Which combination of factors primarily increases cardiac output during exercise?

• A. Heart rate and blood pressure
• B. Stroke volume and respiratory rate
• C. Heart rate and stroke volume
• D. Blood viscosity and heart rate

Answer: (C)

Cardiac output is how much blood the heart pumps each minute, and it depends on two things: how fast the heart beats (heart rate) and how much blood is ejected with each beat (stroke volume). During exercise, both of these rise, so the overall output increases.

The heart speeds up because the fight-or-flight signals raise heart rate, and the heart contracts more forcefully, increasing stroke volume. Venous return also grows as muscles squeeze veins and help push blood back to the heart, making the heart fill more and pump out more with each beat.

Other factors listed don’t drive the increase in cardiac output. Blood pressure itself rises as a consequence of the higher output, not the cause. Respiratory rate helps with oxygen delivery and venous return to some extent, but it isn’t the primary factor boosting cardiac output. Higher blood viscosity makes it harder for blood to flow and actually would tend to reduce cardiac output, not increase it.

So, the combination that most directly increases cardiac output during exercise is a rise in both heart rate and stroke volume.