Human Heart: Have you ever wondered which organ is the main reason for the survival of life? How do you feel when you place your hands on your chest? Do you feel like something is knocking inside your body? Yes, the heart is the main organ responsible for sustaining the life of all living organisms. The human heart is the only pumping organ that pumps 5.7 litres of oxygenated blood every day. It weighs about 280 to 340 grams in males and 230 to 280 grams in females. 

A rib cage protects the heart, and it is present just behind and slightly left of the breastbone. The study of the heart and its functions is called cardiology.  A person who studies and treats heart diseases is called a cardiologist. The medical term of the heart is called the cardiovascular system. Read on more to understand the structure and functions of the heart in the below article.

What is a Human Heart?

The human heart is an important muscular pumping organ that pumps blood from the heart to the body through the circulatory system.

Fig: Heart

Human Heart: Shape and Size

The shape of the heart is similar to the triangular shape. It is a hollow cone with a broad base at the end and narrow in shape near the apex. Heart size can be normally determined by folding the fist of the left hand (closed fist). It is 12 cm in length and 9 cm in width generally.

Human Heart: Position and Location

The mature human heart is about the size of a closed fist. The heart is located in the chest cavity slightly towards the left, enclosed in a double-walled sac called the pericardium. Pericardial fluid is present between the heart wall and pericardium. It is made of muscle cells called cardiac muscle fibres.

Fig: Location of the Heart

Structure of Human Heart

The structure of the heart is divided into two sections- the external and internal structure of the heart.

External Structure of the Heart

The external structure of the heart is covered by a protective membrane known as the pericardium.

Protective Covering Membranes of the Heart

The pericardium is divided into two important layers called the fibrous pericardium layer and the serous pericardium layer. The pericardial fluid acts as lubrication between the two layers during contraction and relaxation.

1. Fibrous Pericardium Layer: It is the most superficial layer of the pericardium and is made up of dense, and loose connective tissue that provides protection for the heart. This layer helps in anchoring it to the surrounding walls and preventing it from overfilling with blood.
2. Serous Pericardium Layer: It is divided into two layers- the visceral layer and the parietal layer.
   (a) Visceral Layer: The inner (visceral) layer of the serous pericardium lines the outer surface of the heart itself.
   (b) Parietal Layer: The inner surface of the pericardium is lined by the outer (parietal) layer of the serous pericardium. The parietal layer forms a sac-like structure in the external region of the heart that contains the pericardial fluid in the pericardial cavity.

Layers of the Heart: The heart wall is covered by three main layers called epicardium, myocardium and endocardium.

(a) Epicardium: It is the external layer of the heart. It is composed of a thin-layered membrane that serves to lubricate and protect the outer section.
(b) Myocardium: The thickest and middle layer of the heart is the myocardium. It is made up of large cardiac muscle cells. It is responsible for pumping action.
(c) Endocardium:  It is the innermost layer of the heart wall. The endocardium is connected to the myocardium with a thin layer of connective tissue. The endocardium lines the chambers where the blood circulates and covers the heart valves. It helps in preventing the sticking of walls to each other and also helps in preventing blood clots.

Fig: Longitudinal Section of the Heart

Internal Structure of Heart

(a) Chambers of Heart: The upper two chambers constitute the right and left auricles or atria (singular atrium), and the lower two chambers form the right and left ventricles. The left and right sides are divided and do not communicate. The left auricle opens into the left ventricle by a left auriculo-ventricular aperture. Similarly, the right auricle opens into the right ventricle through a right auriculo-ventricular aperture.

1. The atria are the blood-receiving chambers that are fed by the large veins.
2. Ventricles are broad and more muscular chambers responsible for pumping and pushing blood out to the circulation. These are linked to larger arteries that deliver blood for circulation.

(b) Blood Vessels: There are three types of blood vessels- arteries, veins, and capillaries, that are connected to form a continuous system.

1. Arteries: Arteries are fairly wide vessels that carry blood from the heart to different organs of the body. Arteries have thick, elastic and muscular walls and narrow lumen (without a valve). All arteries, except the pulmonary artery, carry oxygenated blood. The arteries are separated into smaller vessels, called arterioles, which themselves divide regularly until they form a dense network of microscopic vessels called capillaries.
2. Veins: Veins carry blood from different organs to the heart. Veins have thin, less muscular walls and wide lumen. All the veins, except the pulmonary vein, carry deoxygenated blood. Small veins are called venules. Venules are formed from capillaries and join to form veins. They also have valves in them which permit the unidirectional flow of blood and thus prevent blood from flowing away from the heart.
3. Capillaries: Capillaries are microscopic vessels about 8 microns in diameter. The wall of capillaries consists of a single layer of squamous epithelial cells (endothelium). Capillaries lie in contact with the body tissues.

(c) Valves in the Heart: The valves permit the flow of blood in one direction only and not in the reverse direction, preventing the backflow of blood. The heart has four valves:

1. Tricuspid Valve: It guards the opening between the right auricle and the right ventricle.
2. Bicuspid Valve: It guards the opening between the left auricle and the left ventricle.
3. Aortic Semilunar Valves: It is located at the point of origin of an aorta from the left ventricle and is three in number.
4. Pulmonary Semilunar Valve: It is located at the opening of the right ventricle into the pulmonary artery and is three in number.

