When the heart beats, an electrical signal passes from the top of the heart, near the atria (upper chambers), down through the ventricles (lower chambers), and the chambers contract in that order.

But in the process, the heart ADAPTS and changes. Its left and right ventricles enlarge. It begins to look quite different than a non-athlete's heart.

Atrioventricular valves lie between the atria and ventricles, and semilunar valves are located at the bases of the two large vessels leaving the heart.