The Cardiac Cycle - PCL 2

The cardiac cycle describes the events that occur during one beat of the heart.



The cycle can be divided into 5 stages in general.

Stage 1 (Atrial Systole)

The cardiac cycle starts when the SA node initiates atrial systole, causing the atria to contract and to pump blood into the ventricles. This contributes only the final 20% of ventricular filling as most of the ventricles have been filled passively due to venous pressure at the end of the previous cardiac cycle.

Stage 2 (Isovolumetric Contraction of the Ventricles)

Electrical impulses subsequently reaches the ventricles, intiating ventricular systole and causing the ventricles to contract. This increases the ventricular pressure to a level above the atrial pressure and causes the AV valves to close. The closing of the AV valves produces S1, the 1st heart sound. At the same time, the semilunar valves remain closed as pressure in the pulmonary artery and the aorta are still higher than the ventricular pressure. The ventricles are hence sealed and their volumes are constant.

Stage 3 (Ventricular Ejection)

As the ventricles continue to contract, the ventricular pressure increases until it is higher than the pressure in the aorta and pulmonary artery. This opens the semilunar valves, ejecting blood into the aorta and pulmonary artery respectively. Subsequently, ventricular contraction stops and ventricular pressure starts to fall. The ejection of blood out of the ventricles also contribute to the fall in ventricular pressure.

Stage 4 (Isovolumetric Relaxation of the Ventricles)

The ventricles then start to relax causing a further fall in ventricular pressure. Eventually, ventricular pressure falls below the aortic and pulmonary arterial pressure and the semilunar valves close. This gives rise to S2 or the 2nd heart sound. Concurrently, the AV valves remain closed as the atrial pressure is still lower than the ventricular pressure. Once again, the ventricles are sealed and are of constant volumes.

Stage 5 (Passive Ventricular Filling)

As the ventricles continue to relax, ventricular pressure falls below atrial pressure(which has been increasing concurrently due to venous return to the heart)and the AV valves open. Blood flows from the atria into the ventricles without the atria contracting. At this point in time, the whole heart is relaxed.

Stage 5 marks the end of a cardiac cycle and a new cycle begins when the SA node initiates Atrial Systole again.

Note: Stages 5 and 6 shown in the diagram above have been grouped together as Stage 5 in my explanation. The term diastasis refers to the last stage of diastole in the heart, just before atrial systole is initiated.

Contributed by John Lee