An electrocardiogram (ECG), seen on the right, represents movement of electrical current (depicted in green) through the heart during a heartbeat. The first wave of the ECG, designated P, represents initiation of the heartbeat in the upper chambers of the heart (atria). The QRS complex represents movement of the electrical current through the lower chambers of the heart (ventricles). The T wave represents the recovery phase, in which the electrical current spreads back over the ventricles in the opposite direction. The heart, seen on the left, is beating in time with the ECG. Oxygen-depleted (blue) blood is pumped from the heart to the lungs, which supply oxygen. Oxygen-enriched (red) blood returns from the lungs to the heart and is pumped throughout the body.