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The Cardiac cycle 

  • The function of the heart is to maintain a constant circulation of blood throughout the body. 
  • The heart acts as a pump and its action consists of a series of events known as the cardiac cycle.  During each heart beat, or cardiac cycle, the heart contracts and then relaxes.
  • The period of contraction is called systole and that of relaxation, diastole.


Phases of Cardiac Cycle

 The cardiac cycle is usually divided into seven phases.


 Phase 1 - Atrial Contraction -
 Phase 2 - Isovolumetric Contraction. 
Phase 3 - Rapid Ejection.
 Phase 4 - Reduced Ejection.
Phase 5 - Isovolumetric Relaxation 
Phase 6 - Rapid Filling.
Phase 7 - Reduced Filling


Atrial Contraction (Phase 1)

  •  This is the first phase of the cardiac cycle.
  • Represents electrical depolarization of the atria. 
  • Atrial depolarization initiates contraction of the atrial musculature. 
  •  As the atria contract, the pressure within the atrial chambers increases, which forces more blood flow across the open atrioventricular (AV) valves, leading to a rapid flow of blood into the ventricles.  
  • A-V Valves Open; 
  • Semilunar Valves Closed.


Isovolumetric Contraction (Phase 2) 

  • This phase of the cardiac cycle begins with the ventricular depolarization.
  • This triggers excitation-contraction coupling, myocyte contraction and a rapid increase in intraventricular pressure. 
  • All Valves Closed.
  • The AV valves close when intraventricular pressure exceeds atrial pressure.
  • Ventricular contraction also triggers contraction of the papillary muscles with their chordae tendineae that are attached to the valve leaflets.
  •  This tension on the AV valve leaflets prevent them from bulging back into the atria and becoming incompetent (i.e., "leaky").
  • Closure of the AV valves results in the first heart sound (S,). This sound is normally split (~ 0.04 sec) because mitral valve closure precedes tricuspid closure.
  • During the time period between the closure of the AV valves and the opening of the aortic and pulmonic valves, ventricular pressure rises rapidly without a change in ventricular volume (i.e., no ejection occurs). 
  •  Ventricular volume does not change because all valves are closed during this phase. Contraction, therefore, is said to be "isovolumic" or "isovolumetric."


Rapid Ejection (Phase 3) 

  •  This phase represents initial, rapid ejection of blood into the aorta and pulmonary arteries from the left and right ventricles, respectively.
  •  Ejection begins when the intraventricular pressures exceed the pressures within the aorta and pulmonary artery, which causes the aortic and pulmonic valves to open. Aortic and Pulmonic Valves Open; 
  •   AV Valves Remain Closed
  • No heart sounds are ordinarily noted during ejection because the opening of healthy valves is silent.
  • Left atrial pressure initially decreases as the atrial base is pulled downward, expanding the atrial chamber. 
  • Blood continues to flow into the atria from their respective venous inflow tracts and the atrial pressures begin to rise.


Reduced Ejection (Phase 4)

  •  Aortic and Pulmonic Valves Open; AV Valves Remain Closed.
  • After the beginning of ventricular contraction, ventricular repolarization occurs.
  •  Repolarization leads to a decline in ventricular active tension and pressure generation; Therefore, the rate of ejection (ventricular emptying) falls.
  • Left atrial and right atrial pressures gradually rise due to continued venous return from the lungs and from the systemic circulation, respectively.


Isovolumetric Relaxation (Phase 5) 

  • All Valves Closed.
  •  When the intraventricular pressures fall sufficiently at the end phase 4, the aortic and pulmonic valves abruptly close (aortic precedes pulmonic) causing the second heart sound (S2) and the beginning of isovolumetric relaxation. 
  • Althoughh ventricular pressures decrease during this phase, volumes do not change because all valves are closed.
  • The volume of blood that remains in a ventricle is called the end-systolic volume and is ~ 50 ml in the left ventricle. 
  • Differencee The difference between the end-diastolic volume and the end-systolic volume is ~ 70 ml and represents the stroke volume.

Rapid Filling (Phase 6) 

  • A-V Valves Open.
  •  As the ventricles continue to relax at the end of phase 5, the intraventricular pressures will fall at some point below their respective atrial pressures. 
  • When this occurs, the AV valves rapidly open and passive ventricular filling begins. 
  • Despitee the inflow of blood from the atria, intraventricular pressure continues to fall briefly because the ventricles are still undergoing relaxation. 
  • Oncee the ventricles are completely relaxed, their pressures will slowly rise as they fill with blood from the atria.

Reduced Filling (Phase 7)

  •  A-V Valves Open As the ventricles continue to fill with blood and expand, they become less compliant and the intraventricular pressures rise.
  •  The increase in intraventricular pressure reduces the pressure gradient across the AV valves so that the rate of filling falls late in diastole. 
  • Normall In normal, resting hearts, the ventricle is about 90% filled by the end of this phase. 
  • Inn other words, about 90% of ventricular filling occurs before atrial contraction (phase 1) and therefore is passive.
  • Aortic and pulmonary arterial pressures continue to fall during this period.


Heart sounds • 

  • Two sounds, separated by a short pause, can be clearly distinguished.
  •  They are described in words as 'lubdup'. The first sound, 'lub', is fairly loud and is due to the closure of the atrioventricular valves. 
  • This corresponds with ventricular systole. The second sound, 'dup', is softer and is due to the closure of the aortic and pulmonary valves.
  • This corresponds with atrial systole.