Anaerobic , meaning without oxygen , usually sports or exercises requiring shorts bursts of intense energy. Basically this means that you can do more physical activity for a longer length of time. The heart actually becomes larger. The size of the left ventricle or the pumping size of the heart increases. The stroke volume of the heart increases.
More fluid is pumped per heartbeat at work or at rest.
The heart at rest will beat slower. One of the beneficial side affects of exercise is that your heart rate is lower at rest. The amount of beats saved during the 23 hours when you are not exercising are greater than the extra beats used during exercising. The body uses oxygen more efficiently. This is true not only at a physical level but also a biological or cellular level.