a level Biology

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For the heart to function effectively, there must be a fine control and balance of events that take place during the cardiac cycle described above.
Heart tissue is myogenic which means it would initiate its own contraction allowing the heart to contract without needing to be connected to the body. If cardiac muscle was left to contract by itself this could bring some problems to the heart. Since the atria contract faster than the ventricles this could lead to fibrillation hence there is a need for a way to control the cardiac cycle.
In the heart there are two nodes that are responsible to keep the cardiac cycle running correctly. A heart beat starts off at the region of tissue called the sinoatrial node (SAN) which is located above the right atrium. The SAN acts as the hearts pacemaker and ensures the heart is beating at a constant regular rate. This is achieved by the SAN giving out regular electrical signals spread throughout the heart and spreads throughout the atrial muscles that causes the atria to contract (atrial systole). This is what starts the contraction of the atria.

The second node is the known as the atrioventricular node (AVN) which is located near the AV valve. The role of the AVN is to pass the electrical signal to the middle of the heart also known as the septum. There is also a delay in the electrical pulse at the AVN (between the atria and ventricles contracting) which allows the atria to fully empty before the ventricles control. The electrical pulse next passes the signal into extremely insulated fibres called the Bundle of His. The insulation of these fibres ensures the electrical signal cannot escape. The Bundle of His carries the electrical signal to the apex, bottom of the heart. At the bottom of the heart the Bundle of His splits into two. The non-insulated fibres called Purkinje fibres spread up the walls of the ventricles and cause the ventricles to contract from bottom of the heart to cause the blood to be forced out from the ventricles, out of the semilunar valves and towards the aorta and pulmonary artery.