The cardiac cycle defines the sequence of mechanical and electrical events in one complete heartbeat. The filling of the heart with blood, the forward movement of blood, and preparation for the next beat are included in its explanation. Such is the coordination that even a small mistake in the timing of any event affects cardiac output.

The understanding of the cardiac cycle is crucial for MBBS students, and it forms the foundation for topics under cardiovascular pathology. Hence, mastering these will make clinical correlations quite easier.

Read on to understand it step by step!

What is the Cardiac Cycle? 

A cardiac cycle is regarded as the cycle that takes place in the heart from one beat to the next, encompassing phases such as diastole, or the filling of the heart, systole, or the contraction of the heart, and then a brief pause. These phases of the heart cycle are prompted by electric pulses that trigger the contractions of the heart.

A cardiac cycle is basically the sequence of events that happen in the heart as it beats. It is the process from the start of one beat to the next. It is basically a series of events related to the pressure that is responsible for the blood flowing through the heart and into the body. These events do not occur haphazardly. This is brought about by the electric and mechanical currents of the myocardium, which result in the heart beating.

Moreover, heart valves also play a vital role. These heart valves open and shut with the help of pressure differences created in the heart, causing blood to move forward and not backwards. Thus, the heart cycle is also responsible for a programmed way of blood circulation from one heart chamber to the other and then to the body.

The cardiac cycle is made up of three phases:

• Diastole: This refers to the relaxation phase, where the heart is filled with blood.
• Systole: This is the contraction phase, where blood is ejected.
• Intervening Pause: A pause that falls between beats. 

All these stages make up one beat of the heart. If an average heart beats 72 times in a minute, then there are approximately 72 heart cycles in a minute. Hence, one heart cycle takes about 0.8 seconds.

How Does Electrical Activity Trigger the Heart Cycle?

A cardiac cycle starts at the cellular level. Contractions are caused by electricity, which means that the heart is not capable of functioning without proper electrical conduction.

Here’s how it happens:

• An electrical impulse starts at the sinoatrial (SA) node
• Both atria contract together
• The impulse briefly pauses at the atrioventricular (AV) node.
• Both ventricles now contract together.

This pause is significant as it allows the ventricles to fill before they contract. Moreover, the electrical event of depolarisation is not simultaneous with muscle contraction. This is because there is a lag between depolarisation and muscle contraction, which occurs before the muscle contraction takes place.

All these electrical and mechanical activities result in changes in the pressure-volume state of the heart chambers. The entrance of calcium ions (Ca++), along with other factors, helps sustain the contraction of cardiac muscle. 

Due to the regular pattern of these pressure changes, they can be depicted using a graphical representation called the Wiggers diagram.

A simplified table is given below: 

Aspect	What It Represents
Electrical activity	Seen on ECG (e.g., QRS complex)
Mechanical response	Contraction of atria or ventricles
Pressure changes	Rise and fall within chambers
Clinical relevance	Helps interpret heart sounds and murmurs

For instance, in the Wiggers diagram, the QRS complex is shown before the onset of ventricular systole. This is important, as any confusion between the timing of the QRS complex and that of ventricular systole could potentially produce a mistake in clinical practice.

What are the Phases of the Cardiac Cycle? 

The heart cycle consists of stages that ensure the smooth flow of blood. It begins with atrial diastole, a phase that allows the heart to relax as the ventricles fill with blood. Next is the atrial systole, which forces the remaining blood into the ventricles. This is followed by the ventricular systole, which increases the pressure to ensure that the blood is pumped into the arteries. Finally, there is the ventricular diastole, which allows the ventricles to relax as they fill with blood. 

A heart cycle is divided into distinct phases that occur in a definite order. These stages help understand the filling phase of the heart with blood, its subsequent ejection phase, the phase where it pushes the blood forward, and then prepares for the next cycle. 

This is a consecutive process, but it would help to understand it phase by phase since a phase is closely related to either the change in pressures or the functioning of valves.

From a broad perspective, the heart cycle comprises diastole and systole. Within these phases, distinct phases of the cycle occur. These are explained below: 

1. Atrial Diastole (Relaxation of the Atria)

When the cardiac cycle begins, both the atria and the ventricles are relaxed. This is a passive phase, and it is critical. Here’s what happens:

• Blood enters the right atrium through the superior vena cava, inferior vena cava, and coronary sinus.
• Blood enters the left atrium through the four pulmonary veins.
• The atrioventricular (AV) valves, the tricuspid and mitral, are open. 
• Semilunar valves, namely the aortic and pulmonary valves, remain closed.

