The Cardiac Cycle

• "Systole" means contracting
• "Diastole" means relaxing
• 1. Blood flows into the atria via the vena cava and pulmonary vein, causing pressure in the atria to rise. At one point, the pressure is greater in the atria than in the ventricles. This causes the atrioventricular valve to open, allowing blood to flow from the atria to the ventricles. Atrial systole then takes place, pushing the remaining blood form the atria down to the ventricles

  
• 2. After a short time, the ventricles contract. (ventricular systole).

  • pressure in the ventricles rises
  • Ventricular pressure is now greater than atrial pressure so AV valves close. Backflow to the atria from the ventricles is prevented
  • Semilunar valves in the PA and aorta open, blood is pumped from the ventricles out of the heart

  
• 3. Finally, the ventricles enter ventricular diastole and the pressure in the ventricles falls below that of the PA and aorta

  • This causes the semilunar valves to shut, preventing backflow of blood
  • The heart then repeats this cardiac cycle