Describe the cardiac cycle, including all the relevant electrical and valvular events.

Starting in the right atrium, the sinoatrial node (SAN) will send a signal to contract both atria, opening the atrioventricular valves (tricuspidand bicuspid/mitral) and forcing blood in to both ventricles. The signal from the SAN would propogate towards the atrioventricular node (AVN) in the superior aspect of the ventricular septum, but will pause briefly to allow complete atrial emptying/ventricular filling. Then the signal travels from the AVN down the bundle of Hiss and the left and right bundle branches to initiate a contraction from the apex up, this change in pressure as the contraction starts quickly closes the atrioventricular valves, resulting in the first heart sound, and opens the pulmonary and aortic valves. The ventricular contraction is sustained until all the muscles of the ventricles have depolarised, before repolarising and relaxing, the change in pressure between the ventricles and the aorta and pulmonary artery closes the aortic and pulmoanry valves, leading to the second heart sound. During this time the atria would have refilled from the vena cava and pulmonary veins, and will passively fill the ventricles as the atrioventricular pressure gradient increases, before the initiation of the next cardiac cycle stimulated by the SAN firing.