The Cardiac Cycle

The cardiac cycle comprises all of the physiological events associated with a single heartbeat, including electrical events, mechanical events (pressures and volumes), and heart sounds.

The atria and ventricles alternately contract in each cardiac cycle.  The pressures in the chambers change greatly over the course of the cardiac cycle.

The cardiac cycle is essentially split into two phases, systole (the contraction phase) and diastole (the relaxation phase).  Each of these is then further divided into an atrial and ventricular component.

The cardiac cycle therefore proceeds in four stages:

  1. Atrial systole: lasts about 0.1 seconds - both atria contract and force the blood from the atria into the ventricles.
  2. Ventricular systole: lasts about 0.3 seconds - both ventricles contract, blood is forced to the lungs via the pulmonary trunk, and the rest of the body via the aorta.
  3. Atrial diastole: lasting about 0.7 seconds - relaxation of the atria, during which the atria fill with blood from the large veins (the vena cavae).
  4. Ventricular diastole: lasts about 0.5 seconds - begins before atrial systole, allowing the ventricles to fill passively with blood from the atria.

click here to view the full cardiac cycle in picture form.


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