Cardiac Cycle

• Systole


  Contraction (e.g., atrial systole)
• Diastole


  Relaxation (e.g., ventricular diastole)
• At the end of the previous cardiac cycle, both the atria and ventricles are in diastole (relaxed state)
• Blood flow into the atria

  1. From the vena cava and pulmonary vein

  2. Causing atrial pressure to rise
• Atrial pressure exceeds ventricular pressure

  1. Atrioventricular valves open

  2. Blood flows from the atria into the ventricles
• Atrial contraction (atrial systole)


  Pushing remaining blood into the ventricles
• Ventricular contraction (ventricular systole)

  1. Rapidly increasing ventricular pressure

  2. Atrioventricular valves close to prevent backflow

  3. Semilunar valves in the pulmonary artery and aorta open

  4. Blood pumped out of the heart by the contracting ventricles
• During ventricular contraction


  The atria relax
• Ventricular diastole (relaxation)


  1. Ventricular pressure falls below that of the pulmonary artery and aorta

  2. Semilunar valves close to prevent blood from re-entering the ventricles
• At this point, the heart is ready for the next cardiac cycle