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. Initially all the four chambers of heart are in a relaxed state, i.e., joint diastole. Tricuspid and bicuspid valves are open resulting in inflow of blood into the left and right ventricles from pulmonary veins and vena cava. Semilunar valves are closed at this stage. SAN (sinoatrial node) generates an action potential resulting in simultaneous contraction of both atria – 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. This causes contraction of ventricles (ventricular systole) and relaxation of atria (diastole). Ventricular Systole increases the ventricular pressure causing closure of tricuspid and bicuspid valves. As pressure increases, the semilunar valves of pulmonary artery and aorta are forced open, allowing blood to flow through them into circulatory pathways.
The ventricles now relax (ventricular diastole) and semilunar valves close. As the ventricular pressure declines further, tricuspid and bicuspid valves are pushed open by the pressure in the atria. The blood now once again moves freely to the ventricles. The ventricles and atria are now again in a relaxed (joint diastole) state, as earlier and the process continue.
The duration of a cardiac cycle is 0.8 seconds and during this period each ventricle pumps about 70 ml of blood.