As the AV valves are open, there is a free flow of blood from the atria into the ventricles. Hence, the major contribution to the filling of the ventricles (approximately 70-80%) takes place without any active contraction. 

This phase clearly indicates that the heart does not purely depend on force, as the work is being done by the various pressure forces.

2. Atrial Systole (Atrial Contraction)

Atrial systole refers to the electrical depolarisation of the atria, represented by the P wave on the ECG. In this, the atria are said to be actively contracting.

Atrial contraction pushes the remaining blood into the ventricles, a contribution known as the atrial kick, which completes ventricular filling. However, closure of the atrioventricular valves does not occur due to atrial systole but rather when rising ventricular pressure exceeds atrial pressure at the beginning of ventricular systole.

Although brief, atrial systole is important because it ensures that the ventricles are optimally filled before they contract. Following this, the atria relax and return to diastole. They will not contract again until the next cycle.

3. Ventricular Systole (Ventricular Contraction)

This phase starts following ventricular depolarisation, evident by the QRS complex in the ECG. It does not make any sense if it is taken as one event alone; therefore, it is classified into two phases:

1. Isovolumic Contraction

• Contractions of the ventricles start.
• Pressure in the ventricles increases.
• The mitral and tricuspid valves shut to ensure no backflow into the atria.
• The semilunar valves remain closed.

As no valves are open, there is no blood leaving the ventricles. So, volume remains the same while the pressure increases. That is why this phase is called isovolumic contraction.

2. Ventricular Ejection

As the ventricular pressure increases further:

• Pressure is higher than that of the aorta or pulmonary trunk.
• Aortic and pulmonary valves open.
• Blood is released into the systemic circulation as well as the pulmonary circulation. 

Both ventricles produce the same amount of blood, called stroke volume. But the left ventricle produces a far higher pressure since it supplies the entire body.

4. Ventricular Diastole (Ventricular Relaxation)

Following contraction, the ventricles start to relax. This phase is similarly subdivided into two parts because valve movements differ, along with pressure changes:

1. Isovolumic Relaxation

• Ventricles relax, and pressure falls.
• Blood in the aorta and pulmonary artery tries to flow back.
• The semilunar valves close, leading to the dicrotic notch.
• AV valves are still closed.

Since all the valves are closed, the ventricular volume does not change. The pressure drops rapidly; hence, this phase prepares the ventricles for filling.

2. Ventricular Filling

Once ventricular pressure falls below atrial pressure:

• The opening of the mitral and tricuspid valves occurs.
• Blood flows from the atria into the ventricles.
• Both chambers are in diastole.

This completes the cardiac cycle. There is continuous blood flow from the veins to the atria so that the next cycle does not have to restart, but can rather continue without an irregularity. The heart shouldn’t skip or disturb the sequence of the cardiac cycle, because even with a minor mistake in timing, cardiac output may be affected.

The summary for the stages of the cardiac cycle is given below: 

Phase	Key Event
Atrial diastole	Passive filling of atria and ventricles
Atrial systole	Active atrial contraction
Isovolumic contraction	Ventricular pressure rises, no ejection
Ventricular ejection	Blood pumped into the arteries
Isovolumic relaxation	Ventricles relax, valves closed
Ventricular filling	Blood flows into the ventricles

What is the Duration of the Cardiac Cycle? 

The duration of the cardiac cycle is 0.8 seconds when one is resting. Atrial systole is very brief, then ventricular systole, with diastole being the longest. All phases overlap, meaning that the heart beats throughout. Even slight variations in timing would influence heart functioning.

The duration of the cardiac cycle is defined as the time for one complete heartbeat from the beginning of one cycle to the start of the next. In an average healthy adult human at rest, the heart beats about 72 times per minute. This rate is reasonably consistent, so the time for one cardiac cycle can be estimated easily.

At 72 beats per minute:

• 1 ÷ 72 = 0.0139 minutes per beat
• This is about 0.8 seconds per cardiac cycle.

