Heart- Definition, Anatomy, Structure, Functions, Diseases

The heart is one of the essential organs in every organism. The heart is made up of special cardiac fiber muscles.

Definition of Heart: The heart is the essential organ in the cardiovascular system that is used for the pumping of the oxygenated and deoxygenated blood throughout the body.

Human Heart
Human Heart
  • The heart is attached to different blood vessels where the pulmonary blood vessel transports the blood between the heart and the lungs and the blood is transported between the heart and the whole body tissue by the systematic blood vessels.
  • The heart consists of an SA node, AV node, Bundle of hiss, and Purkinje fiber. The Sinoatrial node (SA node) plays a vital role in the initiation of electrical impulses to stimulate contraction which is a special myocardial structure present at the right atrial wall in the junction between the superior vena cava and right atrium.
  • Whenever the SA node fails to work properly then the artificial pacemaker is added instead of it for the electrical conductivity.
  • The generation of the action potential repeatedly triggers heart contraction.
  • The heart is located in between the lungs or at the center of the chest slightly left to the sternum into the cavity of the left lung.
  • Double-wall sac-like structure called pericardium covers the heart and the layer is filled with the fluid called pericardial fluid.
  • Pericardial fluid helps to protect the heart from external injury and mechanical shocks. Also, it acts as a lubricant between the two layers during the contraction and relaxation of the heart.
  • Normally the size of the heart is considered to be the size of the fist in the case of humans, but the size may vary according to the person and location. So on average, the weight of the heart is supposed to be 250-350 grams in adult males and 200-250 grams in adult females. 
  • The heart rate differs according to age, where the resting heart rate in children above the age of 10 has the rate of 60-100 bpm (beats per minute). Similarly, an athlete resting heart rate may be below 60 bpm. While talking about the newly born babies of 1 month their normal heart rate may be up to 70 – 190 bpm (beats per minute).

What is Heart Rate?

The number of times the heart beats per minute in space is called the heart rate. Similarly, the rate of the heart differs according to the situation. A normal healthy person’s heart rate is normal and pumps the blood at the normal rate while the individual suffering from different diseases may have a slightly higher heart rate. Whenever a person is frightened then their heart rate is increased because of the production of adrenaline hormone which makes the heart rate faster. When the heart rate is faster than it starts to pump a higher amount of blood which makes the body ready for action during frightening with the supply of a higher amount of oxygen into the body.

Similarly, blood pressure is the amount of pressure exerted on the surface or wall of the blood arteries. Thus, the pulse is related to the heartbeat, and the number of times the heart contacts or expands is equal to the pulse rate. The blood pressure is higher when the person starts running or performing other activities as the heart rate increases and people’s blood pressure is lower when they do nothing.

Human Heart Anatomy

The heart anatomy consists of different parts:

  • Heart walls 
  • Chambers
  • Valves 
  • Blood vessels
  • Electrical conduction system
Human Heart Anatomy
Figure: Human Heart Anatomy.

Heart Walls

The heart wall is made up of cardiac muscles which contract and relaxes to send and receive the blood throughout the body (oxygenated and deoxygenated blood). The septum is the muscular tissue that divides the left and the right part of the heart. The heart wall consists of the following three layers:

  • Endocardium: the innermost part of the heart which lines up the heart and extends over the structures like valves, chordae tendineae, and papillary muscles is the endocardium. It helps to regulate the activity of cardiac muscles and directs the flow of blood through the heart along with the regulation of the heartbeat. 
  • Myocardium: The layer that is present in between the endocardium and epicardium which is responsible for the contractile function of cardiac muscle.
  • Epicardium: Epicardium is the outermost protective layer. Epicardium is protected by the membrane layer called a pericardial membrane and this pericardial layer is filled with the pericardial fluid.
Heart Walls
Figure: Heart Walls. Created with biorender.

Heart Chambers

The heart is divided into four chambers, two at the top (called an atrium) and two at the bottom (called ventricle). 

