Cardiac Cycle

The cardiac cycle consists of a series of events that take place in the heart during one complete heartbeat. It includes both mechanical and electrical activities, which facilitate the pumping of blood throughout the body.

1. Overview of the Cardiac Cycle

The cardiac cycle is divided into two main phases:

• Systole: Contraction phase, during which blood is pumped out of the heart chambers.
• Diastole: Relaxation phase, allowing chambers to refill with blood.

The cardiac cycle is usually described in relation to the left side of the heart because the events are similar in both sides, though the pressures are higher on the left.

2. Phases of the Cardiac Cycle

There are six main phases in the cardiac cycle:

A. Atrial Systole

• Duration: About 0.1 seconds.
• Event: The atria contract, pushing the remaining blood into the ventricles.
• Valves: The atrioventricular (AV) valves (mitral and tricuspid) are open, and semilunar valves (aortic and pulmonary) are closed.
• Purpose: Ensures that ventricles are filled to their maximum capacity, a phase often referred to as “active filling.”

B. Isovolumetric Contraction

• Duration: About 0.05 seconds.
• Event: The ventricles begin to contract, increasing pressure within the ventricles.
• Valves: All valves are closed, so there is no blood flow out of or into the ventricles.
• Purpose: The pressure rises sharply, preparing the ventricles to eject blood. This is the initial phase of ventricular systole.

C. Ventricular Ejection

• Duration: About 0.3 seconds.
• Event: The pressure in the ventricles exceeds the pressure in the aorta and pulmonary artery, causing the semilunar valves to open and blood to be ejected.
• Valves: Semilunar valves are open; AV valves are closed.
• Purpose: This is the main phase where blood is pumped into the systemic (from the left ventricle) and pulmonary (from the right ventricle) circulations.

D. Isovolumetric Relaxation

• Duration: About 0.05 seconds.
• Event: The ventricles begin to relax after contraction.
• Valves: All valves are closed again, as pressure drops but remains higher than in the atria.
• Purpose: This phase decreases ventricular pressure to prepare for passive filling in diastole. Ventricular pressure continues to drop until it is lower than atrial pressure.

E. Ventricular Filling (Early Diastole)

• Duration: About 0.2 seconds.
• Event: As the ventricular pressure drops below atrial pressure, the AV valves open, and blood flows passively from atria to ventricles.
• Valves: AV valves are open, semilunar valves are closed.
• Purpose: Allows blood to fill the ventricles passively. This phase accounts for the majority of ventricular filling.

F. Diastasis (Late Diastole)

• Event: This is the last phase of ventricular filling, where the blood flow from atria to ventricles slows down.
• Valves: AV valves remain open; semilunar valves are still closed.
• Purpose: Completes the ventricular filling phase until the cycle begins again with atrial systole.

3. Pressure and Volume Changes

• During systole, ventricular pressure increases and eventually exceeds arterial pressures, leading to ejection.
• During diastole, ventricular pressure falls, allowing for passive filling from the atria.
• Volume Changes: The end-diastolic volume (EDV) is the maximum volume in the ventricles at the end of filling. The end-systolic volume (ESV) is the remaining volume after contraction.

4. Heart Sounds

• First Heart Sound (S1): Occurs during the beginning of systole due to the closure of the AV valves.
• Second Heart Sound (S2): Marks the end of systole, caused by the closure of the semilunar valves.
• Additional Sounds: In some conditions, S3 (rapid filling) and S4 (atrial contraction) can be heard.

5. Electrical Conduction and the Cardiac Cycle

• The SA node initiates the impulse, spreading through the atria (causing atrial systole).
• The impulse reaches the AV node, then travels to the Bundle of His and Purkinje fibers, triggering ventricular contraction.
• The ECG can be correlated with phases of the cardiac cycle:
  • P Wave: Atrial depolarization (atrial systole).
  • QRS Complex: Ventricular depolarization (ventricular systole).
  • T Wave: Ventricular repolarization (beginning of ventricular relaxation).

6. Summary of Key Points

• The cardiac cycle includes atrial contraction (systole), ventricular contraction (systole), and ventricular relaxation (diastole).
• Systolic phase involves blood ejection, and diastolic phase involves filling of the ventricles.
• Heart sounds (S1 and S2) are markers of valve closure.
• The cardiac cycle is controlled by electrical impulses originating in the SA node and traveling through the heart’s conduction system.

Summary table of the cardiac cycle phases:

Note:

• S1: Caused by the closure of the AV valves, marking the start of ventricular systole.
• S2: Caused by the closure of the semilunar valves, marking the end of ventricular systole.
• S3 and S4: Sometimes heard in abnormal conditions, associated with rapid filling (S3) and atrial contraction (S4).

About the Author

Dr. Akif Baig, DM in Cardiology, is a distinguished cardiologist and the founder of Infusion Med Zone. With a strong foundation in advanced cardiology, he is committed to both clinical excellence and medical education, striving to make complex concepts in cardiology accessible to learners and professionals. His work reflects a dedication to advancing cardiovascular knowledge and care.