Study The Cardiac CycleCredit: movements happen within the heart as it performs it vital function. Endless cycle of contraction, the pushing of blood out the chamber, and relaxation, allowing the chamber to be filled with blood, happens. The common term we hear which translates to contraction and to relaxation is systole and diastole. 
Mechanical EventsMechanical Events - The Cardiac CycleCredit:
The term cardiac cycle encompasses all events that are associated with the flow of blood through the heart. The events mentioned happen in one complete beat. To categorize the mechanical events, atrial systole must happen and then followed by diastole; at the same time, ventricular systole must happen then followed by diastole. These mechanical events are reflected in an ECG. 
The Cycle
A dynamic change in pressure and volume clearly depicts what happens in the heart during a cardiac cycle. Because the blood flows continuously, the heart never stops beating and the cycle never ceases.
Ventricular Filling - The Cardiac CycleCredit: Filling – The starting pressure is low. The deoxygenated blood from the various parts of the body is returning passively through the inferior and superior vena cava. The blood then is returned to the heart by way of the atria. The atrioventricular valve opens and allows the flow of blood from the atria to the ventricles. About 80% of ventricular filling happens at this period. The stage is now set for atrial systole. After depolarization, the atria contracts which then compresses the blood in the chamber. This can be seen as the P wave of an ECG tracing. The sudden rise in pressure as the atria contracts pushes the blood towards the ventricles. The atria then relax and it’s the ventricles turn to depolarize. This event is seen as the QRS complex of an ECG tracing. 
Ventricular Systole - The Cardiac CycleCredit: Systole – The ventricles begin to contract as soon as the atria relax. Similar to what happened in the atria, pressure begins to mount as the ventricles contract. The rising pressure results in the closure of the AV valves. This certain point where the AV valve is closed and the pulmonic SL valve is also closed, the blood volume inside the ventricles remains constant for a split second. This phenomenon is called isovolumetric contraction phase. As the pressure continues to rise, the SL valves open and the blood is expelled. During this ejection phase, the pressure in the aorta reaches 120 mm Hg.   
 Isovolumetric Relaxation - The Cardiac CycleCredit:
Isovolumetric Relaxation – A phase after the T wave of an ECG tracing signifies the relaxation of the ventricles. Since the residual blood in the chambers of the heart is no longer compressed, there is a drop in pressure within the ventricular chambers which in turn closes the semilunar valves. In this phase, the ventricles are again closed chambers since the SL and AV valves are all closed.
As ventricular systole is happening, the atria have been filling with blood. As the blood pools in the atria, pressure slowly begins to mount. As the pressure exceed the limit of which the AV valve can handle, the AV valves open up and ventricular filling happens again thereby completing the cycle.