The heart act as a pump and it's action consists of a series of mechanical event (such as relaxation and contraction of the Atria and ventricle),because they all repeat cyclically that's why it is known as cardiac cycle. As a heart of 72/min, an individual cardiac cycle last for 0.8sec.

Each cycle consists of following agents:
1. Atrial systole- contraction of Atria. 
2. Ventricle systole- contraction of ventricle.
3. Complete cardiac diastole- relaxation of Atria and ventricle.
 The sequences of event's during the cardiac cycle---

  1. Atrial Systole- 
     * The superior venacava and inferior venacava transport deoxygenated blood to right atrium, simultaneously four pulmonary veins convey oxygenated blood into the left atrium.
    * The atrio ventricular valves open and blood flows towards the corresponding ventricles.
   * The SA node emits an impulse which stimulants a wave of contraction that spreads over the myocardium of Atria,  emptying the Atria and completing ventricular feeling.
   * This event last for 0.1sec and termed as atrial systole.

   2. Ventricle systole-
    * When the wave of contraction reaches AV node, it is stimulated to emit an impulse which quickly spread to the ventricular muscle via bundle of His and purkinj fibre.
   *  This results in a wave of contraction which sweeps upwards from the apex of the heart and across the walls of the both ventricle and pumping the blood into the pulmonary trunk and the aorta.
   * The events lasting for 0.3sec is termed as ventricular systole.

3. Complete cardiac diastole-
   * After contraction of the ventricles there is a complete cardiac diastole. Lasting for about 0.4 Dec during which both atria and ventricle relax.