Lecture Notes: Cardiac Cycle

Overview of the Cardiac Cycle

• Definition: The cardiac cycle encompasses all mechanical events where blood flows through different chambers of the heart.
• Duration: On average, it takes about 0.8 seconds.
• Key Concepts:
  • Atrial vs. Ventricular Pressure
  • Arterial vs. Ventricular Pressure
  • Atrioventricular (AV) Valves
  • Semilunar Valves (Pulmonary and Aortic)
  • EKG Components

Phases of the Cardiac Cycle

1. Mid to Late Ventricular Diastole

• Diastole: Refers to heart relaxation.
• Blood Flow: Blood returns to the heart from:
  • Inferior and Superior Vena Cava
  • Coronary Sinus
  • Pulmonary Veins
• Pressure Dynamics:
  • Atrial Pressure > Ventricular Pressure
  • AV Valves open (tricuspid and bicuspid/mitral valves)
• Blood Movement:
  • 70-80% of blood flows passively into the ventricles.
• EKG: P wave indicates atrial depolarization.
• Event Name: Period of Ventricular Filling

2. Isovolumetric Contraction

• Phase Definition: Also known as isovolumetric systole.
• Ventricular Activity: Ventricles begin to depolarize and contract.
• Pressure Dynamics:
  • Ventricular Pressure < Arterial Pressure
  • AV Valves close
  • Semilunar Valves (Pulmonary and Aortic) remain closed
• Heart Sound: "Lub" (S1) is heard when AV valves snap shut.
• EKG: QRS complex indicates ventricular depolarization.

3. Ventricular Ejection

• Blood Ejection: Ventricles contract, pushing blood into the aorta and pulmonary trunk.
• Pressure Dynamics:
  • Ventricular Pressure > Arterial Pressure
  • Semilunar Valves open
  • AV Valves remain closed
• Event Name: Mid to Late Ventricular Systole or Ventricular Ejection Phase
• EKG: Continues with QRS complex

4. Isovolumetric Relaxation

• Phase Definition: Ventricles begin diastole, or relaxation.
• Pressure Dynamics:
  • Arterial Pressure > Ventricular Pressure
  • Semilunar Valves close
  • AV Valves remain closed
• Heart Sound: "Dub" (S2) occurs when semilunar valves snap shut.
• EKG: T wave indicates ventricular repolarization.
• Event Name: Also known as Isovolumetric Relaxation Phase.

Summary

• Cardiac Cycle Continuation: After the cycle completes, it restarts with ventricular diastole.
• Cycle Importance: Ensures continuous blood circulation through systemic, pulmonary, and coronary circuits.

Key Terms

• EDV (End Diastolic Volume): Volume of blood in the ventricles at the end of diastole.
• ESV (End Systolic Volume): Volume of blood remaining in the ventricles after systole.