During the P wave, the atria of the heart undergo through a process called depolarization. This is an electrical event that prepares the atria for contraction. The depolarization of the atria is initiated by the sinoatrial (SA) node, which is the heart's natural pacemaker located in the right atrium. The electrical impulse travels through the atria, causing the muscle cells to contract almost simultaneously. This contraction pushes blood from the atria into the ventricles. The P wave is the graphical representation of this atrial depolarization on an electrocardiogram (ECG). After the P wave, the next significant wave on the ECG is the QRS complex, which represents the depolarization of the ventricles, preparing them for contraction and the subsequent pumping of blood to the lungs and the rest of the body. Following the QRS complex, the T wave is observed, which signifies the repolarization of the ventricles. This is the electrical resetting of the ventricles in preparation for the next heartbeat. read more >>

The first wave is the P wave, which represents the depolarization of the atria. This happens right before the atria contract and push blood into the ventricles. The next wave is called the QRS wave. ... The T wave comes next, and this represents the repolarization of the ventricles.read more >>