Heart Physiology
Cardiac Cycle

The arrows indicate the blood flow into and out of the heart.

'Cardiac cycle' is a term that describes the sequence of events occurring during the cardiac activity in one heartbeat. In each cycle the atria and ventricles contract (systole) and relax (diastole) alternately, pumping the blood from areas of high pressure to those of low pressure. The blood is ejected from the atria to the ventricles through the atrioventricular (AV) valves, and from the ventricles to the aorta and pulmonary trunk through the semilunar valves (aortic and pulmonary valves).

The cardiac cycle can be divided into three phases: isovolumetric relaxation, ventricular filling and ventricular systole.

Isovolumetric relaxation (early diastole)

During this phase the ventricles relax, causing a drop in the pressure there. The blood flows back towards the ventricles from the aorta and the pulmonary trunk, closing the semilunar valves (aortic and pulmonary). Rebound of blood off the closed valve causes a brief rise in the aortic pressure, which is called the 'dicrotic wave' in the aortic pressure curve. The closure of the semilunar valves is followed by a short period in which all the valves (the AV and semilunar) are closed, causing no change in the volume of the blood in the ventricles; this is what is called isovolumetric relaxation. This relaxation of the ventricles continues, causing a quick drop in the ventricular pressure. As the ventricular pressure becomes less than the atrial pressure, the AV valves open and the ventricular filling starts.

Ventricular filling (mid to late diastole)

In this phase the blood flows from the atria to the ventricles. The major part of the ventricular filling takes place just after the AV valves open. The first third of ventricular filling is rapid and is therefore called 'rapid ventricular filling'. The middle third is called 'diastasis' and is when a smaller amount of blood is ejected into the ventricles. All three phases – isovolumetric relaxation, rapid ventricular filling and diastasis – happen while all the chambers of the heart (the atria and ventricles) are in diastole. This period is called the relaxation period and lasts for 0.4 seconds. The last third of ventricular filling, which lasts for 0.1 seconds, is when atrial systole (contraction) takes place. During this third, only 20–25 ml of blood is pumped to the ventricles. At the end of the ventricular diastole, each ventricle contains 130 ml of blood. This amount is referred to as the 'end diastolic volume' (EDV). In this phase the AV valves are open and the semilunar valves are closed.

Ventricular systole

This phase lasts for 0.3 seconds, and is characterised by the relaxation of the atria and the contraction of the ventricles. When ventricle systole starts, the blood is pushed backwards, causing closure of the AV valves. At this stage the four valves (the AV and semilunar) are closed for 0.05 seconds; this is referred to as 'isovolumetric contraction'.

As ventricular contraction continues, the ventricular pressure rises sharply. When the pressure in the left ventricle becomes higher than the aortic pressure (80 mm Hg), the aortic semilunar valve opens and the blood is ejected to the aorta. In the right side the semilunar pulmonary valve opens and the blood is ejected to the pulmonary trunk, where the pressure in the right ventricle surpasses the pulmonary trunk pressure (20 mm Hg). The pressure climbs to about 120 mm Hg in the left ventricle and 30 mm Hg in the right ventricle. The semilunar valves stay open for 0.25 seconds; this period is called 'ventricular ejection'. Then the ventricles start to relax, the pressure declines and another relaxation phase starts. The amount of blood that remains in each ventricle at the end of ventricular systole is called 'end systolic volume' (ESV). At rest ESV is about 60 ml. The volume of blood that is ejected by the ventricle in each beat is called stroke volume.

Stroke volume = end diastolic volume – end systolic volume
  = 130 ml – 60 ml
  = 70 ml

Text Written by Dr Nameer Abdulahad
Text edited by Jean Shapcott