Heart conduction system. Artwork showing the impulse conduction system (red) of the human heart. The sinoatrial (SA) node 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. Contraction of the atria causes blood to move into the lower chambers (ventricles, lower frame). 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.