explain cardiac cycle 5 marks

Cardiac cycle is the sequence of events which occur from the beginning of one heart beat to the beginning of the next heart beat. Mechanism of cardiac cycle In the beginning, all the 4 chambers of the heart are in a state of joint diastole (relaxation). Tricuspid and bicuspid valves open and blood from the veins and the vena cava flow into the atria, and then into the ventricles because of the opening of the valves. SAN generates an action potential, and both atria undergo contraction (Atrial systole). The flow of blood into the ventricles increases by 30%. The action potential is conducted towards the ventricles through the AVN and the AV bundles, from where the bundle of His transmits this action potential over the entire cardiac musculature. The ventricles contract (ventricular systole) and the atria relax (atrial diastole) as a result of the conduction of action potential. Ventricular pressure increases. Hence, bicuspid and tricuspid valves close, to prevent the backflow of blood into the atria. Further increase in pressure in the ventricles leads to the opening of the semilunar valves. Blood from the ventricles flow into the pulmonary artery and the aorta, and subsequently into the circulatory pathways. Consequently, the ventricles relax (ventricular diastole), ventricular pressure falls, and the semilunar valves close to prevent the backflow of blood into the ventricles. Ventricular pressure further falls. As a result, the bicuspid and tricuspid valves open. This is because pressure is exerted on the atria by the blood entering them through the veins. Once again, joint diastole is experienced and the entire cycle is repeated. Regards