Cardiac cycle



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CARDIAC CYCLE 

Cardiac cycle is defined as the succession of coordinated events taking place in the heart 

during each beat. 

 The period of contraction that the heart undergoes while it pumps blood into 

circulation is called systole 

 The period of relaxation that occurs as the chambers fill with blood is called diastole 

Both the atria and ventricles undergo systole and diastole and it is essential that these 

components be carefully regulated and coordinated to ensure blood is pumped efficiently to 

the body. All these changes are repeated during every heartbeat, in a cyclic manner. 

Heart rate 

The occurrence of a cardiac cycle is illustrated by a heart rate, which is naturally indicated as 

beats per minute A healthy human heart beats 75 times per minute which states that there are 

75 cardiac cycles per minute. The cardiac cycle involves a complete contraction and 

relaxation of both the atria and ventricles and the cycle lasts approximately 0.8 seconds. 

Atria: 

 Systole is about 0.1 sec. 

 Diastole is about 0.7 sec. 

Ventricular: 

 Systole is about 0.3 sec. 

 Diastole is about 0.5 sec. 

PHASES OF THE CARDIAC CYCLE 

Events of the cardiac cycle are divided into 3 main phase 

 Atrial systole 

 ventricular systole 

 ventricular diastole 


t the beginning of the cardiac cycle, both the atria and ventricles are relaxed (diastole). 

Blood is flowing into the right atrium from the superior and inferior vena cava and the 

coronary sinus. 

 Blood flows into the left atrium from the four pulmonary veins. 

 The two atrioventricular valves, the tricuspid and mitral valves, are both open, so 

blood flows unimpeded from the atria and into the ventricles. 

 Approximately 70–80 percent of ventricular filling occurs by this method. 

 The two semilunar valves, the pulmonary and aortic valves, are closed, preventing 

backflow of blood into the right and left ventricles from the pulmonary trunk on the 

right and the aorta on the left.

ATRIAL SYSTOLE 

 Before atrial systole begins, the ventricles are already 70 percent filled because of 

passive blood flow from the atria. As the atria contract, the increased pressure causes 

the atrioventricular (AV) valves to open completely and the ventricles to fill. 

 Atrial contraction contributes the remaining 20–30 percent of filling. 

VENTRICULAR EVENTS 

Ventricular systole is divided into two subdivisions 

1. Ventricular systole 

 Iso-volumetric contraction 

 Ventricular Ejection period 

2. Ventricular diastole 

 Iso-volumetric relaxation 

 Ventricular filling 

Ventricular systole follows the depolarization of the ventricles and is represented by the QRS 

complex in the ECG. 

VENTRICULAR SYSTOLE 

This initial phase of ventricular systole is known as isovolumetric contraction 

Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber 

rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be 

ejected from the heart. 

However, blood pressure quickly rises above that of the atria that are now relaxed and in 

diastole. This increase in pressure causes blood to flow back toward the atria, closing the 

tricuspid and mitral valves. Since blood is not being ejected from the ventricles at this early 

stage, the volume of blood within the chamber remains constant. This volume is known as 

the end diastolic volume (EDV) or preload. 

Ventricular ejection phase 

In the second phase of ventricular systole, the ventricular ejection phase, the contraction of 

the ventricular muscle has raised the pressure within the ventricle to the point that it is greater 

than the pressures in the pulmonary trunk and the aorta. 

Blood is pumped from the heart, pushing open the pulmonary and aortic semilunar valves. 

Pressure generated by the left ventricle will be appreciably greater than the pressure 

generated by the right ventricle, since the existing pressure in the aorta will be so much 

higher. Nevertheless, both ventricles pump the same amount of blood. This quantity is 

referred to as stroke volume. 

Stroke volume will normally be in the range of 70–80 mL. Since ventricular systole began 

with an EDV of approximately 130 mL of blood, this means that there is still 50–60 mL of 

blood remaining in the ventricle following contraction. This volume of blood is known as the 

end systolic volume (ESV). 


VENTRICULAR DIASTOLE 

Isovolumic ventricular relaxation phase 

Ventricular relaxation, or diastole, follows repolarization of the ventricles and is represented 

by the T wave of the ECG. 

During the early phase of ventricular diastole, as the ventricular muscle relaxes, pressure on 

the remaining blood within the ventricle begins to fall. 

The semilunar valves close to prevent backflow into the heart. Since the atrioventricular 

valves remain closed at this point, there is no change in the volume of blood in the ventricle, 

so the early phase of ventricular diastole is called the isovolumic ventricular relaxation 

phase, also called isovolumetric ventricular relaxation phase.

Ventricular filling 

As the ventricular muscle relaxes, pressure on the blood within the ventricles drops even 

further. Eventually, it drops below the pressure in the atria. When this occurs, blood flows 

from the atria into the ventricles, pushing open the tricuspid and mitral valves. As pressure 

drops within the ventricles, blood flows from the major veins into the relaxed atria and from 

there into the ventricles. Both chambers are in diastole, the atrioventricular valves are open, 

and the semilunar valves remain closed. 

Summary: 

Atrial Systole: At this phase, blood cells flow from atrium to ventricle and at this period 

atrium contracts. 

Isovolumic Contraction: At this stage, ventricles begin to contract. The atrioventricular 

valves, aortic valve, and pulmonary artery valves close but there won’t be any transformation 

in volume. 

Ventricular Ejection: Here ventricles contract and emptying. Pulmonary artery and aortic 

valve opens. 

Isovolumic Relaxation: In this phase, no blood enters the ventricles and consequently forth 

pressure decreases, ventricles stop contracting and begin to relax. Now due to the pressure in 

the aorta – pulmonary artery and aortic valve close. 

Ventricular Filling Stage: In this stage, blood flows from atria into the ventricles. Both 

chambers are in diastole, the atrioventricular valves are open, and the semilunar valves 

remain closed.

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