Lecture on the Cardiac Cycle

Overview

• The cardiac cycle consists of mechanical events where blood flows through the heart chambers.
• Average duration: 0.8 seconds.
• Key focus areas:
  • Differences in atrial vs. ventricular pressure.
  • Differences in arterial vs. ventricular pressure.
  • Behavior of AV (atrioventricular) valves.
  • Behavior of semilunar valves (pulmonary and aortic).
  • EKG correlations with stages of the cardiac cycle.

Key Stages of the Cardiac Cycle

1. Mid to Late Ventricular Diastole

• Diastole: Defined as relaxation.
• Blood returns to the heart via:
  • Inferior vena cava
  • Superior vena cava
  • Coronary sinus
  • Pulmonary veins (into the left atrium)
• AV Valves Open: Due to more pressure in atria than ventricles.
  • Tricuspid valve (right atrium to ventricle)
  • Mitral valve (left atrium to ventricle)
• Blood Flow: 70-80% of blood passively flows into ventricles.
• Pressure Dynamics:
  • Atrial pressure > Ventricular pressure.
  • Arterial pressure > Ventricular pressure (semilunar valves closed).
• EKG: P wave indicates atrial depolarization.
• Phase Name: Period of ventricular filling.

2. Isovolumetric Contraction (Systole)

• Ventricles Start Depolarizing & Contracting: Blood moves upwards towards pulmonary trunk/aorta.
• Pressure Dynamics:
  • Ventricular pressure < Arterial pressure (semilunar valves closed).
  • Ventricular pressure > Atrial pressure (AV valves close).
• Heart Sound: First heart sound (S1, "lub") from AV valves snapping shut.
• EKG: QRS complex indicates ventricular depolarization.

3. Ventricular Ejection (Mid to Late Ventricular Systole)

• Ventricular Pressure: Exceeds arterial pressure, opening semilunar valves.
  • Left ventricle: 120 mmHg (aorta pressure ~80 mmHg)
  • Right ventricle: 25 mmHg (pulmonary trunk ~10 mmHg)
• Blood Ejection: Blood flows out of ventricles.
• AV Valves Remain Closed: Due to higher ventricular pressure than atrial pressure.
• EKG: Continues with QRS complex.

4. Isovolumetric Relaxation

• Ventricles Start Relaxing: Blood ejected, semilunar valves close due to pressure change.
• Pressure Dynamics:
  • Arterial pressure > Ventricular pressure (closing semilunar valves creates aortic pressure rise, known as dichrotic notch).
  • Ventricular pressure > Atrial pressure (AV valves closed).
• Heart Sound: Second heart sound (S2, "dub") from semilunar valves snapping shut.
• EKG: T wave indicates ventricular repolarization.

Conclusion

• The cardiac cycle involves coordinated pressure changes, valve operations, and electrical activity as reflected in the EKG.
• The cycle repeats approximately every 0.8 seconds, maintaining continuous blood circulation.