  • Right atrium: It is present at the right top corner of the heart and receives deoxygenated blood from the upper and lower part of the body. The collected blood is then pumped to the right ventricle.
  • Right ventricle: It is located on the right bottom corner and pumps blood to the lungs through the pulmonary artery.
  • Left atrium: It is located on the left top side which receives the oxygenated blood from the lungs through the pulmonary vein.
  • Left ventricle: It is the fourth chamber of the heart which pumps the oxygenated blood through the body. It is slightly larger than that of the right atrium.

Heart Valves

The heart valve is the opening between the two chambers of the heart which allows and controls the flow of the blood between the chambers. The valves that open and close between the two chambers of the heart are atrioventricular valve and includes:

  • Tricuspid valve: The valve is present between the right atrium and right ventricle.
  • Mitral/ Bicuspid valve: The valve that is present between the left atrium and left ventricle is the mitral valve.

There is another valve that opens when the blood flows from the ventricles called a semilunar valve (SL) and includes:

  • Aortic Valve: Aortic valve opens when the blood flows from the left ventricle to the aorta carrying the oxygenated blood.
  • Pulmonary valve: It opens when the blood passes from the right ventricle to the pulmonary artery carrying the impure blood to the lungs.

Blood Vessels

The heart pumps blood through different types of blood vessels which include:

  • Arteries: It carries the oxygenated blood through the body except for the pulmonary artery.
  • Veins: It carries the deoxygenated blood to the heart except for the pulmonary vein.
  • Capillaries: It is a type of blood vessel where the oxygenated and the deoxygenated blood are exchanged.

Electrical Conduction System

The electrical system of the heart consists of rhythmic contraction and relaxation. It includes:

  • Sinoatrial (SA) node: It is the mass of the cardiac muscle which sends the signal for the heartbeat. 
  • Atrioventricular (AV) node: It carries the electrical activity from the upper part of the heart to the lower part of the heart.
  • Left bundle branch: It carries the electrical impulses to the left ventricle.
  • Right bundle branch: The electrical impulse is sent to the right ventricle.
  • Bundle of hiss: It sends the electrical impulses to the Purkinje fibers from the Atrioventricular node.
  • Purkinje fiber: It helps the ventricle contract and pumps out the blood.
Arterial Blood Vessels
Arterial Blood Vessels

The human heart consists of two structures called external structure and internal structure. While performing the anatomy of the heart the outermost structure that is seen is called the external structure of the heart and the exposed structure after dissection of the heart is the internal structure.

External Structure of the human heart

The heart is concaved shaped, dark red in color which is enclosed by the double-layered membrane called the pericardium. The outermost pericardium layer is called the fibrous pericardium while the inner layer membrane is called the serous pericardium. The outermost fibrous pericardium layer acts as the protective covering and is thicker than that of the inner layer membrane.

The inner serous membrane layer is divided into two layers called the parietal and visceral layers, enclosing the pericardial fluid between these layers. The function of the pericardial fluid is to protect the heart from shocks and injuries caused due to mechanical injuries.

The wall of the heart is composed of different layers including the epicardium made of the epithelial cells called mesothelium, the myocardium made up of cardiac muscles, and the inner layer endocardium made up of epithelial cells called the endothelium.

Two thin-walled auricles at the upper part of the heart and thick-walled ventricles at the lower part are divided by a groove called transverse groove and coronary sulcus. The human heart consists of two auricles and two ventricles.

Two large vessels, superior vena cava, and inferior vena cava open into the right atrium carrying the deoxygenated blood from the upper and lower part of the body respectively. Similarly, the left auricle Is attached to the pulmonary veins. From the right ventricle, the pulmonary aorta arises whereas from the left ventricle systemic aorta arises. Similarly, right and left coronary arteries arise from the systemic aorta which carries the oxygenated blood to the heart.

Internal structure of the human heart

The internal structure of the heart shows the internal components and the mechanism of blood flow.

