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Each heartbeat can be divided into series of five distinct phases termed the cardiac cycle (Figure 3.5). Each phase is perfectly timed and coordinated by the cardiac conductive system to ensure that the chambers of the heart contract and relax at the correct time, allowing the heart to function as an efficient pump.

Figure 3.5 Phases of the cardiac cycle

Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://opentextbc.ca/anatomyandphysiology/chapter/19-3-cardiac-cycle

The first phase is called the passive ventricular filling stage, where for a short time both the atria and the ventricles of the heart are in diastole and the atrioventricular valves (bicuspid and tricuspid) are open. This allows approximately 70 per cent of atrial blood volume to flow passively from the atria into the dilated ventricles under the influence of gravity and the elastic recoil of the atria.

This is followed by phase two, atrial systole, where the atria contract, forcing the remaining 30 per cent of atrial blood volume into the dilated ventricles.

In phase three, isovolumetric contraction, the ventricles are rapidly undergoing systole, forcing closure of the atrioventricular valves. This phase is very fast, occurring in around 0.05 seconds. The term isovolumetric means at constant volume, reflecting the fact that the volume of the left and right ventricle is identical.

Phase four is known as ventricular ejection; as the pressure in the ventricles increases, the aortic and pulmonary valves are forced open and blood is ejected into the pulmonary and systemic circuits.

The final fifth phase is isovolumetric relaxation; here the ventricles undergo diastole and blood begins to flow back against the aortic and pulmonary valves, snapping them shut. Pressure continues to fall within the ventricles until it is below that of the atria. The elastic recoil of the atria and effects of gravity push blood onto the atrioventricular valves which open, returning the heart back to phase one, passive filling.