Describe how the cardiac cycle is coordinated with reference to the sinoatrial node (SAN), the atrioventricular node (AVN), and Purkinje tissue.

The SAN is the pacemaker, situated at the top of the right atrium. The SAN initiates a wave of excitation at regular intervals. The wave of excitation quickly travels over the walls of both atria. As it passes, it causes the muscle cells to contract. At the base of the atria is a layer of non-conducting tissue, so the only route through to the ventricles is via the AVN, which is at the top of the septum. The excitation is delayed here to allow for the atria to finish contracting and for the blood to flow into the ventricles. The wave is then passes away from the AVN down specialised conduction tissue known as Purkinje tissue. At the base of the septum, the wave of excitation spreads out over the walls of the ventricles. As the excitation spreads upwards from the apex, the muscles contract, pushing blood up to the major arteries at the top of the heart.