Lecture Notes: The Cardiac Cycle

Overview

• The cardiac cycle includes all mechanical events for blood flow through the heart chambers.
• Average duration: 0.8 seconds.
• Focuses on:
  • Atrial vs. ventricular pressure
  • Arterial vs. ventricular pressure
  • AV (atrioventricular) valves status
  • Semilunar valves status
  • EKG components of each stage

Key Structures

• AV Valves: Tricuspid (right), Bicuspid/Mitral (left)
• Semilunar Valves: Pulmonary, Aortic

Phases of the Cardiac Cycle

1. Mid to Late Ventricular Diastole (Ventricular Filling)

• Definition: Ventricles are relaxed, blood returns to the heart.
• Blood Sources: Inferior vena cava, Superior vena cava, Coronary sinus, Pulmonary veins.
• Pressure and Valves:
  • Atrial pressure > Ventricular pressure ➔ AV valves open
  • Arterial pressure > Ventricular pressure ➔ Semilunar valves closed
• EKG: P wave (Atrial depolarization)

2. Isovolumetric Contraction (Systole)

• Definition: Ventricles begin to depolarize and contract, but no blood is ejected.
• Pressure and Valves:
  • Ventricular pressure < Arterial pressure ➔ Semilunar valves closed
  • Ventricular pressure > Atrial pressure ➔ AV valves close
• Heart Sound: S1 or "Lub" (AV valves closing)
• EKG: QRS Complex

3. Ventricular Ejection

• Definition: Blood is ejected from ventricles into arteries as ventricular pressure exceeds arterial pressure.
• Pressure and Valves:
  • Ventricular pressure > Arterial pressure ➔ Semilunar valves open
  • Ventricular pressure > Atrial pressure ➔ AV valves remain closed
• EKG: Continues as QRS Complex

4. Isovolumetric Relaxation

• Definition: Ventricles relax after ejection; no blood enters or leaves the ventricles.
• Pressure and Valves:
  • Arterial pressure > Ventricular pressure ➔ Semilunar valves close (Dicrotic notch observed)
  • Ventricular pressure > Atrial pressure ➔ AV valves remain closed
• Heart Sound: S2 or "Dub" (Semilunar valves closing)
• EKG: T wave (Ventricular repolarization)

Additional Notes

• End Diastolic Volume (EDV): Volume of blood in ventricles at the end of diastole.
• End Systolic Volume (ESV): Volume of blood remaining in ventricles after contraction.
• The cycle continuously repeats approximately every 0.8 seconds.