Cardiac Cycle

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•  Pressure changes in the left atrium, left ventricle and aorta during the cardiac cycle

The cardiac cycle describes the series of events that take place in the heart over the duration of a single heart beat

  • It is comprised of a period of contraction (systole) and relaxation (diastole)


  • Blood returning to the heart will flow into the atria and ventricles as the pressure in them is lower (due to low volume of blood)
  • When ventricles are ~70% full, atria will contract (atrial systole), increasing pressure in the atria and forcing blood into ventricles
  • As ventricles contract, ventricular pressure exceeds atrial pressure and AV valves close to prevent back flow (first heart sound)
  • With both sets of heart valves closed, pressure rapidly builds in the contracting ventricles (isovolumetric contraction)
  • When ventricular pressure exceeds blood pressure in the aorta, the aortic valve opens and blood is released into the aorta


  • As blood exits the ventricle and travels down the aorta, ventricular pressure falls 
  • When ventricular pressure drops below aortic pressure, the aortic valve closes to prevent back flow (second heart sound)
  • When the ventricular pressure drops below the atrial pressure, the AV valve opens and blood can flow from atria to ventricle
  • Throughout the cycle, aortic pressure remains quite high as muscle and elastic fibres in the artery wall maintain blood pressure

Overview of Cardiac Cycle Events

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Pressure Changes in Left Side of Heart During the Cardiac Cycle

cardiac cycle

Link:  Interactive Cardiac Cycle Tutorial