^BHeart conduction system.^b Artwork showing the impulse conduction system (yellow) of a human heart. The sino-atrial (SA) node (oval, upper left) consists of self-excitatory muscle cells which 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 (round, centre left). 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 branching purkinje fibres. The ventricles contract, sending blood to the lungs and to the rest of the body.