Taken from my own lab: using cited information not cited here.

The effects of aerobic exercise and conditioning are thus:

Aerobic conditioning is defined as having worked out aerobically for the past 2 months, at least 3-4 times weekly (definition loosely based). With conditioning, the cardiovascular system increases its ability to deliver oxygen to muscles, which can also utilize oxygen more easily. This is because with long-term training, capillary density in muscles increases allowing a greater extraction of oxygen from blood. Conditioning is also marked with an increase of capillary density in the muscles, an increase of mitochondria in muscle cells, and an increase in left ventricular volume.

Overall, aerobic exercise increases the body’s cardiovascular endurance and efficiency so that the lungs and heart can, with less exertion exchange more air and pump more blood, therefore increasing the efficiency of the system. The pumping effectiveness of the heart is actually 40 to 50 percent greater in a conditioned person so that the heart can pump more blood with fewer heartbeats. Trained athletes also have a lower resting heart rate and greater stroke volume.