Describe cardiac output and how heart rate and stroke volume interact to influence it during exercise.

• A. Cardiac output = heart rate × stroke volume; as intensity increases, both HR and SV rise to increase CO
• B. Cardiac output equals stroke volume only; HR remains constant
• C. Cardiac output equals heart rate plus stroke volume; CO decreases with intensity
• D. Cardiac output is a fixed value; HR and SV do not change with exercise

Answer: (A)

Cardiac output is how much blood the heart pumps each minute, and it depends on both how often the heart beats (heart rate) and how much blood is ejected with each beat (stroke volume). The key relationship is CO = HR × SV. During exercise, your body needs more blood to supply muscles with oxygen, so CO rises. This happens because heart rate increases as the nervous system stimulates the heart to beat faster, and stroke volume also tends to rise early in exercise as more blood returns to the heart (greater preload) and the heart contracts more forcefully (increased contractility). In addition, working muscles dilate their blood vessels, which can reduce the resistance the heart pumps against (afterload), helping stroke volume rise further. As exercise intensity continues to climb, heart rate keeps increasing and stroke volume typically continues to rise or plateaus later, but the overall cardiac output keeps climbing because both components are contributing. This makes the idea that CO is simply the sum or a fixed value incorrect; the correct concept is that CO is the product of heart rate and stroke volume, and both increase to meet the greater demands of exercise.