CARDIAC CYCLE: PHASES AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The cardiac cycle comprises a repetitive sequence of events of contraction, relaxation, and filling of the ventricles that occur during the heartbeat. These phases are usually generalized in systolic and diastolic function. The first refers to the contraction of the heart and the second to the relaxation of the organ.

The cycle can be studied using different methodologies. If an electrocardiogram is used, we will be able to differentiate different types of waves, namely: P waves, QRS complex, T waves and finally U waves, where each one corresponds to a precise event of the electrical cycle of the heart, associated with depolarization phenomena and repolarization.

Source: DanielChangMD revised original work of DestinyQx

The classic graphic way of representing the cardiac cycle is called a Wiggers diagram.

The function of the cardiac cycle is to achieve the distribution of blood throughout all tissues. For this body fluid to achieve effective circulation through the body's vessel system, there must be a pump that exerts enough pressure for its movement: the heart.

From a medical point of view, the study of the cardiac cycle is useful for the diagnosis of a series of cardiac pathologies.

Historical perspective

Studies related to the cardiac cycle and the function of the heart date back to the early 18th century, where researcher Harvey first described the movements of the heart. Later, in the 20th century, Wiggers represented these movements graphically (more on this graph later).

Thanks to the contribution of these scientists, the cardiac cycle was defined as the time period where the phenomena of systoles and diastoles occur. In the first, the contraction and ejection of the ventricle occurs and in the second, relaxation and filling occurs.

Subsequent research using isolated muscle as an experimental model has transformed the traditional concept of the cardiac cycle initially proposed by Wiggers.

The change was not made in terms of the essential steps of the cycle, but in terms of the two phenomena mentioned - systoles and diastoles - which develop continuously.

For the reasons previously stated, Brutsaert proposes a series of modifications more in line with the experimental model, including relaxation phenomena.

Heart anatomy

To achieve a better understanding of the cardiac cycle, it is necessary to know certain anatomical aspects of the heart. This pumping organ is present in the animal kingdom, but differs highly depending on lineage. In this article we will focus on the description of the typical heart model of a mammal.

The heart present in mammals is characterized mainly by its efficiency. In humans it is located in the thoracic cavity. The walls of this organ are called the endocardium, myocardium, and epicardium.

It consists of four chambers, two of which are atria and the remaining two are ventricles. This separation ensures that oxygenated and deoxygenated blood do not mix.

Blood is able to circulate inside the heart thanks to the presence of valves. The left atrium opens to the ventricle through the mitral valve, which is bicuspid, while the opening of the right atrium to the ventricle occurs through the tricuspid valve. Finally, between the left ventricle and the aorta we have the aortic valve.

Properties of the heart muscle

The nature of cardiac muscle is quite similar to skeletal muscle. It is excitable under the application of a wide range of stimuli, namely: thermal, chemical, mechanical or electrical. These physical changes lead to a contraction and a release of energy.

One of the most outstanding aspects of the heart is its ability to emit an automatic rhythm, in an orderly, repetitive, constant manner and without the help of any external entity. In fact, if we take the heart of an amphibian and place it in a physiological solution (Ringer's solution) it will continue to beat for a while.

Thanks to these properties, the heart can function in a sequential repetition of events collectively called the cardiac cycle, which we will describe in depth below.

What is the cardiac cycle?

The heart works by following a basic pattern of three phenomena: contraction, relaxation, and filling. These three events occur incessantly throughout the life of animals.

Ventricular ejection is called systolic function and diastolic function refers to the filling of blood. This entire process is orchestrated by the sinoatrial or sinus node.

The cycle can be studied using different methodologies and can be understood from various points of view: such as electrocardiography, which refers to the sequence of electrical signals; anatomofunctional or echocardiographic; and the hemodynamic that is studied by pressurometry.

Anatomical and functional vision

Five events can be specified in each heart beat: isovolumic ventricular contraction and ejection corresponding to systoles - generally known as systoles or contraction of the heart; followed by isovolumic ventricular relaxation, passive atrial filling, and active ventricular filling (atrial systole), which together are known as diastoles or muscle relaxation and blood filling.

With the ultrasound approach, it is done using echoes, which describes the passage of blood through the valves through the chambers of the heart. The hemodynamic, for its part, consists of the introduction of a catheter inside the heart and measuring the pressures during each phase of the cycle.

Active ventricular filling

The cycle begins with the contraction of the atria due to an action potential. Immediately the blood is expelled to the ventricles thanks to the opening of the valves that connect both spaces (see anatomy of the heart). When the filling is complete all the blood will be contained in the ventricles.

Ventricular contraction

Once the ventricles have filled, the contraction phase begins. During this process, the valves that were open when filling were closed, to prevent the return of blood.

