Electrical conduction of the heart. Artwork showing the impulse conduction system (yellow) of the human heart (left) and an electrocardiogram (ECG) of a normal heart rate (right). The sinoatrial (SA) node (yellow, upper left) consists of self-excitatory muscle cells that contract rhythmically at around 70 beats per minute. Following each contraction, an impulse spreads through the heart's upper chambers (atria, upper frame) to the atrioventricular (AV) node (yellow, centre left). Contraction of the atria causes blood to move into the lower chambers (ventricles, lower frame). This contraction is seen as the P wave on the ECG. The AV node transmits the impulse to the ventricles, via the bundle of His. The ventricles contract, sending blood to the lungs and to the rest of the body. The ventricular contraction is seen as the QRS wave complex on the ECG. The T wave is the relaxation of the ventricles.