Cardiac Cycle- Phases, Diagram, and Physiology of the Cardiac Cycle

The cardiac cycle is a sequential event in the heart that is cyclically repeated. The cardiac cycle diagram shows the cardiac cycle phases consisting of systole and diastole of both the atria and ventricles. The cardiac cycle class 11 is an important concept that is also studied in higher classes.

It is regulated by electrical signals from the sinoatrial (SA) node and atrioventricular (AV) node, which ensures the circulation of oxygenated blood throughout the body. The heart beats 72 times per minute, that is many cardiac cycles are performed per minute. In this article, we will cover the cardiac cycle - steps, diagram, and physiology of the cardiac cycle.

Cardiac Cycle Definition

The cardiac cycle is the complete sequence of events in a single heartbeat, including ventricular contraction and relaxation, ensuring blood circulation. It is controlled by electrical signals from the SA and AV nodes.

What is the Cardiac Cycle?

The cardiac cycle represents the series of events that occur to complete a heartbeat. The electrical impulse is started by the SA node, which causes the atria and ventricles to contract and relax. During this process, the heart muscles undergo changes, involuntarily pumping blood through the circulatory system.

During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood which is called the stroke volume. The stroke volume multiplied by the heart rate gives the cardiac output. Therefore, the cardiac output can be defined as the volume of blood pumped out by each ventricle per minute and averages 5000 mL or 5 litres in a healthy individual. Each cardiac cycle consists of a complete contraction and relaxation of both the atria and ventricles and each cardiac cycle lasts approximately 0.8 seconds.

In the cardiac cycle, the heart contracts during systole, expelling blood, and relaxes during diastole, filling with blood. When the atria contract, they push blood into the ventricles through the valves, creating the "lubb" sound when the valves close. Then, the ventricles force blood into the pulmonary and systemic arteries, producing the "dubb" sound as these arteries' valves close.

Cardiac Cycle Diagram

The cardiac cycle diagram class 11 is an important concept that we study in our biology syllabus and even in higher studies. Different phases of the cardiac cycle are represented in the diagram below:

Physiology of the Cardiac Cycle 

The human heart is a muscular organ with four chambers. These chambers are divided into left and right halves in which the top chambers are the atrium and the lower chambers are the ventricles. The right atrium receives the deoxygenated blood from the body and then passes it on to the right ventricle which pumps the blood towards the lungs through the pulmonary artery.

On the other hand, the left atrium receives the oxygenated blood from the lungs and then passes it on to the left ventricle which then pumps the blood towards the body through the aorta.

Cardiac Cycle Phases

Cardiac cycle phases ensure that the heart efficiently pumps blood and maintain the circulation of oxygenated blood throughout the body. Systole of the cardiac cycle refers to the contraction of heart muscles that result in the pumping of the blood.

Diastole refers to the relaxation of the heart muscles that result in the relaxing and filling of the chambers of the heart. Both the atrium and ventricles of the heart undergo systole and diastole simultaneously.

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