Cardiac Cycle

• Cardiac cycle


  The sequence of events that occur and repeat with every heartbeat
• Cardiac cycle


  1. Systole

  2. Diastole
• Systole


  Ventricular contraction
• Diastole


  Ventricular relaxation
• Blood flows from higher to lower pressure
• Contraction increases the pressure within a chamber, while relaxation lowers the pressure
• AV valves open when atrial pressures are higher than ventricular pressures and close when the pressure gradient is reversed
• Semilunar valves open when ventricular pressures are higher than aortic/pulmonary pressures, and close when the reverse is true
• Cardiac cycle initiation


  1. Firing of SA node

  2. Atria depolarize
• Atrial contraction


  1. Increases atrial pressure

  2. Forces blood into ventricles
• Atrial contraction only accounts for a fraction of ventricular filling, because the ventricles are already almost full due to passive blood flow
• AV valve closure


  1. Atrial pressure falls

  2. Pressure gradient across AV valves reverses

  3. AV valves close
• S1 heart sound


  Marks the beginning of systole
• Ventricular contraction


  1. Ventricular depolarization (QRS complex)

  2. Ventricles start to contract

  3. Pressures rapidly build up
• Isovolumetric contraction


  Ventricles contract within a closed space, no blood is ejected and ventricular volume is unchanged
• Ventricular ejection


  1. Ventricular pressures exceed aortic and pulmonary pressures

  2. Semilunar valves open

  3. Blood is ejected out of the ventricles
• Rapid ejection phase


  Ventricular ejection
• End of systole


  1. Ventricular repolarization (T-wave)

  2. Ventricular pressure falls

  3. Semilunar valves close
• S2 heart sound


  Marks the end of systole and beginning of diastole
• Isovolumetric relaxation


  1. Ventricles relax with all valves closed

  2. Ventricular pressure drops rapidly but volume remains unchanged
• Ventricular filling


  1. Atrial pressures rise

  2. Ventricular pressures drop below atrial pressures

  3. AV valves open

  4. Blood flows down ventricles passively

  5. Atrial contraction finishes filling
• The cardiac cycle repeats itself