The Cardiac Cycle & the Pathway of Blood

• What is the cardiac cycle?

  All of the events taking place within a single heartbeat
• Diastole

  The heart muscle relaxes and the chamber fills with blood
• Systole

  • The muscle walls of the heart chambers contractand blood is forced out
  • Atrial systole forces blood out of the atria and into the ventricles
  • Ventricular systole forces blood out of the ventricles and into the arteries
• Diastole (right)

  Deoxygenated blood from the body enters the right atrium via the vena cava
• Diastole (left)

  Oxygenated blood from the lungs enters the left atrium via the pulmonary vein
• Atrial systole (right)

  Blood is forced from the right atrium into the right ventricle
• Atrial systole (left)

  Blood is forced from the left atrium into the left ventricle
• Ventricular systole (right)

  Blood is forced from the right ventricle, through the pulmonary artery, to the lungs where gas exchange occurs
• Ventricular systole (left)

  Blood is forced from the left ventricle into theaorta, which carries blood to the rest of the body
• The role of Valves

  • Valves inside the heart prevent the backflow of bloodbetween the ventricles and the atria, and between the arteries and the ventricles
• Valves open due to pressure differences between the different area of the heart

  • E.g. when the atria contract the pressure in the atria is higher than the pressure in the ventricles; the valves between the atria and ventricles are forced open
• Valves also close due to pressure differences:

  • E.g. when the ventricles contract the pressure in theventricles is higher than the pressure in the atria; this forces the valves between the atria and the ventricles to close, preventing backflow
• Blood flows through the left and right sides of the heart at the same time