When the heart is dissected to visualize the internal parts then the left and the right side of the heart are divided by a cardiac layer called the septum. The heart consists of four chambers, two on the upper side called atria and two on the lower side called ventricles. The ventricle wall is thicker than that of the atrium.

The right atrium is located on the superior right side of the heart and is separated from the left atrium by the thin layer called the inter-atrial septum. The right atrium is comparatively larger and thinner than that of the left atrium, receiving the deoxygenated blood from the superior and inferior vena cava. Two ventricles are separated by the thick layer called the ventricular septum. Similarly, the atrium and the ventricle are separated by a thick fibrous tissue called the atrioventricular septum.

In between the right atrium and the right ventricle, there is an opening and the valve called the tricuspid valve for the control of the blood flow. Whereas, between the left atrium and the left ventricle, a bicuspid or the mitral valve is present. The pulmonary artery opens from the right ventricle that carries the blood to the lungs and the aorta arises from the left ventricle carrying the oxygenated blood throughout the body. The aorta and the pulmonary artery are provided with the semilunar valve and these valves help in the unidirectional flow of the blood.

Mechanism of blood circulation in the human heart

The heart pumps both the oxygenated and the deoxygenated blood to the body and the lungs. The primary step is the collection of the impure blood from the upper and the lower part of the body which is completed by the superior vena cava and the inferior vena cava respectively.

The impure blood is collected from the upper parts of the body like the arms, head, brain, and shoulder, and from the lower parts of the body like the thigh, abdomen, hip, etc. Now the deoxygenated blood gets collected into the right atrium and passes to the right ventricle. The flow of the blood is controlled by the tricuspid valve. Then the blood is pumped to the lungs for purification by the pulmonary artery.

Now the purification of the blood takes place in the lungs. When the air is inhaled then the lungs are equipped with an abundance amount oxygen and this oxygen from the air comes in contact with the deoxygenated blood. The blood consists of oxygen and at the same time, the blood releases the waste matter in the form of carbonic acid, making the blood purified. 

Then, the oxygenated blood is carried out by the pulmonary vein and is collected in the left atrium. After that, the pure blood is collected in the left ventricle and the flow of the blood is controlled by the bicuspid or the mitral valve.

Finally, the left ventricle pumps the blood to the aorta for the distribution of the blood to the whole body. The left ventricle has a high-pressure pump and produces enough pressure to pump the blood to the body. The aorta is divided into different arteries including the coronary artery supplying the pure blood to the heart, the brachiocephalic artery carrying the blood to both arms and head, and the iliac artery carrying the blood to the pelvis and lower part of the legs.

Functions of the heart

  • Maintains the blood pressure
  • Controls the rhythm of the heart 
  • Pump the oxygenated blood to the different parts of the body
  • Pumping the hormones and other necessary substances into the different parts of the body
  • Receiving the deoxygenated blood for purification in the lungs

What is Electrocardiogram (ECG)?

An electrocardiogram (ECG) is an instrument that is used for the measurement of electrical conductivity or the movement of the heart. There are sensors attached to the device that senses the electrical signals produced by the heart when it beats. ECG is measured when the heart activity is imbalanced or when it does not perform the rhythmic contraction and relaxation.

ECG tracing
ECG tracing

An ECG can help to detect different problems associated with the heart that include:

  • Arrhythmias: when the heart starts to beat abnormally (either too slowly, too quickly, or irregularly). On the basis of the heart beating, arrhythmias are divided into the following:
  • Bradycardia: when the heart starts to beat slowly (lower than 60 beats per minute). It is normal during sleep but can be induced by certain drug ingestion and a low body temperature.
  • Tachycardia: when the heart starts to beat at a faster rate (greater than 100 beats per minute). It is normal during exercise but can be induced by high body temperature, certain drugs, heart diseases, anemia, or shock.
  • Coronary heart diseases: sometimes the blood vessels are blocked by fatty substances or interrupted by unwanted particles, then the supply of blood is hampered leading to coronary heart diseases.
  • Heart attacks: Heart attack occurs when the supply of blood in the blood vessels is suddenly stopped.
  • Cardiomyopathy: the wall of the heart becomes enlarged or thickens up leading to cardiomyopathy.

Risk factors for Heart Diseases

Heart diseases are one of the major causes of death for millions of people. There are different risk factors for heart disease including:

  • Age: The arteries get damaged and narrowed as the individual grows older. Also, the heart muscles get thickened.
  • Sex: Males are found to be at greater risk than females. The heart disease risk increases in females during menopause.
  • Heredity: Family history is also another factor for heart diseases. If the parents have genetic diseases, then their spouse may have the disease as well.
  • Poor diet: Poor food habits, and high fat, sugar, or cholesterol content in the food may lead to heart disease.
  • High blood pressure: continuous high blood pressure can lead to thickening of the arteries and narrowing of the blood vessels.
  • Obesity: when the weight of the body increases, it increases the heart disease risk factor.
  • High blood cholesterol level: when the level of cholesterol increases in the bloodstream then it leads to heart diseases.

Besides this, there are several other factors for heart diseases including stress.

Heart Diseases- Types, Causes, and Symptoms

  • Coronary artery disease (CAD): coronary heart disease is one of the most common diseases which may lead to the blocking of the coronary artery. Due to this, the blood is not transferred into the blood vessels. There are different factors that cause CAD including age, genetics, obesity, high blood pressure, smoking, stress, etc. The symptoms may be a pain in the chest or a heart attack.
  • Heart Failure: heart failure may occur because of coronary heart disease, thyroid, high blood pressure, or heart muscle disease.
Organ Dysfunction in Acute Heart Failure
Organ Dysfunction in Acute Heart Failure
  • Heart valve disease: The heart consists of different valves that pump the blood from one location to another and prevent the backflow of the blood. Thus, any defect in the valve to open and close may lead to blocking of the blood flow. Rheumatic fever, high blood pressure, congenital heart diseases, or coronary artery diseases are the causes of heart valve failure.
  • Pericardial diseases: Pericardial diseases occur when there is any kind of damage or diseases associated with the pericardium. Pericarditis or the inflammation of the pericardium is the most common disease. The main cause of the pericardial disease is the inflammation of the pericardium, infection with a virus, rheumatoid arthritis, or injury of the pericardium.
  • Cardiomyopathy (Heart Muscle Diseases): The disease that is associated with the heart muscle or myocardium is called cardiomyopathy. The myocardium layer gets stretched or thickened because of which the blood is not pumped properly. The causes of this disease are reactions to certain kinds of drugs, alcohol, genetic heart conditions, and infections from viruses. There are other different causes and doctors can’t find the exact cause.
Cardiomyopathy (Heart Muscle Diseases)
Cardiomyopathy (Heart Muscle Diseases)
  • Congenital Heart Diseases: when the baby’s heart starts to develop in the womb then sometimes something may go wrong and because of this congenital heart disease may occur. One of the causes of this disease is septal abnormalities (which are holes in the region that separate the left and the right heart). Another cause of this disease is the pulmonary abnormality called pulmonary stenosis (narrowing of the valve that leads to a decrease in the blood flow into the lungs).

What is a Stroke?

A stroke is a condition caused by the blockage of the flow of blood to the brain. While a heart attack is a condition that appears due to blockage of the blood flow in the heart. The flow of the blood is disturbed in the brain either by blockage or ruptured blood vessels. Because of the lack of blood supply, the brain cells are deprived of oxygen leading to the death of the brain cells.


Some of the symptoms of stroke include:

  • Severe headache,
  • One or both of the eyes gets blurred,
  • Weakness and the limbs or the face gets numb (mostly one side of the body),
  • Difficult to understand and communicate with others,
  • Dizziness and loss of balance.


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