Human Heart: Structure, Functions and Disorders, Practice Problems and FAQs

Let’s do a small activity. Place your palm on your chest, can you feel the thumping? You must be knowing that it is your heart beating but do you know why it beats relentlessly throughout the day and night even when the brain decides to sleep at night?

This is because unlike plants our cells cannot directly take in oxygen from the atmosphere and give out carbon dioxide to the atmosphere. Exchange of gases in humans occurs with the help of the lungs and the blood helps in carrying the inhaled oxygen from the lungs to the tissues and the carbon dioxide to be exhaled from the tissue to the lungs. But do you know what causes the blood to flow throughout the body at all times? It’s the incessant beating of the heart. The heart beats to pump blood throughout the body so that our cells can receive oxygen and nutrients and also get rid of waste.

The pumping of the heart can be heard as a lub-dub sound. In a healthy human adult, the heart beats at an average of 72 times per minute and you must have seen doctors trying to listen to it with the help of a stethoscope.

But what does the heart look like? How does it pump blood? What can happen if the heart doesn’t function properly? Let us get our answers as we read through this article.

GIF: Heartbeat

Table of contents

• Human heart
• Structure of the human heart
• Blood vessels
• Cardiac conduction system
• Cardiac cycle
• Functions of the human heart
• Disorders of the human heart
• Practice Problems
• FAQs

Human heart

The human heart is considered one of the most essential organs that is responsible for sustaining life. It is defined as a hollow, muscular organ having four chambers. It is derived from mesodermal tissue. The size of the human heart is about the size of a clenched fist. The heart is one of the most powerful and labour-intensive muscles in the human body that continues to beat throughout a person's lifetime.

In addition to humans, all other vertebrates have hearts that circulate blood throughout their bodies. Even invertebrates like grasshoppers and cockroaches have a pumping organ that resembles a heart, though it does not work in the same manner that a human heart does.

GIF: Human heart

Position of heart in the human body

The human heart is situated in the thoracic cavity, just to the left of the sternum (breastbone), between the lungs. The lower conical apex of the heart lies on the diaphragm, in between the 5^th and 6^th ribs.

GIF: Position of heart in the human body

Structure of the human heart

The human heart is roughly the size of a human fist and is classified into four chambers, two ventricles, and two atria. The atrium and ventricles are the blood-receiving and pumping chambers, respectively. Among these, the "right heart" is composed of the right atrium and ventricle, and the "left heart" is composed of the left atrium and ventricle.

Pericardium

The pericardium is a fibroserous bag-like structure that is situated as an external covering of the heart. It has an outer fibrous layer and an inner serous layer which is further divided into an outer parietal and inner visceral layers. In order to lubricate the heart and lessen friction between the surrounding organs, the serous layer produces a fluid called pericardial fluid which fills up the space between the parietal and visceral serous layers. Along with providing lubrication, the pericardium aids in keeping the heart in place and maintaining a hollow region for the heart to expand into when it is full and also prevents unnatural expansion of the heart.

Fig: Pericardium

Layers of the human heart

The wall of the human heart is composed of three layers -

• Epicardium
• Myocardium
• Endocardium

Epicardium

The outermost layer of the heart wall is the epicardium which lies in contact with the pericardium on the outer side and the myocardium on the inside.

Myocardium

The myocardium is the heart's muscular wall made up of cardiac muscles, connective tissues and blood vessels. It contracts to pump blood from the heart and then relaxes when the blood returns to fill the heart.

Fig: Myocardium

Endocardium

It is the innermost layer made up of squamous epithelium, covering the heart valves and lining the inner heart chambers. Additionally, it stops the blood from clotting, which can be dangerous, on the inside walls.

Fig: Endocardium

Chambers of the human heart

Based on the number of chambers, vertebrate hearts can be divided into different categories. All reptiles except crocodiles and amphibians have three chambers, whereas the majority of fishes have two-chambered hearts. Four chambers make up the hearts of crocodiles, birds and mammals. Being mammals, we have four heart chambers which are:

• Left atrium
• Right atrium
• Left ventricle
• Right ventricle

Atria

The small upper chambers of the heart are collectively known as the atria or auricles. The atria have thin and less muscular walls. They are smaller in size as compared to the ventricles. The atria are divided into two chambers based on their position: left atrium and right atrium. The left atrium is slightly smaller than the right atrium and has comparatively thicker walls. Both the atria are separated through a thin muscular interatrial septum.

An oval depression, known as the fossa ovalis, in the septum is a remnant of an embryogenic aperture known as the foramen ovale which allowed blood to pass from the right atrium to the left atrium during the embryonic stage.

