The Cardiac Cycle

Phases of the Cardiac Cycle (0.8s)

  1. Atrial Systole (0.1s): Atria contract, pushing blood into ventricles.
  2. Ventricular Systole (0.3s):
    • Isovolumetric Contraction: Ventricles contract, all valves closed.
    • Ventricular Ejection: Semilunar valves open, blood is ejected.
  3. Ventricular Diastole (0.5s):
    • Isovolumetric Relaxation: Ventricles relax, all valves closed.
    • Rapid & Slow Filling: AV valves open, ventricles fill passively.

Heart Sounds

  • S1 ("Lub"): Closure of the AV valves (tricuspid and mitral) at the beginning of ventricular systole.
  • S2 ("Dub"): Closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole.

Aortic Pressure Curve

  • Systolic Pressure: The peak pressure in the aorta during ventricular ejection (~120 mmHg).
  • Diastolic Pressure: The lowest pressure in the aorta before the next ejection (~80 mmHg).
  • Dicrotic Notch: A small dip in pressure caused by the closure of the aortic valve.

ECG Correlation

  • P-wave: Atrial depolarization (leads to atrial systole).
  • QRS complex: Ventricular depolarization (leads to ventricular systole).
  • T-wave: Ventricular repolarization (occurs during ventricular diastole).

Summary of the Cardiac Cycle

Phase Key Events Valves Heart Sound
Atrial Systole Atria contract AV open, SL closed -
Isovolumetric Contraction Ventricles contract All closed S1 ("Lub")
Ventricular Ejection Blood ejected SL open, AV closed -
Isovolumetric Relaxation Ventricles relax All closed S2 ("Dub")
Ventricular Filling Ventricles fill AV open, SL closed -