Explain the sequence of events that occur in the control of the cardiac cycle.

The sino-atrial node depolarises which causes a wave of excitation to pass through the atria (right prior to left) before reaching the atrio-ventricular node. The wave of excitation causes systole of the atria in turn pushing blood through the atrioventricular valves filling the ventricles. At the atrioventricular node, there is a delay in the conduction of the impulse to give time for the ventricles to fill.The wave of excitation then moves along the bundle of His to the purkinje fibres where it starts systole of the ventricles from the apex. As the pressure is higher in the ventricles than the atria, this causes the atrioventricular valves to close. This causes blood to be pushed through the semilunar and aortic valve and therefore into the aorta and pulmonary artery.