Blood Vessels

(a) Entering the Heart:

1. Superior Vena Cava (also called anterior vena cava or precaval): It carries deoxygenated blood from the upper half of the body, including the head, chest, and arms, to the heart’s right atrium.
2. Inferior Vena Cava (also called posterior vena cava or postcaval): It carries deoxygenated blood from the lower half of the body, including legs and abdomen, into the right atrium of the heart.
3. Pulmonary Vein: Pulmonary veins carry oxygenated blood from the lungs back to the left atrium of the heart.

(b) Leaving the Heart:

1. Pulmonary Artery: It arises from the right ventricle and carries deoxygenated blood to the lungs for oxygenation.
2. Aorta: It arises from the left ventricle and carries oxygenated blood to be distributed to all the body parts.

Coronary Arteries: Two coronary arteries, right and left, arise from the base of the aorta to supply the heart muscles.

Working of the Human Heart

To start with, all the four chambers of the heart are in a relaxed state (joint diastole). Blood from the pulmonary veins and vena cava flows into the left and the right auricles, respectively. As the tricuspid and bicuspid valves are open, blood passes into the ventricle easily.

1. Both the atrium undergo a simultaneous contraction (the atrial systole). This increases the flow of blood into the ventricles. When the ventricles contract (ventricular systole), the auricles relax. Ventricular systole increases the ventricular pressure causing the closure of two cuspid valves to prevent backflow of blood into the atria.
2. As the ventricular pressure increases, the semilunar valves guarding the pulmonary artery and aorta are forced open, and blood flows to these vessels. Now the ventricles relax (ventricular diastole).
3. The ventricular pressure falls, causing the closure of semilunar valves, which prevent the backflow of blood into the ventricles. Due to decreased ventricular pressure, tricuspid and bicuspid valves are pushed open by the pressure in the atria. The blood now once again movesly to the ventricle.
4. This sequential event in the heart which is cyclically repeated, is called the cardiac cycle. It consists of systole and diastole of both the atria and ventricles.

Types of Circulation

There are two types of circulations in the human body. As the blood flows two times through the heart, it is called double circulation.

1. Pulmonary Circulation: The circulation of blood from the right ventricle to the lungs and from the lungs to the left auricle is called pulmonary circulation. In pulmonary circulation, deoxygenated blood from the right ventricle is pumped through pulmonary arteries to both the lungs. Oxygenated blood comes back again to the heart through pulmonary veins.
2. Systemic Circulation: Systemic circulation is the circulation of oxygenated blood from the left ventricle through the aorta to all the body parts and the circulation of deoxygenated blood from body parts by veins to the right auricle.

The Heart Sounds

During each cardiac cycle, two distinct sounds are performed, which can be easily heard through a stethoscope. The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves, whereas the second heart sound (dub) is associated with the closure of the semilunar valves.

Heart Beat and Pulse Rate

Heartbeat is due to the rhythmic relaxation and contraction of heart muscles. One relaxation (diastole) and One contraction (systole) of the heart constitute one heartbeat.

1. The normal heartbeat in human beings is 70 per minute.
2. The rhythmic contraction of the heart is felt in certain regions like the wrist in the form of pulsations, and this is called a pulse.
3. The pulse rate is the same as the heartbeat rate. One complete heartbeat (or cardiac cycle) thus, consists of one systole and one diastole.

Fig: Pulse Rate

Functions of Human Heart

(a) The heart pumps oxygenated blood to all the parts of the body.
(b) The blood supplies hormones,  oxygen, glucose, and other components to different parts of the body, including the human heart.
(c) Blood also helps in the removal of waste products and carbon dioxide from the cells, which are further eliminated or excreted from the body.
(d) The heart regulates and maintains the appropriate blood pressure.

Diseases of the Heart

1. Heart Attack: The blockage in the coronary arteries or in any of their branches, deadening of the corresponding area of the heart muscle occurs, causing myocardial infarction, commonly called heart attack.
2. Arrhythmia: The abnormality of heart rhythm.
3. Congenital Heart Diseases: The abnormality of the heart that has been developed before childbirth.
4. Atherosclerosis: The thickening and hardening of the arteries lead to abnormal pumping of blood.

Summary

The heart is the muscular organ that pumps blood all over the body. It has four main chambers called the left atrium, right atrium, left ventricle, and right ventricle. Atria are thin and consist of muscular walls. They are smaller than the ventricles. The atria receive deoxygenated blood from all parts of the body. The ventricles are the muscular chambers that pump oxygenated blood to all parts of the body.

There are three main blood vessels that help in carrying blood in and out from the heart; they are arteries, veins, and capillaries. The heartbeat is measured by an instrument called a stethoscope. The hearts make sounds called lub-dub due to the action of contraction and relaxation. The main function of the heart is to supply hormones and nutrients to tissues, oxygenated blood to various parts of our body, and also maintain and regulate blood pressure.

FAQs

Q.1. What are the main functions of the heart?
Ans: The main function of the heart is to maintain and regulate blood pressure and pump oxygenated blood to all parts of the body.

Q.2. What are chambers of the heart?
Ans: There are four chambers of the heart, and are two atria and two ventricles.

Q.3. What are the 3 layers of the heart?
Ans: Epicardium, myocardium, and endocardium are the 3 layers of the heart.

Q.4. Which artery is the largest?
Ans: Aorta is the largest artery.

Q.5. What causes heart sound?
Ans: The opening and closing of the cardiac valves during the cardiac cycle cause the heart sound lub-dub.

Learn About Double Circulation Here

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