The stages of the cardiac cycle

April 30, 2024

The stages of the cardiac cycle
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Cardiac Cycle

The cardiac cycle describes a complete heartbeat from the moment of production until the start of the subsequent pulse. It consists of the diastole, the systole, and the interval in between. Heart rate, which is expressed naturally in beats per minute, illustrates the occurrence of a cardiac cycle. 

Table of Contents

  1. Human Heart Anatomy
  2. Understanding the Cardiac Cycle
  3. Stages of the Cardiac Cycle
  4. Function of the Cardiac Cycle
  5. Conclusion
  6. Frequently Asked Questions

Human Heart Anatomy

The heart is the organ that circulates blood throughout the body. In a single cardiac cycle between its left and right sides, it removes deoxygenated blood from the body, pumps it to the lungs for oxygenation, returns oxygenated blood, and pumps it back to the remainder of the body. 

The left and right portions of the human heart are divided into four chambers. The heart has two chambers on top: the left and right atria, and two chambers below: the left and right ventricles. The right ventricle’s primary job is to force blood depleted of oxygen into the lungs through the pulmonary trunk and arteries. On the other hand, the left ventricle is in charge of pushing fresh blood with oxygen into the body via the aorta.

Understanding the Cardiac Cycle

The cardiac cycle is the duration of time between the atria’s contraction and the ventricles’ relaxation. 

Systole and diastole are processes that the atria and ventricles both go through. Systole is the contraction phase that the heart goes through when it circulates blood. Diastole is the term for the relaxing phase that follows the blood chambers filling up. 

To guarantee that blood is pumped to the body effectively, these parts must be meticulously controlled and synchronised.The cardiac cycle of a healthy heart, beating 70–75 times per minute, lasts approximately 0.8 seconds.

Stages of the Cardiac cycle

Every cardiac cycle involves alternating contractions of the ventricles and atria. Throughout the cardiac cycle, there is a significant shift in the chamber pressures.

The contraction phase, or systole, and the relaxation phase, or diastole, are the two main divisions of the cardiac cycle. The atrial and ventricular components of each of them are then further separated.

Following are the stages of the cardiac cycle:

  • ATRIAL SYSTOLE & DIASTOLE : In this stage, depolarization is followed by contraction of the atria. Pressure builds within the atria when the atrial muscles contract from the superior section of the atria toward the atrioventricular septum. Blood is then pumped into the ventricles through the open tricuspid, mitral, and bicuspid atrioventricular valves. When the atrial muscle returns to diastole, atrial systole, which lasts for around 100 ms, ends before ventricular systole.During the roughly 0.1-second-long atrial systole, both atria contract, forcing blood into the ventricles.
  • VENTRICULAR SYSTOLE: In this stage, the blood pressure inside the chamber rises initially when the ventricle’s muscles contract, but it is not yet high enough to cause the pulmonary and aortic semilunar valves to open and allow blood to exit the heart. 

Nevertheless, blood pressure surpasses the diastolic and relaxed atria’s level rapidly. The tricuspid and mitral valves close as a result of blood flowing back into the atria due to this pressure rise. Blood is pumped into the aorta once the ventricular muscle contraction has increased the pressure to a level higher than the pressures in the pulmonary trunk.

There are three additional sub-phases inside the ventricular systole:

ISOVOLUMETRIC VENTRICULAR CONTRACTION: During the first stage of ventricular systole, muscle tension increases, initiating ventricular contraction while maintaining constant volume. As ventricular pressure surpasses atrial pressure, both atrioventricular (AV) valves and semilunar (SL) valves close, preventing blood from exiting the ventricles. This phase, lasting approximately 0.05 seconds, elevates ventricular pressure without altering volume.

RAPID VENTRICULAR EJECTION: During this stage, pressure disparities prompt the semilunar (SL) valves to open, facilitating the ejection of roughly 70% of ventricular blood into the aorta and pulmonary arteries within 0.13 seconds. With a pressure of approximately 80 mmHg in the left ventricle compared to 8 mmHg in the right ventricle, blood is propelled more forcefully into the aorta than the main pulmonary artery.

REDUCED VENTRICULAR EJECTION: During this stage, the remaining 30% of ventricular blood is ejected, typically lasting about 0.09 seconds. Ventricular pressure decreases as blood reaches smaller arteries.

  • PROTODIASTOLE: The transitional phase signifies the conclusion of the systolic phase and the onset of the diastolic phase. The ventricular pressure will be below the blood pressure inside the major arteries due to the total reduction caused by the ventricular ejection. These variations in pressure will cause the SL valves to close. There are roughly 0.04 seconds in this level.
  • VENTRICULAR DIASTOLE: During ventricular diastole, relaxation occurs following ventricular repolarization. Divided into two phases and lasting approximately 430 ms, the early phase sees a drop in ventricular muscle pressure, causing blood to flow back toward the heart. This is known as the isovolumic ventricular relaxation phase, as atrioventricular valves remain closed, maintaining constant blood volume within the ventricle.

In the late phase of ventricular diastole, as ventricular muscle relaxes further, pressure decreases, dropping below atrial pressure. This prompts blood to flow from the atria into the ventricles, opening the tricuspid and mitral valves. As ventricular pressure continues to decline, blood flows from major veins into relaxed atria, then into ventricles. With both chambers in diastole and atrioventricular valves open, while semilunar valves remain closed, the cardiac cycle concludes.

Function of the Cardiac Cycle

The cardiac cycle’s main function is to circulate blood throughout the body. To do this, the heart alternates between contraction and relaxation cycles, pumping blood into the blood vessels so that it can be distributed throughout the body.


The cardiac cycle orchestrates the vital function of circulating blood throughout the body, achieved through the heart’s alternating contraction and relaxation cycles. Comprising two main phases, systole and diastole, the cycle delineates specific stages: atrial systole, ventricular systole, atrial diastole, and ventricular diastole. Together, these stages ensure the efficient pumping of blood into the blood vessels, facilitating its distribution across the body to sustain life.

Frequently Asked Questions

1. What takes place in the systole?

The heart contracts during systole, which results in the blood being ejected into the pulmonary trunk and aorta.

2. What takes place in the systole?

The diastolic phase of the cardiac cycle is when the heart refills its reservoir following the systolic period of blood ejection.

3. Which are the main symptoms of diastolic dysfunction?

The main signs and symptoms of diastolic dysfunction include shortness of breath, dizziness, nausea, loss of appetite, wheezing or coughing.

Disclaimer: We recommend consulting a Doctor before taking any action based on the above shared information.


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