The sequential event in the heart which is cyclically repeated is called the cardiac cycle and it consists of systole and diastole of both the atria and ventricles.
Duration of a cardiac cycle is 0.8 seconds.
During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood which is called the stroke volume.
The stroke volume multiplied by the heart rate gives the cardiac output.
Cardiac output can be defined as the volume of blood pumped out by each ventricle per minute and averages 5000 mL or 5 litres in a healthy individual.
As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and right atria and the semilunar valves are closed at this stage.
The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction – the atrial systole.
The action potential is conducted to the ventricular side by the AVN and AV bundle from where the bundle of HIS transmits it through the entire ventricular musculature, which causes the ventricular muscles to contract, (ventricular systole), the atria undergoes relaxation (diastole), coinciding with the ventricular systole.
Ventricular systole increases the ventricular pressure causing the closure of tricuspid and bicuspid valves.
As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.
The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing the closure of semilunar valves which prevents the backflow of blood into the ventricles.
As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by the veins and the blood now once again moves freely to the ventricles, leading the ventricles and atria again in a relaxed (joint diastole) state.
Again, the SAN generates a new action potential and the events are repeated.
During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope.
The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves, whereas the second heart sound (dub) is associated with the closure of the semilunar valves.