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Cardiac Cycle – Definition, Phases, Diagram, FAQs

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  • Last Updated : 07 Aug, 2022
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The basic function of the heart is to circulate blood throughout the body in a cycle. The human heart beats 100,000 times every day. The cardiovascular system, which consists of the heart, arteries, veins, and blood capillaries, is responsible for the transfer of many chemicals in humans and includes systemic and pulmonary circulation. The valves of the heart regulate blood flow, resulting in organized blood propulsion to the next chamber. The cardiac cycle is a sequence of heart contractions that pressurize different chambers of the heart, causing blood to flow in only one direction. To have a better grasp of the cardiac cycle, meaning, length, and stages, continue reading.

Cardiac Cycle 

The cardiac cycle is the series of events that occur as the heart beats.”

The sequence of events that occur throughout a heartbeat is known as the cardiac cycle. Automaticity and rhythmicity are features of the (Sinoatrial) SA node. As a result, action potentials are generated in the atrial and ventricular muscle fibers. Depolarization and repolarization occur as a result. Following then, the heart goes through a series of modifications that are repeated from beat to beat.

The cardiac muscle cells, or cardiomyocytes, that comprise the heart muscle are in charge of pumping blood. Cardiomyocytes are distinct muscle cells that are striated like skeletal muscle yet pump involuntarily and rhythmically like a smooth muscle.

During each cardiac cycle, the heart contracts (systole), pushing blood out and pumping it around the body; this is followed by a relaxation phase (diastole), during which the heart fills with blood—at the same time, the atria contract, pumping blood into the ventricles through the atrioventricular valves. As the atrioventricular valves shut, a monosyllabic “lub” sound is created. After a small delay, the ventricles contract at the same time, pushing blood via the semilunar valves into the aorta and the artery supplying blood to the lungs.

Physiology of the Cardiac Cycle 

The human heart has four chambers, divided into left and right halves. The two top chambers are the left and right atria, while the lower two chambers are the right and left ventricles. The right ventricle’s primary purpose is to transport deoxygenated blood to the lungs through the pulmonary arteries and trunk. The left ventricle is in charge of pumping freshly oxygenated blood to the body via the aorta.

Phases of the Cardiac Cycle 

Phases of Cardiac Cycle


The cardiac cycle events outlined here track the route of blood from the time it enters the heart to the time it is pumped out of the heart and into the rest of the body. Systole refers to contraction and pumping, whereas diastole refers to relaxation and filling. The atria and ventricles of the heart both go through diastole and systole phases, which occur concurrently.

Atrial Diastole 

The semilunar valves close and the atrioventricular valves open during atrial diastole. The left atrium is filled with oxygenated blood from the pulmonary veins, whereas the right atrium is filled with blood from the venae cavae. The SA node contracts once again, causing both atria to do the same.

Atrial contraction causes the contents of the left atrium to empty into the left ventricle and the contents of the right atrium to empty into the right ventricle. The mitral valve, which is situated between the left atrium and the left ventricle, prevents oxygenated blood from returning to the left atrium.

Atrial Systole  

The atrioventricular valves close and the semilunar valves open during atrial systole. The ventricles receive contraction signals. The left ventricle pumps oxygenated blood to the aorta, and the aortic valve prevents the oxygenated blood from returning to the left ventricle.

At this moment, oxygen-depleted blood is also pushed from the right ventricle to the pulmonary artery. Through systemic circulation, the aorta extends out to give oxygenated blood to all regions of the body. De-oxygenated blood returns to the heart via the venae cavae after its journey through the body.

Ventricular Diastole 

The atria and cardiac ventricles are relaxed during ventricular diastole, and the atrioventricular valves are open. Following the final cardiac cycle, oxygen-depleted blood returns to the heart via the superior and inferior vena cavae and travels to the right atrium.

The atrioventricular valves (tricuspid and mitral) that are open allow blood to flow from the atria to the ventricles. The sinoatrial (SA) node delivers impulses to the atrioventricular (AV) node, which provides a signal that causes both atria to contract. The right atrium discharges its contents into the right ventricle as a result of this contraction. The tricuspid valve, which is situated between the right atrium and the right ventricle, prevents blood from returning to the right atrium.

Ventricular Systole  

The right ventricle, which is filled with blood from the right atrium, gets impulses from fibre branches (Purkinje fibres) transmitting electrical impulses that force it to contract at the start of the ventricular systole period. The atrioventricular valves close and the semilunar valves (pulmonary and aortic valves) open as a result.

