Cardiac Cycle Notes

Cardiac Cycle

The cardiac cycle is the sequence of events that occur and repeat with every heartbeat. It consists of two major phases: systole and diastole. Systole refers to ventricular contraction, while diastole refers to ventricular relaxation.

Key Principles

  • Blood Flow: Blood always flows from areas of higher pressure to areas of lower pressure.
  • Contraction: Contraction increases pressure within a chamber.
  • Relaxation: Relaxation lowers pressure within a chamber.
  • AV Valves: Open when atrial pressures are higher than ventricular pressures, and close when the pressure gradient is reversed.
  • Semilunar Valves: Open when ventricular pressures are higher than aortic/pulmonary pressures, and close when the reverse is true.

Phases of the Cardiac Cycle

1. Atrial Depolarization
  • The cycle begins with the firing of the SA node, which stimulates the atria to depolarize.
  • This depolarization is represented by the P-wave on the ECG.
2. Atrial Contraction
  • Atrial contraction starts shortly after the P-wave begins.
  • This contraction increases the pressure within the atria, forcing blood into the ventricles.
  • Atrial contraction only accounts for a fraction of ventricular filling because the ventricles are already almost full due to passive blood flow.
3. AV Valve Closure (S1)
  • As atrial contraction completes, atrial pressure begins to fall, reversing the pressure gradient across the AV valves.
  • This causes the AV valves to close, producing the first heart sound, S1.
  • S1 marks the beginning of systole.
  • Ventricular depolarization, represented by the QRS complex on the ECG, is halfway through at this point.
4. Isovolumetric Ventricular Contraction
  • The ventricles begin to contract, rapidly building up pressure.
  • The semilunar valves (aortic and pulmonic) remain closed.
  • This phase is called isovolumetric contraction because no blood is ejected, and ventricular volume remains unchanged.
5. Ventricular Ejection
  • Ventricular ejection starts when ventricular pressures exceed the pressures within the aorta and pulmonary artery.
  • The aortic and pulmonic valves open, and blood is ejected out of the ventricles.
  • This is the rapid ejection phase.
6. Semilunar Valve Closure (S2)
  • As ventricular repolarization, reflected by the T-wave on the ECG, begins, ventricular pressure starts to fall and the force of ejection is reduced.
  • When ventricular pressures drop below aortic and pulmonary pressures, the semilunar valves close.
  • Closure of the semilunar valves produces the second heart sound, S2.
  • This marks the end of systole and the beginning of diastole.
7. Isovolumetric Ventricular Relaxation
  • The first part of diastole is isovolumetric.
  • The ventricles relax with all valves closed.
  • Ventricular pressure drops rapidly, but ventricular volume remains unchanged.
  • Meanwhile, the atria are being filled with blood, and atrial pressures rise slowly.
8. Ventricular Filling
  • Ventricular filling starts when ventricular pressures drop below atrial pressures.
  • This causes the AV valves to open, allowing blood to flow passively down the ventricles.
9. Atrial Contraction (Final Filling)
  • The atria contract to finish the filling phase before the cycle repeats.