Cardiac output

Cardiac output is how much blood leaves the heart per minute, calculated as heart rate times stroke volume. At rest it is roughly five litres a minute; in a trained endurance athlete at maximum effort it can exceed thirty.

It matters because it is the central limit on aerobic capacity. VO2 max is the product of how much oxygenated blood you can deliver and how much oxygen the working muscle can extract from it — and in healthy people it is the delivery side, cardiac output, that runs out first.

In practice it improves largely through stroke volume rather than heart rate, since maximum heart rate barely changes with training and falls with age. That is why endurance training lowers your resting and submaximal heart rate: a bigger stroke volume moves the same blood in fewer beats.