Ejection

With the increase in pressure in the ventricles, the valves open so that blood can access the vessels and continue on its way. At this stage, a significant decrease in ventricular pressure is noted.

Ventricular ratio

In the previous stage, we have concluded the phenomenon of systole, and with the initiation of ventricular relaxation we give way to diastole. As its name indicates, what happens in this phase is the relaxation of the ventricle, reducing the pressures in the area.

Passive ear filling

In the stages described above, we have created a pressure gradient that will favor the passive entry of blood. This gradient will favor the passage of blood from the atria to the ventricles, generating pressure in the corresponding valves.

When this filling process is complete, a new systole can begin, thus ending the five phases that occur in one heartbeat.

Electrocardiographic vision

An electrocardiogram is a record of the local currents involved in the transmission of action potentials. In the tracing produced by the electrocardiogram, the different stages of the cardiac cycle can be clearly distinguished.

The waves that are detected in an electrocardiogram have been arbitrarily designated, namely: P waves, QRS complex, T waves and finally U waves. Each one corresponds to an electrical event in the cycle.

The P wave

These waves represent the depolarization of the arterial muscles, which spread radially from the sinoatrial node to the atrioventricular (AV) node. The average duration is about 0.11 seconds, and the amplitude is about 2.5 mm.

The PR interval

The delay in the transmission of the impulse from the AV node is recorded on the electrocardiogram as a segment lasting about 0.2 seconds. This event occurs between the start of the P wave and the start of the QRS complex.

The QRS complex

This interval is measured from the beginning of the Q waves to the S wave. The stage represents a depolarization event that expands. The normal range for this stage is 0.06 seconds to 0.1.

Each wave in the complex is characterized by having a particular length. The Q wave occurs due to the depolarization of the septum and lasts about 0.03 seconds. The R wave ranges from 4 to 22 mm in height with a duration of 0.07 seconds. Lastly, the S wave is about 6mm deep.

The ST interval

This interval corresponds to the duration of a state of depolarization and repolarization. However, most electrocardiograms do not show a true ST segment.

The T wave

This stage represents the repolarization wave of the ventricle. It measures approximately 0.5 mm.

One of the characteristics of T waves is that they can be affected by a series of physiological factors, such as drinking cold water prior to the exam, smoking, medications, among others. Emotional factors can also alter the T wave.

U wave

It represents the period of greatest excitability of the ventricles. However, the interpretation becomes complicated, since in most electrocardiograms the wave is difficult to visualize and analyze.

Graphical representations of the cycle

There are different graphic ways to represent the different stages of the cardiac cycle. These graphs are used to describe the changes that occur throughout the cycle in terms of different variables during a beat.

The classical diagram is called the Wiggers diagram. These figures represent the pressure changes in the color chambers and the aorta, and the volume variations in the left ventricle throughout the cycle, the noises, and the recording of each of the electrocardiogram waves.

The phases are assigned their names depending on the contraction and relaxation events of the left ventricle. For reasons of symmetry, what is true for the left portion also holds for the right.

Duration of cycle phases

Two weeks after conception, the newly formed heart will begin to beat in a rhythmic and controlled manner. This heart movement will accompany the individual until the moment of death.

If we assume that an average heart rate is in the order of 70 beats every minute, we will have that diastole exhibits a duration of 0.5 seconds and systole of 0.3 seconds.

Cardiac cycle function

Blood is considered the body fluid responsible for the transport of various substances in vertebrates. In this closed transport system, nutrients, gases, hormones and antibodies are mobilized, thanks to the organized pumping of blood to all body structures.

The efficiency of this transport system is responsible for maintaining a homeostatic mechanism in the body.

Clinical study of cardiac function

The simplest approach that a healthcare professional can use to assess cardiac function is to listen to the sound of the heart through the chest wall. This test is called auscultation. This cardiac evaluation has been used since time immemorial.

The instrument for this test is a stethoscope that is placed on the chest or back. By means of this instrument two sounds can be distinguished: one corresponds to the closure of the AV valves and the next to the closure of the semilunar valves.

Abnormal sounds can be identified and associated with pathologies, such as murmurs or abnormal valve movement. This occurs due to the pressure flow of blood trying to enter through a closed or very narrow valve.

Medical applicability of the electrocardiogram

In case of any medical condition (such as arrhythmias) it can be detected in this test. For example, when the QRS complex has an abnormal duration (less than 0.06 seconds or more than 0.1) it may be indicative of a heart problem.

An atrioventricular block, tachycardia (when the heart rate is between 150 and 200 beats per minute), bradycardia (when the beats per minute are less than expected), ventricular fibrillation (a disorder that affects the contractions of the heart and normal P waves are replaced by small waves), among others.

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