the cardiac cycle

• the cardiac cycle pumps blood around the body
• the cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating round the body
• the volume of the atria and ventricles change as they contract and relax, altering the pressure in each chamber, causes valves to open and close which directs blood flow in the heart
• cardiac cycle 3 stages

  1. ventricles relax, atria contract
  2. ventricles contract, atria relax
  3. ventricles relax, atria relax
• stage 1 - ventricles are relaxed, atria contract.

  • decreases volume and increases pressure
  • pushes blood into ventricles through atrioventricular valves
  • slight increase in ventricular pressure and volume as ventricles receive the blood from the contracting atria
  • semi lunar valves closed
• stage 2 - ventricles contract, atria relax

  • atria relax and ventricles contract ( decreasing volume ) increasing their pressure
  • pressure higher in ventricles which forces atrioventricular valves shut to prevent backflow
  • high pressure in ventricles opens semi lunar valves
  • blood forced out of pulmonary artery and aorta
• stage 3 - ventricles relax, atria relax

  • ventricles and atria both relax.
  • high pressure in pulmonary artery and aorta causes semi lunar valves to close preventing back flow
  • atria fill with blood due to high pressure in vena cava and pulmonary vein
  • as ventricles relax pressure falls below the pressure in the atria
  • causes atrioventricular valves to open and blood flows passively into ventricles from atria which then contracts
• cardiac output = heart rate x stroke volume
• in diastole the heart relaxes. the atria and ventricles fill with blood. volume and pressure of the blood in the heart build as the heart fills, pressure in arteries is at minimum
• in systole the atria contract, followed by the ventricles. pressure in the heart increases dramatically and blood is forced out of the right side of the heart to the lungs and from the left side to the main body circulation

  vol and pressure of blood are low at the end of systole, blood pressure in arteries is maximum