In other words, each heartbeat takes less than a second. That may sound brief, but in that brief span of time, a lot happens, and that is where the need for such precision in timing comes into play.

Duration of Individual Phases of the Cardiac Cycle

Although the total time for one complete cardiac cycle is approximately 0.8 seconds, the timeframe covered by each event of the cycle is unequal because the heart must fill before ejecting, and filling doesn’t happen instantaneously.

Atrial systole is brief due to the fact that it is only intended to provide the final push into the ventricles. Ventricular systole, though, will take more time since blood needs to be strongly pumped into both the pulmonary artery and the aorta. 

Meanwhile, ventricular diastole takes up the largest portion of the cycle because proper filling is of importance, and an incomplete relaxation will not allow the heart to function well.

Phase	Approximate Duration
Atrial systole	0.1 seconds
Ventricular systole	0.3 seconds
Atrial diastole	0.7 seconds
Ventricular diastole	0.5 seconds

The overlap between atrial and ventricular phases keeps the blood continuously circulating. For instance, during ventricular systole, atrial diastole extends well into it in order for the atria to refill while the ventricles are contracting. Thus, this makes the heart work in a very efficient manner without pauses.

Thus, understanding the sequence and duration of events in the cardiac cycle provides insight into the normal rhythmic heart beating, variations in heart rate, and why the heart cannot hasten or prolong any period of the cardiac cycle. Small changes in the timing can lead to altered cardiac output during conditions such as exercise or disease states.

What is the Haemodynamics of the Cardiac Cycle? 

Haemodynamics is concerned with the study of blood circulation through the heart and arteries or veins during the heart cycle. Blood circulation is aided by the presence of differences in pressure; therefore, it is filled and ejected effectively. Stages involved in haemodynamics include relaxation, filling, contraction, and ejection phases.

Haemodynamics is basically the study of the flow of blood through the heart and blood vessels of a body or individual organism during the cardiac cycle. It can be simply described as the understanding of the process that allows the heart to fill with blood and then release it effectively. 

This mechanism can be analysed using a graph called the pressure-volume curve of the heart, which actually represents the left side of the heart since it works at high pressures, whereas the same process takes place on the right side of the heart at low pressures.

Pressure-Volume Variations During the Cardiac Cycle

There are four phases of ventricular functioning. All of these phases are governed by the principles of haemodynamics. This implies that there is a certain amount of difference in the pressure between phases, resulting in blood flowing from the higher to the lower-pressure areas.

Phase	Valve Status	What Happens	Why It Matters
Isovolumic Relaxation	Aortic valve closed (S2 heard); Mitral valve closed	Ventricular pressure falls rapidly, but volume doesn’t change	The pressure drop creates a gradient, so the mitral valve can open. Without this, blood won’t enter the ventricle
Ventricular Filling	Mitral valve open; Aortic valve closed	Blood flows from the atrium into the ventricle, increasing ventricular volume	Filling occurs due to the elastic recoil of blood vessels; hence, the heart doesn’t actively pull blood in
Isovolumic Contraction	Mitral valve closed (S1 heard); Aortic valve closed	Ventricular pressure rises rapidly, but blood can’t leave the ventricle	Pressure builds up to a level high enough for ejection
Rapid Ventricular Ejection	Aortic valve open; Mitral valve closed	Blood is ejected into the aorta; over 60% of ventricular blood is expelled	This phase ensures effective blood delivery to the body

Pressure Differences between Heart Chambers

Blood circulation in the heart cycle is driven by the differences in pressure between the heart chambers and blood vessels. It is characterised by the following values of blood pressure:

Heart Chamber / Vessel	Pressure (mmHg)
Left ventricle	120 / 15
Right ventricle	25 / 5
Right atrium	Mean 4–5
Pulmonary arteries / Left atrium	25 / 10
Aorta	120 / 80

Haemodynamics helps clarify “why heart sounds occur, what heart valves do, or why blood keeps flowing.” Furthermore, because each step must work precisely to achieve the right pressure change, the tiniest disturbances can affect cardiac output. 

Hence, the study of haemodynamics is critical in understanding heart sound phenomena and pressure-volume loops, among other heart conditions.

Heart Sounds and Cardiac Auscultation

Cardiac auscultation involves listening for the heart sounds, with emphasis on the sound of the valves closing. S1 or “lub” is responsible for systole, while S2 or “dub” is responsible for diastole. Both murmurs, S3 or S4, can reveal improper heart functioning.

