Cardiac cycle:
Cardiac cycle refers to the sequence of events that occur during one heart beat. The cardiac cycle involves the atrial systole, the ventricular systole and the cardiac diastole.
The impulse that starts a heart beat is initiated by a patch of muscles located in the right auricle called sino-atrial node (SAN). As SAN initiates and regulates the heart beat, it is also referred to as the 'pacemaker'.
Atrial systole:
The SAN initiates an impulse of contraction that is relayed all over the auricular muscles resulting in atrial systole. When the auricles contract, the blood from the auricles is pumped into the relaxed ventricles.
The atrial systole lasts for 0.15 seconds.
Ventricular systole:
The impulse initiated by SAN is transmitted to the auriculo-ventricular node (AVN), which relays the wave of contraction to the muscles of the ventricles via the Bundle of His and Purkinje fibres, resulting in ventricular systole. When the ventricles contract, the pressure in the ventricles increases and closes the auriculo- ventricular valves, which prevents the backflow of blood into the auricles. The closing of the auriculo-ventricular valves produces the first sound of the heart beat, 'lub'. The sound 'lub' is low pitched and of long duration. The pressure in the ventricles ensures the opening of the semilunar valves, resulting in blood being pumped into the aorta and the pulmonary artery. The ventricular systole lasts for 0.30 seconds.
Cardiac diastole:
During cardiac diastole, all the chambers of the heart, that is, the auricles and the ventricles are relaxed. The high pressure developed in the aorta and the pulmonary artery results in the closure of the semilunar valves, thus preventing the back flow of blood into the ventricles. The closing of the semilunar valves causes the second sound of the heart beat called 'dub'. The sound 'dub' is high pitched, louder and of shorter duration as compared to 'lub'.
The atrial systole, followed by ventricular systole, which is then followed by cardiac diastole, completes one cardiac cycle and lasts for 0.85 seconds.
The number of heart beats for human beings at rest, ranges from 60 -80 heart beats per minute. During strenuous exercise and stress, the heart beat may increase to 140-180 heart beats per minute.