M04.03.04 The Cardiac Cycle

The cardiac cycle represents the sequence of events occurring in the heart during one complete heartbeat. It involves the contraction (systole) and relaxation (diastole) of the atria and ventricles. Understanding the cardiac cycle is crucial for comprehending how blood circulates through the heart and body.

Phases of the Cardiac Cycle

The cardiac cycle can be divided into several phases:

1. Atrial Systole
2. Isovolumetric Contraction
3. Ventricular Ejection
4. Isovolumetric Relaxation
5. Ventricular Filling

Detailed Description of Each Phase

1. Atrial Systole
   • Description: This phase begins with the atrial contraction initiated by the sinoatrial (SA) node. The atrial pressure increases, pushing the remaining blood into the ventricles through the open atrioventricular (AV) valves (tricuspid and mitral valves).
   • Key Points:
     • Atrial systole contributes to the final ventricular volume.
     • The P wave on an electrocardiogram (ECG) corresponds to atrial depolarization.
   
2. Isovolumetric Contraction
   • Description: This phase occurs after atrial systole. The ventricles begin to contract, but no blood is ejected because the semilunar valves (aortic and pulmonary valves) are still closed.
   • Key Points:
     • The QRS complex on an ECG represents ventricular depolarization.
     • Ventricular pressure increases sharply.
3. Ventricular Ejection
   • Description: Once the ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open, and blood is ejected from the ventricles.
   • Key Points:
     • The stroke volume is the amount of blood ejected per beat.
     • The T wave on an ECG corresponds to ventricular repolarization.
4. Isovolumetric Relaxation
   • Description: Following ventricular ejection, the ventricles begin to relax. As the ventricular pressure drops below the arterial pressure, the semilunar valves close, marking the end of systole.
   • Key Points:
     • The second heart sound (S2) is associated with the closure of the semilunar valves.
     • Ventricular volume remains constant as all valves are closed.
5. Ventricular Filling
   • Description: This phase starts when the ventricular pressure falls below atrial pressure, causing the AV valves to open. Blood flows passively from the atria to the ventricles.
   • Key Points:
     • The rapid filling phase is followed by a slow filling phase (diastasis).
     • Atrial systole provides the final portion of ventricular filling.

Side Notes with Key Points to Memorize

• Atrial Kick: Provides the final 20-30% of ventricular filling.
• QRS Complex: Represents ventricular depolarization.
• Stroke Volume: Amount of blood ejected by the ventricles during systole.
• Second Heart Sound (S2): Closure of the semilunar valves.
• Diastasis: The slow-filling phase of the ventricles during diastole.

Bibliography:

1. Berne, R. M., Levy, M. N., Koeppen, B. M., & Stanton, B. A. (2018). Physiology (6th ed.). Elsevier.
2. Mohrman, D. E., & Heller, L. J. (2018). Cardiovascular Physiology (10th ed.). McGraw-Hill Education.
3. Hall, J. E. (2016). Guyton and Hall Textbook of Medical Physiology (13th ed.). Elsevier.
4. Klabunde, R. E. (2011). Cardiovascular Physiology Concepts (2nd ed.). Lippincott Williams & Wilkins.
5. Lilly, L. S. (2012). Pathophysiology of Heart Disease (5th ed.). Wolters Kluwer Health.