How is the cardiac cycle regulated by the heart itself?

The heartbeat is started at a specialised area of the right atrium, known as the sinoatrial node (SAN); the SAN sends out the waves of depolarisation, resulting in contraction of the cardiac muscle. These waves spread across the walls of both atria from the top to the bottom, causing contraction from top to bottom. There is a band of fibres at the border between the atria and the ventricles which stops the waves of depolarisation from passing, this forces the waves to pass through a specialised area of the septum called the atrioventricular node (AVN). The AVN causes a short delay of approximately 0.1s to ensure that the atria have completed contraction before the ventricles begin contraction. The AVN passes the excitation on down to the base of the heart via the Bundle of His (in the septum), which branches into the right bundle and the left bundle, these lead to the purkinje fibres. The purkinje fibres then pass the excitation up through the walls of the ventricles causing contraction of the ventricles from the bottom to the top, this forces the blood up and out of the heart.