Stroke volume

Stroke volume is the blood pumped out of the left ventricle in a single contraction — typically around 70 millilitres at rest, rising substantially during exercise as the heart fills more completely and contracts more forcefully.

It matters because it is the trainable half of cardiac output. Endurance training enlarges the left ventricle's chamber and improves filling, so each beat moves more blood. That single adaptation is what produces the low resting heart rate of trained endurance athletes and much of their higher aerobic ceiling.

In practice this is one reason a falling resting heart rate over months of consistent training is a meaningful fitness signal rather than a curiosity — the same circulation is being achieved with fewer, larger beats.