The Cardiac Cycle, Animation

Introduction

This tutorial provides a comprehensive overview of the cardiac cycle, explaining its phases and key events. Understanding the cardiac cycle is essential for students of cardiology and those preparing for exams like the USMLE.

Step 1: Initiation of the Cardiac Cycle

• The cardiac cycle begins with the firing of the sinoatrial (SA) node, which acts as the heart's natural pacemaker.
• This electrical impulse stimulates the atria to depolarize, represented by the P-wave on an ECG.
• Atrial contraction follows shortly after the P-wave, causing an increase in atrial pressure that forces blood into the ventricles.
• Note that atrial contraction accounts for only a fraction of ventricular filling; most filling occurs passively through open atrioventricular (AV) valves.

Step 2: Atrial Contraction and Closure of AV Valves

• As atrial contraction completes, atrial pressure begins to fall, reversing the pressure gradient across the AV valves.
• This pressure change causes the AV valves to close, marking the beginning of systole.
• The closure of the AV valves produces the first heart sound (S1).

Step 3: Ventricular Depolarization and Isovolumetric Contraction

• At this stage, ventricular depolarization is represented by the QRS complex on the ECG.
• The ventricles begin to contract, rapidly increasing pressure inside them.
• During this phase, known as isovolumetric contraction, all valves are closed, and no blood is ejected, leading to an unchanged ventricular volume.

Step 4: Ventricular Ejection Phase

• When ventricular pressures exceed those in the aorta and pulmonary artery, the aortic and pulmonic valves open.
• Blood is then ejected out of the ventricles during the rapid ejection phase.
• As the ventricles begin to repolarize (indicated by the T-wave), ventricular pressure starts to fall, which reduces the force of ejection.

Step 5: Closure of Semilunar Valves and Beginning of Diastole

• Once ventricular pressures drop below those in the aorta and pulmonary artery, the semilunar valves close.
• This closure marks the end of systole and the beginning of diastole, producing the second heart sound (S2).

Step 6: Isovolumetric Relaxation

• The first part of diastole is characterized by isovolumetric relaxation.
• During this phase, the ventricles relax with all valves closed, leading to a rapid drop in ventricular pressure while the volume remains unchanged.
• Meanwhile, the atria are being filled with blood, causing a gradual rise in atrial pressure.

Step 7: Ventricular Filling

• Ventricular filling begins when ventricular pressures drop below atrial pressures, leading to the opening of the AV valves.
• Blood flows passively down into the ventricles, completing the cycle as the ventricles get ready for the next contraction.

Conclusion

The cardiac cycle consists of distinct phases, including initiation, contraction, ejection, and relaxation. Each phase plays a crucial role in ensuring effective blood circulation throughout the body. Understanding these phases can enhance your grasp of cardiac function and prepare you for further studies in cardiology. For practical application, consider reviewing ECG readings alongside this cycle for a comprehensive learning experience.