The cardiac cycle

• Systole is the contraction of the heart and the emptying of the chambers
• Diastole is the relaxation of the heart and the filling of the chambers
• In diastole

  • heart muscle is relaxed
  • atria and ventricles filling with blood
  • semi-lunar valves are closed preventing the backflow of blood into the arteries
  • pressure in the atrium exceeds the ventricles so atrioventricular valves open
• In atrial systole

  • walls of atria contract simultaneously
  • blood forced into the ventricles
  • muscular walls of the ventricles are relaxed
• In ventricular systole

  • ventricle walls contract
  • atrioventricular valves are forced shut and are prevented from inverting by the tendons
  • pressure in ventricles exceeds blood vessels so blood is forced through the semi-lunar valves
  • pressure in aorta and pulmonary artery exceeds ventricles so semi-lunar valves close preventing backflow
• In diastole

  • AV valves open - atria pressure > ventricle pressure
  • SL valves close - artery pressure > ventricle pressure
• In ventricular systole

  • AV valves close - ventricle pressure > atria pressure
  • SL valves open - ventricle pressure > artery pressure