When the ventricles contract, oxygen-depleted blood from the right ventricle is pushed to the pulmonary artery. The pulmonary valve keeps blood from returning to the right ventricle. The pulmonary artery transports deoxygenated blood to the lungs via the pulmonary circuit. The blood absorbs oxygen there before returning to the left atrium of the heart through the pulmonary veins.

Duration of the Cardiac Cycle 

The heart rate per minute determines the length of a cardiac cycle. The cardiac cycle lasts one second if the heart rate is 60 beats per minute. 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 0.1 seconds.
  • Ventricular systole lasts roughly 0.3 seconds.
  • Atrial diastole lasts around 0.7 seconds.
  • Ventricular diastole lasts roughly 0.5 seconds.

Phases of Cardiac Cycle

  • At the end of the atrial systole, the ventricular systole commences.
  • The ventricular chamber is bursting at the seams with blood. The pressure within the left ventricle is around 5-8 mm Hg. As the chamber compresses, the intraventricular pressure rises.
  • After then, the blood in the ventricle attempts to return to the atrium.
  • The initial cardiac sound is created by the atrioventricular valves shutting, preventing blood from returning to the atrium.
  • Because the aortic pressure is about 80 mm Hg, the semilunar valves at the aortic opening cannot open.
  • As a result, the ventricle now contracts isometrically, like a closed chamber.
  • As a result, intraventricular pressure rises rapidly.
  • The period of ventricular systole during which both the AV and SL valves shut, resulting in a dramatic increase in intraventricular pressure, is known as isovolumetric contraction/isovolumetric ventricular contraction.
  • This phase lasts around 0.05 seconds.
  • The ventricular pressure rapidly increases from around 5 mm Hg to about 80 mm Hg.
  • When the intraventricular pressure surpasses 80 mm Hg, the SL valves are pushed open, resulting in the maximum ejection phase of the ventricular systole.

The following are the various stages of a cardiac cycle:

  • Atrial Diastole: The heart’s chambers are calmed at this period. When the aortic and pulmonary arteries close and the atrioventricular valves open, the chambers of the heart relax.
  • Atrial Systole: During this phase, blood cells travel from the atrium to the ventricle and the atrium contracts.
  • Isovolumic Contraction: The ventricles begin to contract at this stage. The atrioventricular, valve, and pulmonary artery valves all close, but there is no volume change.
  • Ventricular Ejection: Ventricular Ejection occurs when the ventricles contract and empty. Both the pulmonary artery and the aortic valve shut.
  • Isovolumic Relaxation: During this phase, no blood enters the ventricles, lowering pressure and causing the ventricles to cease contracting and relax. Because of the aortic pressure, the pulmonary artery and aortic valve have closed.
  • Ventricular Filling Stage: Blood moves from the atria into the ventricles during this stage. It is referred to as a single stage (first and second stage). Following that, there are three steps that involve the movement of blood from the ventricles to the pulmonary artery.

Frequently Asked Questions

Question 1: What happens during the cardiac cycle’s diastole phase?


The cardiac ventricles relax and the heart fills with blood during the diastole phase. The ventricles contract and pump blood from the heart to the arteries during the systole phase. When the heart chambers fill with blood and blood is pushed out of the heart, one cardiac cycle is accomplished.

Question 2: What is the cardiac cycle’s second to final phase?


The relaxation is isovolumetric. All of the heart valves close during isovolumetric relaxation, the second to the final phase of the cardiac cycle. This means no blood is flowing between the cardiac chambers or out of the heart.

Question 3: When the heart rate is 75 beats per minute, what is the cardiac cycle?


When the heart rate is 75 beats per minute, the cardiac cycle lasts 0.8 seconds. The cardiac cycle events are as follows: The atrial systole lasts 0.1 seconds, during which the atria contract and the ventricles relax. Depolarization of the SA node results in atrial depolarization, which is shown by a P wave on the ECG.

Question 4: What exactly is the cardiac cycle?


The cardiac cycle is described as a cycle that contains all of the events that occur during a single heartbeat. Both the atria and ventricles contract (systole) and relax (diastole) during the cardiac cycle. The atria and ventricles contract and relax during each cardiac cycle.

Question 5: What happens to the blood throughout the course of a cardiac cycle?


The atria and ventricles contract and relax during each cardiac cycle. This alternating contraction and relaxation cause the blood to flow from high-pressure locations to low-pressure areas.

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