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.
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.
Systole is about 0.1 sec.
Diastole is about 0.7 sec.
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
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
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.
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 systole is divided into two subdivisions
1. Ventricular systole
Ventricular Ejection period
2. Ventricular diastole
Ventricular systole follows the depolarization of the ventricles and is represented by the QRS
complex in the ECG.
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).
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.
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.
Atrial Systole: At this phase, blood cells flow from atrium to ventricle and at this period
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
Ventricular Ejection: Here ventricles contract and emptying. Pulmonary artery and aortic
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