Cardiac output

Cardiac output is the volume of blood your heart pumps each minute: heart rate multiplied by stroke volume, the amount ejected per beat. At rest it runs around five litres a minute in an average adult, which is roughly your entire blood volume passing through every minute.

During hard exercise it rises several fold, and in a highly trained endurance athlete it can exceed thirty litres a minute. That increase is the main reason oxygen delivery rises to meet demand, and it is why the heart is usually the limiting factor in aerobic capacity rather than the lungs.

The two components adapt differently. Maximum heart rate is set mostly by age and barely changes with training. Stroke volume is highly trainable, and it is what improves when endurance training makes you fitter: a larger, more compliant left ventricle that fills better and empties more completely.

That is why a trained person has a low resting heart rate. If each beat moves more blood, fewer beats deliver the same output at rest, which is the same arithmetic seen from the other end.

It also explains an early plateau many beginners hit. Cardiac adaptations take longer than the peripheral ones in muscle, so the first few weeks of improvement come substantially from mitochondria and capillaries, with the pump catching up over months.

Clinically, falling cardiac output is what heart failure means in physiological terms, and the symptoms follow from it: breathlessness, fatigue, poor exercise tolerance. Exercise capacity is therefore a genuinely useful measure of cardiac function rather than a fitness vanity metric.

In practice, you cannot measure it outside a lab, but its consequences are visible for free: resting heart rate, how quickly your heart rate falls after a hard effort, and the pace you can hold at a given heart rate. Our VO2max calculator estimates the capacity it produces.