often the heart beats and how much blood is ejected with each beat. In other words, CO = heart rate × stroke volume.

Heart rate is set by autonomic nervous system input and circulating factors. Sympathetic activity (and adrenaline) speeds the heart and can raise CO, while parasympathetic activity slows the heart. However, if the heart rate becomes very high, filling time shortens and stroke volume can drop, limiting CO.

Stroke volume depends on three main factors: preload (venous return stretching the heart), afterload (the pressure the heart must work against to eject blood), and contractility (how forcefully the heart contracts). More venous return increases preload and strengthens contraction via the Frank-Starling mechanism, increasing stroke volume. Higher afterload or weaker contractility can reduce stroke volume.

Because cardiac output is the product of heart rate and stroke volume, both factors determine CO.