Cardiac Cycle: Definition, Diagram, Steps and Facts

What is the Cardiac Cycle?

The cardiac cycle is the process of a single series of heartbeats, which describe the contraction and relaxation of the heart’s chambers to pump blood to the body tissues. This cycle is very important for the continuous circulation of blood where oxygen and nutrients are taken to the tissues and wastes are removed.

The cardiac cycle includes several key phases: systole which essentially involves the contraction of the heart to pump blood to the arterial system and diastole phase which is the period of relaxation of the heart after it has filled the self with blood. The knowledge of the cardiac cycle is important in any practice when it comes to the understanding of the function of the heart in the circulatory system which is a vital part of diagnosing cardiovascular disorders.

Phases of the Cardiac Cycle

The phases of the cardiac cycle are listed below-

Atrial Systole

• Systole atrial is a phase where the atrial muscles contract.

• This contraction forces blood into the ventricles, which finalises the submission of that chamber and raises the pressure in the ventricle.

• The contraction of the atria makes sure that the ventricles are filled with blood in readiness for the next phase of pumping.

Ventricular Systole

• This phase commences with the myocardium of ventricles contracting without an alteration of the volume due to the shutting of the valves.

• Thus, the pressure inside the ventricles starts to rise rapidly, becoming higher than the pressure inside the atria.

• When the pressure in the ventricles becomes higher than in the aorta and the pulmonary arteries this opens the semilunar valves and then the blood is expelled from the ventricles into the arteries.

Diastole

• After ventricular ejection, both ventricles will be in the process of relaxation while the ventricular volumes remain the same and the semilunar valves close while atrioventricular valves are still closed.

• After the valves of the atria are closed, the ventricles start to decompress, and the pressure within them becomes lower than the pressure in the atria, therefore, the atrioventricular valves open and, hence, the blood from the atria flows into the ventricles and the cycle is complete.

Heart Sounds

The heart sounds are discussed below-

S1 – AV Valves Closure (Lub)

It is due to the closure of atrioventricular (AV) valves; tricuspid and mitral resulting from the start of ventricular systole. This sound known as the “lub” is the first phase of the contraction of the heart and is important for the first sound of the heartbeat.

S2 – SL Valves Closure (Dub)

These are created by the shutting of the semilunar valves, pulmonary and aortic, at the end of the ventricular systole. This sound known as ‘dub’ marks the point when the contraction phase of the heart stops and the diastole, which is the relaxation of the ventricles starts.

Additional Heart Sounds: S3 & S4

The additional heart sounds are:

S3 (Third Heart Sound)

After S2, the fast filling of ventricles during the early phase of diastole cause this sound. This is common, in the youthful and healthy populations. However, the older population may experience heart failure or volume overload. It is referred to as “ventricular gallop. ”

S4 (Fourth Heart Sound)

Found immediately before S1 and is related to atrial systole in an attempt to force blood into a stiff or hypertrophied ventricle. This sound is known as the ‘atrial gallop’ and may occur in ailments such as hypertension as well as aortic stenosis.

Murmurs (Systolic, Diastolic, Continuous)

Heart murmurs are just sounds that are produced by the turbulent blood flow in the valves or in the heart chambers.

• Systolic Murmurs: Happen between S1 and S2 and are linked to pathologies that comprise aortic stenosis and/or mitral regurgitation.

• Diastolic Murmurs: Occur between S2 and the following S1, and may be due to mitral stenosis or aortic regurgitation.

• Continuous Murmurs: Lasts throughout the cardiac cycle, that is those seen in conditions like patent ductus arteriosus.

Duration of Cardiac Cycle

A normal person's heart beats at a rate of 72 beats per minute. Thus, one cardiac cycle's duration can be computed as follows:

1/72 beats per minute equals 0.0139 beats per minute.

Each cardiac cycle will last 0.8 seconds at a heart rate of 72 beats per minute.

The following lists the duration of the various cardiac cycle stages:

Conditions Affecting the Cardiac Cycle

The diseases related to defective cardiac cycle are:

Hypertension

Hypertensive disease and hypertension in particular worsen the demand on the heart and radical changes in the cardiac cycle by increasing afterload and thus negatively affect the heart muscle over time.

Heart Failure

A state, in which stroke volume and heart rate are endangered due to the failure of the heart to pump blood efficiently. This can alter the rhythms of the normal cardiac cycle and that can cause such symptoms like fatigue and fluid retention.

Valve Disorders

Diseases such as stenosis or regurgitation relate to the valves and influence the circulating movement of blood through the heart and its capacity to fulfil the work of the cardiac cycle. Heart sounds become abnormal and cardiac output is decreased as a result of poor circulation.

Cardiac Cycle NEET MCQs (With Answers & Explanations)

Types of questions asked from this topic are:

• Phases of cardiac cycle

• Heart sounds

Practice Questions for NEET

Q1. Systole is contraction of

1. AV node

2. SA node

3. Major arteries

4. Atria and ventricle

Correct answer: 4) Atria and ventricle

Explanation:

The heart regularly and rhythmically contracts and relaxes at short intervals throughout life without pause and fatigue The heart functions by collecting blood through both the atria and then distributing it through the ventricles. The action of the heart is a summation of contractions and relaxations of the atria and ventricles. The contraction of the heart is called systole whereas the relaxation of the heart is referred to as a diastole. The atria and ventricles of the heart contract alternately. A single contraction of the heart followed by relaxation of the heart constitutes a heartbeat. A heartbeat consists of contraction and relaxation of atria and ventricles. The phase of contraction is referred to as systole while the phase of relaxation is called diastole.

Hence, the correct answer is option 4) Atria and ventricle.

Q2. In a cardiac output of 5250 mL per minute , with 75 heartbeats per minute , the stroke volume is

1. 60 ml

2. 80 ml

3. 55 ml

4. 70 ml

Correct answer: 4) 70 ml

Explanation:

Each ventricle ejects about 70 mL of blood per contraction or the stroke volume in the cardiac cycle, where the pumping of blood by the heart. The volume plays an important role in ensuring an appropriate blood flow, and it is calculated using the basic formula of EDV-ESV difference. Stroke volume varies with cases of preload, afterload, and contractility, and this makes it an important component of overall cardiac output.

Hence, the correct answer is option 4) 70 mL.

Q3. During cardiac cycle, the excitatory impulses are rapidly transmitted from AV node to all parts of the ventricles via

1. Bundle of His and Purkinje fibers

2. Post caval and pre caval

3. AV node

4. Dorsal aorta

Correct answer: 1) Bundle of His and Purkinje fibers

Explanation:

Contraction of atria is initiated and activated by the SA node which then transmits these waves of contraction across the walls of atria through muscle fibers. When this wave of contraction originating from the SA Node reaches the AV node, it gets stimulated. The excitatory impulses are rapidly transmitted from the AV node to all parts of the ventricles via the bundle of His and Purkinje fibers. These impulses cause the contraction of ventricles simultaneously. The ventricles force blood through a long system of arteries and hence exert great pressure on the blood. The excitatory impulses are rapidly transmitted from the AV node to all parts of the ventricles via a bundle of His and Purkinje fibers.

Hence, the correct answer is option 1) Bundle of His and Purkinje fibers.

Also Read:

• MCQ Practice on Cardiac Cycle

• Components of Blood

• Functions of Blood

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