Which component increases to raise maximal cardiac output after endurance training?

• A. Heart rate
• B. Stroke volume
• C. End-diastolic pressure
• D. Capillary density

Answer: (B)

Maximal cardiac output is determined by how much blood the heart can push out per beat (stroke volume) and how fast the heart can beat (heart rate). With endurance training, the biggest change that raises the peak cardiac output is an increase in stroke volume. The heart adapts by becoming better at filling and pumping: blood volume expands with training, boosting venous return and end-diastolic volume, which stretches the ventricle more before it contracts. This leads to a stronger, more complete contraction and a higher amount of blood ejected per beat. Since maximal heart rate changes little or may even go down slightly, the overall peak cardiac output rises mainly because stroke volume rises.

End-diastolic pressure relates to preload and supports the mechanism behind the greater stroke volume, but the direct factor driving the increase in peak cardiac output is the rise in stroke volume. Capillary density improves tissue oxygen delivery rather than directly increasing the amount of blood the heart can eject per beat.