Understanding the Cardiac Cycle Phases

Sep 10, 2024

Cardiac Cycle Breakdown

Overview

  • The cardiac cycle refers to the sequence of events in one heartbeat to the beginning of the next.
  • Series aims to help understand the cardiac cycle and related cardiovascular concepts.

Phases of the Cardiac Cycle

1. Atrial Systole

  • Definition: Contraction of the atria.
  • Blood flow:
    • Right side: Blood returns from the body.
    • Left side: Blood returns from the lungs.
  • Signal for Contraction:
    • SA node (pacemaker) in the right atrium initiates the signal.
    • Signal spreads, shown as a P wave on an ECG.
  • Pressure Changes:
    • Atria contraction increases atrial pressure.
    • Blood pushed through atrioventricular valves into ventricles.
    • Atrial contraction contributes ~25% of ventricular blood volume.
  • End Diastolic Volume: Blood volume in ventricles at the end of diastole.

2. Isovolumetric Contraction

  • Definition: Ventricles contract but blood volume remains constant.
  • Signal for Contraction:
    • Depolarization from the AV node spreads through bundle and Purkinje fibers.
    • Shown as QRS complex on an ECG.
  • Valve Actions:
    • Atrioventricular and aortic valves close.
  • Pressure Changes:
    • Closed valves create a sealed compartment, increasing ventricular pressure.
    • Pressure must exceed aortic pressure (~80 mmHg) to open aortic valve.
  • Heart Sound: First heart sound ('lub') occurs.

3. Ejection Phase

  • Definition: Blood ejected from ventricles into arteries.
  • Valve Actions:
    • Aortic valve opens once ventricular pressure exceeds aortic pressure.
  • Phases:
    • Rapid Ejection: Blood quickly exits ventricles.
    • Slow Ejection: Resistance in arteries slows blood flow, decreasing pressure.
  • Blood Pressure:
    • Normal values: Systolic (120 mmHg), Diastolic (80 mmHg).
  • Stroke Volume: Amount of blood ejected (~70 ml).
  • EKG: T wave shows ventricular repolarization.

4. Isovolumetric Relaxation

  • Definition: Ventricles relax with constant blood volume.
  • Valve Actions:
    • Aortic valve closes.
    • Atrioventricular valves remain closed.
  • Pressure Changes:
    • Ventricular pressure quickly decreases.
  • Heart Sound: Second heart sound ('dub') occurs.
  • Special Notes: Dichrotic notch on aortic pressure curve.

5. Ventricular Filling

  • Definition: Blood flows into the ventricles.
  • Valve Actions:
    • Atrioventricular valves open, semilunar valves closed.
  • Blood Flow:
    • Passive filling from the atria into the ventricles.

Summary

  • Cardiac cycle phases: Atrial systole, isovolumetric contraction, ejection, isovolumetric relaxation, ventricular filling.
  • The cycle is crucial for maintaining blood circulation and eventually starts over after ventricular filling.

Additional Resources

  • Free cardiac cycle guide available for download.