the cardiac cycle

• the cardiac cycle is an ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continuously circulating around the body
• the volume of the atria and ventricles changes as they contract and relax.

  pressure changes also occur, due to changes in chamber volume (e.g. decreasing the volume of a chamber by contraction will increase the pressure of a chamber)
• ventricles relax, atria contract:

  • ventricles are relaxed
  • atria contract
  • decreasing volume inside chambers
  • increasing pressure inside the chambers
  • pushes blood into the ventricles
  • slight increase in ventricular pressure and chamber volume as the ventricles receive ejected blood from contracting atria
• cardiac contraction = systole
• cardiac relaxation = diastole
• ventricles contract, atria relax

  • atria relax
  • ventricles contract
  • decreasing volume and increasing pressure
  • pressure becomes higher in ventricles than atria forces AV valves shut to prevent backflow
  • pressure in ventricles also higher than in the aorta and pulmonary artery - SL valves forced open
  • blood forced out of arteries
• in step 1 - AV valves open

  SL valves closed
• higher pressure in front of a valve - forced shut

  higher pressure behind a valve - forced open
• ventricles relax, atria relax

  • ventricles and atria both relax
  • higher pressure in pulm artery and aorta closes SL valves to prevent backflow into ventricles
  • blood returns to heart and atria begin to fill again due to higher pressure in vena cava and pulm vein
  • this starts to increase pressure in atria
  • ventricles continue to relax - pressure falls below that of the atria and so AV valves open
  • allows blood to flow passively (without being pushed by atrial contraction) into ventricles from atria
  • atria contract - process begins again
• calculating cardiac output:

  stroke volume X heart rate
• cardiac output is the volume of blood pumped by the heart per minute (measured in cm3 min-1)
• the left ventricle has a thicker wall than the right ventricle and so it contracts more forcefully - this means the pressure is higher in the left ventricle and in the aorta
• heart rate = the number of beats per minute (bpm)
• stroke volume = volume of blood pumped during each heartbeat measured in cm3