The term cardiac cycle means one pumping cycle which consists of contraction (systole) and relaxation (diastole) of both the atria and both ventricles. Both the atria contract at the same time and as they relax both the ventricles contract which gives a sequential pumping action to the heart. The atria remain relaxed during part of the ventricular relaxation (diastole) and then the cycle starts over again.

The pressure changes in the left ventricle, aorta and left atrium and the corresponding phase of the ECG, heart sounds and changes in the ventricular volume are shown in the figure below: