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.

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Anatomy Of The Heart

The anatomy of the heart is discussed below-

Heart Chambers

The chambers are divided into:

Atria

All anteriorly situated to the ventricles are the heart's two superior chambers referred to as atria. The right atrium collects deoxygenated blood that drains the body through superior & inferior vena cava while the left atrium collects oxygenated blood pumped by the lungs through pulmonary veins.

Ventricles

The two lower chambers are collectively referred to as atria while the two lower chambers are severally referred to as ventricles. While the right ventricle sends the now low-oxygen blood to the lungs through the pulmonary arteries, the left ventricle sends high-oxygen blood to the rest of the body through the aorta.

Heart Valves

The different types of heart valves are:

Atrioventricular Valves (Tricuspid And Mitral)

• Tricuspid Valve: Located between the right atrium and the right ventricle it prevents the admixture of blood with the atrial blood during ventricle contraction.

• Mitral Valve: Sits in between the left atrium and the left ventricle; it helps the ventricles not to fill with the blood in the atrium whenever it is pumping.

Semilunar Valves (Pulmonary And Aortic)

• Pulmonary Valve: It is situated between the right ventricle and the pulmonary artery to make sure that when the blood is ejected from the right ventricle; it should not flow back in.

• Aortic Valve: According to its location it is positioned between the left ventricle and the aorta and this function of preventing backflow into the ventricle is very crucial.

Blood Vessels

The blood vessels are classified as:

Aorta

The largest artery in the human body; is the blood vessel that carries oxygenated blood from the essential part of the heart known as the left ventricle to the rest of the body.

Pulmonary Arteries And Veins

PA moves deoxygenated blood from the right ventricle to the lungs and PV carries oxygenated blood from the lungs to the left atrium.

Vena Cava

These large veins are the superior and inferior vena cava; these veins move the deoxygenated blood to the right atrium of the heart from the body organs.

Detailed labelled diagram of the heart

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(First Heart Sound)

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(Second Heart Sound)

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

The additional heart sounds are:

S3 (Third Heart Sound)

Mainly said after S2, is caused by the fast filling of ventricles during the early phase of diastole. This is very common, especially 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

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

Valvular diseases can stem from valvular lesions, where the valves are stenotic, regurgitant or there is an abnormal connection between the atria and ventricles.

• 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.

Factors Affecting The Cardiac Cycle

The factors affecting the cardiac cycle are discussed below-

Intrinsic Factors

The intrinsic factors affecting the cardiac cycle are:

Heart Rate

The number of heartbeats within a minute depends on the rate by the sinoatrial node or SA node. The cases of increasing or decreasing the cases of heart rate result in shortening or prolongation of the cardiac cycle respectively.

Stroke Volume

The volume of blood that is pumped by the ventricles in a single contraction or pulse. Thus, preload, afterload, and contractility affect the stroke volume and by extension the cardiac output.

Extrinsic Factors

The extrinsic factors affecting the cardiac cycle are:

Autonomic Nervous System

The sympathetic neurotransmitter, norepinephrine raises both the heart rate and the force of contraction, on the other hand, parasympathetic, particularly through the vagus nerve reduces the heart rate and enables the body to relax. Thus, this balance helps make proper corrections about the needs of the body if any.

Hormones

Some hormones such as adrenaline and thyroid hormones play a role in influencing the rate and contractility of the heart. Adrenaline raises the rate of heart beating and the force with which the heart contracts, thyroid hormones can alter the general metabolism of cardiac muscles.

Conditions And Diseases

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.

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