Cardiac Cycle

  • A cardiac cycle refers to the interval between onset of one heartbeat to the onset of the next heart beat. It has two main phases : Ventricular systole and ventricular diastole.
  • Ventricular systole (or simply systole)
The systolic phase is divided into : -
  1. Isovolumetric contraction : As the ventricular contraction starts, the intraventricular pressure begins to rise, leading to an abrupt closure of AV valves (mitral and tricuspid valves). The closure of AV valves produces first heart sound (S1). the pressure is not enough to push open the semilunar valves (aortic and pulmonary) but causes the closed AV valve to bulge into the atrium, causing a small but sharp rise in atrial pressure called the 'C' wave on jugular venous pulse (JVP). Because both the valves (AV valves an semilunar valves) are closed, there is no change in volume, i.e., there is isovolumetric (isometric) contraction. Isovolumetric contraction ends with opening of semilunar (aortic and pulmonary) valves.
  2. Ventricular ejection : When the steeply rising ventricular pressure exceeds the pressure in aorta and pulmonary artery, the semilunar valves open and ventricular ejection 
  3. begins. The ejection of blood is rapid at first (rapid ejection phase), but slows down during later part of systole (slow ejection phase). During rapid ejection phase, when the ventricles contract, the fibrous partition separating the ventricles from the atria (the AV ring) is pulled down. As a result, the atrial muscles get stretched and the atria dilate which causes a sharp fall in atrial pressure and the X-descent in JVP.
  4. Protodiastole : In this very short phase, ventricles start relaxing and ventricular pressure begins to fall very sharply but the semilunar valves are still open. As a result, the column of blood in aorta (or pulmonary artery for right ventricle) tries to fall into the ventricle, hitting on its way the semilunar (aortic or pulmonary) valves. This causes 
  5. closure of the aortic/pulmonary valves which produces 2" heart sound (S2). The venous blood flow continues to flow in the atria from great veins (SVC and ICC) and there is relaxation of fibrous AV ring due to ventricular relaxation; both of which cause a rise in atrial pressure and production of `V' wave in JVP.
Ventricular diastole (or simply diastole)The diastole phase is divided into : -
  1. Isovolumetric relaxation : - This phase is the period between the closure of semilunar valve and opening of the AV valve. The ventricles continue relaxing and there pressure continues to fall. However, as both valves (AV valves and semilunar valves) are closed, there is no change in volume, i.e., isovolumetric relaxation. Relaxation phase ends with opening of AV valve.
  2. Rapid ventricular filling : - When the ventricular pressure falls below atrial pressure, AV valves open and the accumulated blood in the atria rushes into the ventricle very rapidly. This passive filling contributes to 70% of ventricular filling, normally. This results in a sharp fall in atrial pressure which produces Y descent in JVP.
  3. Diastasis : - After the initial rapid ventricular filling, blood flows slowly and smoothly from the SVC and IVC through the right atrium into the right ventricle without any turbulence anywhere along the path. Similarly, blood from the pulmonary veins flows into the left ventricle without any turbulence. This phase of nonturbulent ventricular filling is called diastasis. The atrial pressure remains slightly greater than the ventricular pressure because inflow to atrium exceeds the outflow the atrium. iv) Last rapid filling phase (atrial systole) : - The atria contract and pump blood rapidly into the ventricles.Atrial systole is associated with sharp rise in atrial pressure which produces a-wave in JVP.
Exam Question
  • Opening of the aortic valve takes place during ventricular systole, especially during the phase of ventricular ejection which occur at the end of isovolumetric contraction.. Each cardiac cycle consist of an atrial and ventricular cycle. Atrial cycle consist of atrial systole and diastole. Ventricular cycle consist of ventricular systole (0.3s) and diastole (0.5s). Ventricular systole consist of isovolumetric contraction and phase of ventricular ejection. Ventricular diastole consist of protodiastole, isovolumetric relaxation, rapid filling phase, diastasis and last rapid filling phase.
  • The P wave represents the wave of depolarization that spreads from the SA node throughout the atria.
  • The QRS complex represents ventricular depolarization. Ventricular rate can be calculated by determining the time interval between QRS complexes.
  • The isoelectric period (ST segment) following the QRS is the time at which the entire ventricle is depolarized and roughly corresponds to the plateau phase of the ventricular action potential.
  • The T wave represents ventricular repolarization and is longer in duration than depolarization.
  • The Q-T interval represents the time for both ventricular depolarization and repolarization to occur and therefore roughly estimates the duration of an average ventricular action potential. 
  • Duration of cardiac cycle (at heart rate of 75/min) is 0.80sec. Out of which duration of systole is 0.27sec and diastole is 0.53sec.
  • Coronary blood flow is maximum during the isovolumetric relaxation phase of cardiac cycle. 
  • Isovolumic relaxation phase of the cardiac cycle ends with opening of AV Valve.
  • Action potential in cardiac muscles
  • Normal RMP in myocardial fibers is about -90 mV The AP in myocardial fibers has 5 phases : 0, 1, 2, 3, 4.
  • Phase 0 :- It is the phase of rapid depolarization and is due to opening of fast sodium channels.
  • Phase 1 :- It is the initial phase of rapid repolarization. It is due to closure of fast Na+ channels.
  • Phase 2 :- This is platen phase. It is due to opening of 'voltage gated slow Ca+ channels, also called calcium-sodium channels which causes calcium influx.
  • Phase 3 :- This phase of final repolarization is due to opening of IC- channels. The membrane potential comes back to resting membrane potential.
  • Phase 4 :- It is the resting phase, i.e., the phase of resting membrane potential.
  • Maximum pressure in left ventricle is seen in ventricular ejection phase of cardiac cycle.
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