cardiac cycle

• pathway of blood in the heart:

  vena carva --> right atrium --> atrioventricular valve --> right ventricle --> semi-lunar valve --> pulmonary artery --> lungs --> pulmonary veins --> left atrium --> atrioventricular valve --> left ventricle --> semi-lunar valve --> aorta
• valves:

  • stop blood flowing the wrong way
  • valves open one way - whether they're open or closed depends on the relative pressure of the heart chambers
  • if there's higher pressure behind a valve, it's forced open
  • if there's higher pressure in front of the valve, it's forced shut
• 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
• stage 1 of the cardiac cycle:

  the ventricles are relaxed. The atria contract, which decreases their volume and increases their pressure. This pushes the blood into the ventricles through the AV valves. There's a slight increase in the ventricular pressure and volume as the ventricles receive the ejected blood from the contracting atria
• stage 2 of the cardiac cycle:

  the atria relax. The ventricles contract, decreasing their volume and increasing their pressure. The pressure becomes higher in the ventricles than the atria which forces the AV valves shut to prevent backflow. The high pressure in the ventricles opens the semi-lunar valves - blood is forced out into the pulmonary artery and aorta
• Stage 3 of the cardiac cycle

  1. The ventricles and the atria both relax

  2. The higher pressure in the pulmonary artery and the aorta causes the semi-lunar valves to close

  3. The atria fill with blood (increasing the pressure) due to the high pressure in the vena carva and the pulmonary vein

  4. As the ventricles continue to relax, their pressure falls below the pressure in the atria

  5. This causes the atrioventricular valves to open and blood flows passively (without being pushed by atrial contraction) into the ventricles from the atria

  6. The atria contract, and the whole process begins again
• cardiac output = heart rate x stroke volume

  cardiac output: volume of blood pumped by the heart per minute

  heart rate: number of beats per minute

  stroke volume: blood pumped during each heartbeat