Table of Content

• Cardiac Cycle Definition
• What is the Cardiac Cycle?
• Cardiac Cycle Diagram
• Physiology of the Cardiac Cycle 
• Cardiac Cycle Phases
• Duration of the Cardiac Cycle 

Phase	Description
Atrial Diastole	The atria (upper chambers) are relaxed, allowing blood to flow in from the lungs (left atrium) and body (right atrium).
Atrial Systole	The atria contract, squeezing blood into the ventricles
Isovolumic Contraction	Ventricles contract, but the valves are closed, so blood volume stays the same within them. Pressure increases.
Ventricular Ejection	Ventricles continue to contract, forcing blood out to the lungs (right ventricle) and body (left ventricle) through open semilunar valves.
Isovolumic Relaxation	Ventricles relax, but the valves are closed, so blood volume stays the same within them. Pressure decreases.
Ventricular Filling	Ventricles relax, and the pressure difference between the atria and ventricles causes blood to flow back into the ventricles through open atrioventricular valves.

Atrial Relaxation (Atrial Diastole)

• The four chambers of heart are in a relaxed state. The semilunar valves are closed at this phase.
• As the tricuspid and bicuspid valves are open, blood from the vena cava and pulmonary veins flows into the right and left ventricle respectively through the right and left atria.
• Now, the SAN generates an action potential which stimulates both the atria to undergo a simultaneous contraction.

Atrial Contraction (Atrial Systole)

• The atria contract again providing an additional push to fill the ventricles with the blood.

Isovolumetric Contraction (Ventricular Systole)

• Now, the ventricles begin to contract. This contraction increases ventricular pressure, leading to the closure of the atrioventricular valves.
• At this point, all valves are closed, and no blood is ejected from the heart.
• This phase is termed “isovolumetric” because no volume change occurs.

Ventricular Ejection

• The blood flow in the ventricle is increased by 30%.
• The AV node and AV bundle conduct the action potential to the ventricles,
• The bundle of His distributes it throughout the ventricular muscle, resulting in the contraction of the ventricular muscles.

Isovolumetric Relaxation

• After ejection, the ventricles relax, and the semilunar valves close to prevent backflow of blood from the arteries into the ventricles.
• During this phase, all heart valves are closed.

Passive Ventricular Filling (Early Diastole)

• As the ventricles continue to relax, pressure decreases, causing the atrioventricular valves (tricuspid and mitral) to open. B
• Blood flows from the atria into the ventricles due to gravity. This phase primarily accounts for the filling of the ventricles with blood.

Duration of the Cardiac Cycle 

The heart rate per minute determines the length of a cardiac cycle. The length of the cardiac cycle and heart rate are inversely related. As a result, if the heart rate is increased to 120 beats per minute, the cardiac cycle will be completed in around 0.5 seconds.

Duration of cardiac cycle ( seconds / beats ) = 60 (seconds / minutes ) / Heart ( beats / minutes )

A normal person’s heartbeat is 72 beats per minute. As a result, the length of one cardiac cycle may be estimated as follows:

1/72 minutes/beat =.0139 minutes/beat

Each cardiac cycle will last 0.8 seconds at a heart rate of 72 beats per minute. The following table shows the duration of the various phases of the cardiac cycle:

• Atrial systole lasts roughly from 0.8 to 0.1 seconds.
• Ventricular systole lasts roughly from 0.1 to 0.3 seconds.
• Atrial diastole lasts around from 0.1 to 0.8 seconds.
• Ventricular diastole lasts roughly from 0.4 to 0.1 seconds.
• Joint diastole lasts roughly from 0.4 to 0.8 seconds.

Conclusion – Cardiac Cycle

In conclusion, the cardiac cycle is an important process in the heart. The cardiac cycle consits of systole and diastole phases that ensure blood circulation. This cycle will be covered in various calsses as it forms an important part of biology syllabus. Cardiac cycle is regulated by electrical signals, maintaining a heart rate of 72 beats per minute. Understanding its phases and physiology helps in learning about heart function and overall health.

Also Read:

• Regulation of Cardiac Activity
• Cardiac Output
• Body Fluids and Circulation

FAQs on Cardiac Cycle

What are the Different Phases of the Cardiac Cycle?

The cardiac cycle phases includes: Atrial diastole, Atrial systole, Isovolumic contraction(ventricle systole), Ventricular ejection, Isovolumic relaxation, Ventricular filling.

What is Cardiac Cycle Length?

Each cardiac cycle comprises of a complete contraction and relaxation of both the atria and ventricles therefore, each cardiac cycle lasts approximately 0.8 seconds.

What Happens During Systole?

The heart’s chambers contract during the systole causing the atria to push blood into the ventricles. Then the ventricles pump blood into the aorta and pulmonary artery. This phase is responsible for blood ejection from the heart.

What are the Major Symptoms of Diastolic Dysfunction?

Major symptoms of diastolic dysfunction include weakness, shortness of breath, especially during physical activity, nausea, dizziness and fatigue, as the heart has difficulty relaxing and filling properly during its resting phase.

What Exactly is the Cardiac Cycle?

The cardiac cycle is the complete sequence of events in one heartbeat. It involves both systole (contraction) and diastole (relaxation) of the heart chambers, ensuring the continuous circulation of blood throughout the body. It is regulated by electrical signals from the sinoatrial (SA) and atrioventricular (AV) nodes.

What is Diastole?

Diastole is the phase of the cardiac cycle when the heart’s chambers relax and fill with blood, allowing for the heart to rest and prepare for the next contraction. It is essential for proper blood circulation and nourishing the body’s tissues and organs.

What are the 4 Stages of the Cardiac Cycle?

The 4 stages of the cardiac cycle are atrial diastole, atrial systole, ventricular systole, and ventricular diastole.

How do you Calculate your Cardiac Cycle?

Cardiac cycle duration is calculated by dividing 60 seconds by the heart rate (beats per minute).

What is the Cardiac Cycle of Blood Flow?

The cardiac cycle of blood flow involves atrial and ventricular contraction and relaxation, ensuring blood circulation throughout the body.

What are the Major Symptoms of Diastolic Dysfunction?

Major symptoms of diastolic dysfunction include weakness, shortness of breath, nausea, dizziness, and fatigue.

What Happens During Systole?

During systole, the heart chambers contract and push blood into the pulmonary artery and aorta.