the cardiac cycle

• Cardiac cycle


  The pressure and volume changes within the heart that control the opening and closing of the valves and ensure blood flows in one direction
• Stages of cardiac cycle

  1. Diastole (relaxation)

  2. Atrial systole (atria contracting)

  3. Ventricular systole (ventricles contracting)
• Pressure and volume changes

  Cause the valves to open and close, ensuring unidirectional blood flow
• Heart valves


  • Semilunar valves (between ventricles and arteries)
  • Atrioventricular valves (between atria and ventricles)
• Semilunar valves


  • Open when pressure is higher behind (in ventricles) than in front (in arteries)
  • Close when pressure is higher in front (in arteries) than behind (in ventricles)
• Atrioventricular valves


  • Open when pressure is higher behind (in atria) than in front (in ventricles)
  • Close when pressure is higher in front (in ventricles) than behind (in atria)
• Diastole


  Both atria and ventricles are relaxed, allowing blood to flow into the atria
• Atrial systole


  Atria contract, increasing pressure and forcing blood into the ventricles
• Ventricular systole


  Ventricles contract, increasing pressure and forcing blood out of the heart
• Cardiac output = Heart rate x Stroke volume
• Interpreting pressure change graphs for the cardiac cycle requires understanding the opening and closing of the valves based on pressure differences
• A common exam question is to calculate heart rate from the duration of one cardiac cycle shown on a graph