Cardiac Cycle

At rest the heart pumps around 5L of blood around the body every minute, but this can increase massively during exercise. In order to achieve this high output efficiently the heart works through a carefully controlled sequence with every heart beat – this sequence of events is known as the cardiac cycle.

Cardiac Cycle |
Cardiac Cycle

                                                                                                                                                                                                                                                                                                                                                                                                                                                 The Cardiac Cycle

The stages of the cardiac cycle can be roughly divided into the four stages:

  • Filling phase– the ventricles fill during diastole and atrial systole
  • Isovolumetric contraction– the ventricles contract, building up pressure ready to pump blood into the aorta/pulmonary trunk
  • Outflow phase– the ventricles continue to contract, pushing blood into the aorta and the pulmonary trunk. Also known as systole
  • Isovolumetric relaxation– the ventricles relax, ready to re-fill with blood in the next filling phase

In this article we will discuss each of the phases in more detail; describing the changes in pressure and the actions of the heart valves in the cardiac cycle.

Filling Phase

The ventricles are filled with blood in two stages – diastole (heart relaxation) and atrial systole (contraction of the atria).

In diastole both the atria and the ventricles are relaxed. Blood flows from the vena cava and pulmonary veins into the right and left atria respectively, before flowing directly into the ventricles. The ventricles fill with blood at a steadily decreasing rate, until the pressure in the ventricles is equal to that in the veins.

At the end of diastole the atria contract, squirting a small amount of extra blood into the ventricles. This increases the pressure in the ventricles so that it is now higher than that in the atria, causing the atrioventricular valves (mitral/tricuspid) to close.

Isovolumetric Contraction

As contraction begins both sets of valves are closed, meaning that no blood can escape from the ventricles. The start of systole therefore massively increases the pressure within the ventricles, ready to eject blood into the aorta and pulmonary trunk. The stage of isovolumetric contraction lasts for approximately 50ms while the pressure builds.

Outflow Phase

Once the pressure in the ventricles exceeds the pressure in the aorta/pulmonary trunk the outflow valves (aortic/pulmonary) open. Blood is pumped from the heart into the great arteries.

At the end of systole, around 330ms later, the ventricles begin to relax and this decrease in pressure compared to the aorta causes the valves to close. As well as this, blood begins to flow backwards through the outflow valves, which also contributes to the closure of the valves. Hello Nursing Students you read about Cardiac Cycle for more query visit our website

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