The cardiac cycle is the sequence of events that occurs during a single heartbeat. During a cardiac cycle, both the atria and ventricles undergo periods of contraction (systole) and relaxation (diastole), which are coordinated by the heart’s pacemaker cells. Blood enters the chambers during the periods of diastole and is ejected from the chambers during the periods of systole. Major veins and arteries allow blood to enter and leave the heart, and the valves help direct blood flow in and out of the chambers.
In a person whose heart is beating at 75 times per minute, the duration of one cardiac cycle is 0.8 seconds.
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3D Cardiac Cycle
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Phases of the Cardiac Cycle
The following diagram shows the periods when the heart chambers are contracting (systole) or relaxing (diastole) during each cardiac cycle. A recording of the heart’s electrical activity (electrocardiogram; ECG) can also be seen. Elevations and depressions in the ECG indicate when the heart muscles are depolarizing or repolarizing.
Each cardiac cycle consists of five repeating phases or stages, which are shown below. The end of one cycle immediately precedes and prepares the heart for the start of the next cycle.
Because it is cyclic, selecting a starting point for the cardiac cycle is somewhat arbitrary, and any phase can serve as the starting point. We have chosen to begin our discussion of phase events with “Atrial systole” because this phase is associated with the beginning of an ECG recording.
Phases of the Cardiac Cycle
a. Atrial Systole Phase Events
During this phase, both atria contract (atrial systole) while both ventricles enter a late stage of relaxation (late ventricular diastole).
Atrial systole follows the P wave on an ECG. This upward deflection in the ECG recording is produced by a wave of depolarization that sweeps across the atrial chambers.
As the atrial muscles contract from the superior portion of the atria toward the atrioventricular septum, pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular (tricuspid, and mitral or bicuspid) valves.
At the start of atrial systole, the ventricles are normally filled with approximately 70 – 80 percent of their capacity due to inflow during ventricular diastole. Atrial contraction, which is also referred to as the “atrial kick,” contributes the remaining 20 – 30 percent of filling.
b. Ventricular Isovolumetric Contraction Phase Events
During the ventricular isovolumetric (isovolumic) contraction phase, the ventricles begin to contract (early ventricular systole) and the atria begin to relax (early atrial diastole).
This phase of the cardiac cycle is initiated by a wave of depolarization that sweeps across both ventricles and is recorded as a QRS complex on an ECG.
At the end of atrial systole and just prior to ventricular contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position. This volume is known as the end diastolic volume (EDV).
Initially, as the ventricles contract, the pressure of the blood in the chambers rises but does not become high enough to open the pulmonary and aortic semilunar valves so blood can be ejected. The ventricular blood pressure, however, quickly rises above that of the relaxing atria. As a result, blood in the ventricles flows back toward the atria, closing the tricuspid and mitral (AV) valves.
Since blood is not being ejected from the ventricles at this early stage, the volume of blood within the chamber remains constant. Consequently, this initial phase of ventricular systole is known as isovolumetric (isovolumic) ventricular contraction.
c. Ventricular Ejection Phase Events
Ventricular ejection occurs as the ventricular chambers continue to contract (= mid to late ventricular systole) and the atria remain in diastole.
On an ECG, the ventricular ejection phase begins near the end of the QRS wave and terminates near the end of the T wave.
The sustained ventricular contraction causes the pressure in the chambers to rise above the pressures in the pulmonary trunk and aorta arteries, which pushes opens the semilunar valves.
The 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).
d. Ventricular Isovolumetric Relaxation Phase Events
Ventricular iIsovolumetric (isovolumic) relaxation occurs during early ventricular diastole and mid atrial diastole. The ventricular muscles start to relax and pressure on the remaining blood within the ventricle begins to fall.
This brief time period corresponds with the end portion of the T wave on an ECG recording.
When the pressure in the ventricles drops below the pressure in both the pulmonary trunk and aorta, blood flows back toward the heart, producing the dicrotic notch (small dip) seen in aortic blood pressure tracings. 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 isovolumetric (isovolumic) ventricular relaxation phase.
e. Ventricular Filling Phase Events
Ventricular filling occurs during mid to late ventricular diastole and late atrial diastole, which corresponds to the time period between the end of one ECG recording and the start of another.
As the ventricular muscles relax, pressure on the blood in the ventricles continues to decrease. 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 (AV) valves.
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 are closed, preventing the backflow of blood into the ventricles from the pulmonary trunk and aorta arteries.
At this stage, all four heart chambers are in diastole, the atrioventricular valves are open, the semilunar valves remain closed, and the cardiac cycle is now complete.
Remember, all of the cardiac cycle phases and events described above repeat every 0.8 seconds when a heart is beating at 75 times per minute. Amazing, right?
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