Cardiac auscultation is the process of listening to heart sounds that are produced at each stage of the cardiac cycle. Since blood actually courses rather smoothly through both chambers and valves of the healthy heart, it cannot be heard. What the clinician hears is the sound of the heart valves snapping shut. 

Heart sounds are thus produced by the movement of blood through the valves and the resultant snapping shut of the valves, rather than by blood flow itself. These sounds reflect the precise timing and coordination of the cardiac cycle.

Normal Heart Sounds

During routine auscultation, two major heart sounds are normally discerned. These are normally described as “lub” and “dub”, names that together apply to one heartbeat.

Heart Sound	When It Occurs	Valve Closure	What It Marks
S1 (Lub)	End of ventricular filling	Atrioventricular valves (mitral and tricuspid)	Beginning of ventricular systole
S2 (Dub)	End of ventricular ejection	Semilunar valves (aortic and pulmonary)	Beginning of ventricular diastole

Thus, these sounds help clinicians locate where the heart is within the cardiac cycle.

Heart Murmurs

Abnormal blood flow may occur when the heart valves are damaged. The valves may thicken and narrow (stenosis) or leak (regurgitation). As a result of that, the flow of the blood becomes turbulent instead of being smooth.

• Turbulent flow creates a humming or whooshing sound.
• These sounds are termed heart murmurs.
• Murmurs can occur during systole, diastole or both. 

Therefore, murmurs are important clues to underlying valve disease, even in the absence of symptoms.

Additional Heart Sounds (S3 and S4)

In certain clinical instances, additional heart sounds may be present apart from S1 and S2. These are less common but highly informative:

Heart Sound	Timing in the Cardiac Cycle	Mechanism	Normal Occurrence	Clinical Significance
S3	Early diastole, just after S2	Rapid deceleration of blood flowing from the left atrium into the left ventricle	Normal in children, young adults, and athletes	In older adults, it often indicates congestive heart failure
S4	Late diastole, just before S1	Atrial contraction against a stiff ventricle	Not considered normal	Associated with reduced ventricular compliance and left ventricular hypertrophy

These sounds shouldn’t be ignored, as they often point toward structural or functional cardiac problems.

FAQs about the Cardiac Cycle

1. What are the various stages of the heart cycle?

A cycle of the heart comprises several distinct stages that occur in a definite order. These stages include atrial diastole, atrial systole, isovolumic contraction, ventricular ejection, isovolumic relaxation, and ventricular filling. All these stages work together to ensure that the heart is filled with blood before it can be pumped. Due to their consecutive dependency, the stages cannot be interrupted for the continuity of circulation.

2. What takes place in the systole phase?

Systole is the phase where the heart chambers contract. During the ventricular systole phase, the blood is pumped into the aorta and pulmonary trunk. This is the phase that creates the pressure for the blood to move onwards; therefore, systole is crucial for the blood to reach the body and lungs.

3. What is the diastole phase? 

Diastole is the relaxation phase of the heart cycle. During this phase, the heart chambers are filled with blood after being pumped out during the systole phase. This phase is important since the amount of blood pumped depends on the efficiency of blood filling into the heart. Therefore, diastole would not be effective if the heart fails to relax correctly.

4. What are the key symptoms of diastolic dysfunction?

When diastolic dysfunction happens, the heart’s ventricles become stiff and lose their ability to relax. It leads to reduced filling of the heart. Symptoms of diastolic dysfunction include:

• Breathing difficulties
• Weakness
• Dizziness
• Nausea 
• Loss of appetite 
• Coughing or wheezing

These signs occur because the heart cannot fill with blood or pump it effectively. 

5. Why is knowing the cardiac cycle important? 

A description of the cardiac cycle is important for understanding heart sounds, blood pressure fluctuations, and heart valve functioning. Furthermore, it is significant for understanding heart failure or heart valve dysfunctions. 

Conclusion 

Understanding the cardiac cycle is fundamental for every medical student. It explains how the heart fills and pumps blood in a precise duration and sequence, and relaxes, allowing blood to flow with ease throughout the body. Because small timing errors can impact cardiac output, a clear grasp of this cycle helps you link physiology with clinical findings and exam concepts.

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