When you exercise, your heart has to pump more blood to supply oxygen to your muscles. The volume of blood that your heart pumps in a minute depends on the heart rate (the number of beats per minute) and the stroke volume (the blood volume pumped by each ventricle of your heart with each beat).

When you exercise, your heart beats faster and contracts more forcefully, pushing out more blood and increasing its stroke volume. During vigorous exercise, the return of blood from your veins to your heart increases, causing the ventricles of your heart to fill more fully with blood, stretching them a bit. Like other muscles, the heart contracts more forcefully if it is stretched a little before contraction. So the increase in returning blood also adds to the stoke volume by increasing the volume of blood in each ventricle and by causing the heart to beat more forcefully.

By training aerobically on a regular basis, you gradually increase the volume of blood that your heart can pump in a minute. This change results from an increase in your heart's stroke volume, since the maximum heart rate isn't changed by training.