Cardiac cycle
Cardiac cycle is defined as sequence of events such as pressure and volume changes, cardiac sounds and changes in the electrical potential occurring during the period between two successive ventricular contractions.
It is characterised by one complete contraction (systole) and relaxation (diastole) and indicates a complete heart beat.
These periods are not equal in duration, when heart rate is 75/minute, one cardiac cycle lasts for about 0.8 sec. in which ventricular systole is 0.3 sec. and diastole is 0.5 sec.
In atria, the systole lasts for 0.1 sec. and diastole 0.7 sec. During ventricular systole, the atria are not contracting and during atrial systole, ventricles are in diastole.