Cardiac Cycle Process

• Cardiac Cycle pt1

  1. cycle initiated with firing of sinoatrial node stimulating atria to depolarise - P wave
  2. atrial contraction starts shortly after P wave begins, causing atrial pressure to increase, opening atrioventricular valves, forcing blood into ventricles
  3. as atrial contraction completes, atrial pressure beings to fall, reversing the pressure gradient, closing atrioventricular valves - atrioventricular valves closing produce the first heart sound, S1, and marks the beginning of systole
  4. ventricular depolarisation (represented by QRS complex) is halfway through and ventricles start to contract, rapidly increasing ventricular pressure
• Cardiac Cycle pt2:

  1. semilunar valves remain closed for a while, so ventricles contract within a closed space - called isovolumetric contraction, where no blood is ejected and ventricular volume is unchanged
  2. ventricular ejection starts when ventricular pressure exceed the pressures within the aorta and pulmonary artery, the aortic and pulmonic valves open and blood is ejected out of the ventricles - called the rapid ejection phase
  3. during ventricular repolarisation (T wave) begins, ventricular pressure decreases, decreasing force of ejection
• Cardiac Cycle pt3:

  1. when ventricular pressures drop below aortic and pulmonary pressures, the semilunar valves close, marking the end of systole and start of diastole
  2. closure of semilunar valves produces the second heart sound, S2
  3. the first part of diastole is isovolumetric, as the ventricles relax with all valves close
  4. ventricular pressure drops rapidly but their volumes remain unchanged, whilst the atria are being filled with blood and atrial pressure increasing
• Cardiac Cycle pt4:

  1. ventricular filling starts when ventricular pressures drop below atrial pressures, causing the atrioventricular valve to open, allowing blood flow down to ventricles passively
  2. atria contract to finish the filling phase and the cycle repeats itself