Fig: Atria

They are blood-receiving chambers supplied by large veins. The right atrium receives blood from the whole body through the superior vena cava (upper parts of the body) and inferior vena cava (lower parts of the body). It also receives blood from the cardiac muscles via the coronary sinus formed when the coronary veins unite. Similarly, the left atrium receives blood from the lungs through the opening of four pulmonary veins. When the auricles/atria contract, the deoxygenated blood from the right auricle is pumped into the right ventricle and the oxygenated blood from the left atrium is pumped into the left ventricle.

Fig: Auricles are the blood receiving chambers

Ventricles

The larger and lower chambers of the heart are known as ventricles. They are more muscular as compared to the atria. They are responsible for pumping and pushing the blood out into circulation to farther distances. They are categorised into two chambers based on their position: left ventricle and right ventricle. A thick fibrous structure known as the atrioventricular septum separates the right atrium and right ventricle. The left and right ventricles are separated by a thick structure called the interventricular septum. The atria open into the ventricles via the atrioventricular apertures which are protected by the tricuspid valve on the right side and the bicuspid valve on the left side. These apertures and their respective valves ensure unidirectional flow of blood from the auricles/atria to the ventricles when the atria contract.

Fig: Ventricles

Both the ventricles are connected to the larger arteries that supply blood for circulation. From the right ventricle arises the pulmonary artery which carries deoxygenated blood to the lungs for oxygenation. From the left ventricle, arises the aorta which branches out into several arteries and carries oxygenated blood from the left ventricle to the entire body.

Fig: Pulmonary artery and aorta

Valves

Valves are described as the flaps of fibrous tissue present in the cardiac chambers and between the veins. They make sure the blood only flows in one way (unidirectional). Additionally, flaps stop the blood from flowing backward.

The eustachian valve guards the opening of the inferior vena cava into the right atrium and the valve of Thebesius guards the entry of the coronary sinus. The opening of the pulmonary veins into the left atrium is not guarded by any valve. Apart from these, the major valves in heart are -

Tricuspid valve

The right ventricle receives deoxygenated blood from the right atria. The tricuspid valve, which is composed of three muscular flaps or cusps, guards the opening between the right atrium and the right ventricle. This valve makes sure that blood does not return to the atrium when the atria relax and the ventricles contract. The chordae tendineae assist these valves in opening and closing and in holding them in position. The chordae tendineae have two attachment points: one on the valve and the other on the papillary muscles of the ventricle.

Fig: Tricuspid valve

Bicuspid valve

The left ventricle receives blood from the left atrium and the passage of blood is guarded by a bicuspid or mitral valve. This valve has two flaps. Similar to the tricuspid valves, the chordae tendineae attached to the papillary muscles hold these valves in position and control their opening and closing.

Fig: Bicuspid valve

Semilunar valve

Once the right ventricle has received the blood, it pumps the blood into the pulmonary artery or pulmonary trunk, which transports the blood to the lungs. The pulmonary semilunar valve controls the opening of the right ventricle to the pulmonary artery.

Fig: Pulmonary semilunar valve

When oxygenated blood reaches the left ventricle, it is pumped into the aorta, which distributes oxygenated blood to the entire body. The passage of blood to the aorta is guarded by the aortic semilunar valve.

Fig: Aortic semilunar valve

The semilunar valves are so called due to their half moon-like shape. They prevent the backflow of blood into the ventricles when they relax.

Grooves or Sulci

Internally, the heart is divided into four chambers. This division creates a depression on the outer surface of the heart. Three types of grooves are formed on the outer surface of the heart:

• Atrioventricular sulcus or coronary sulcus
• Interventricular sulcus
• Interatrial sulcus

Atrioventricular sulcus

It is a type of groove present on the outer surface of the heart and runs in between the atrium and the ventricle. It is present on the anterior side of the heart.

Fig: Atrioventricular sulcus

Interventricular sulcus

The interventricular septum, which is thick and runs between the two ventricles, creates a groove on the anterior surface of the heart that is referred to as the interventricular sulcus.

Fig: Interventricular sulcus

Interatrial sulcus

The interatrial sulcus is present between the two atria and is present on the posterior side of the heart.

Fig: Interatrial sulcus

Blood supply to the heart

Oxygenated blood from the left ventricle is brought to the heart wall by a pair of coronary arteries which arise from the systemic aorta. Coronary veins carry deoxygenated blood from the heart wall to the coronary sinus which pours it into the right atrium.

Blood vessels

The heart pumps blood through three types of blood vessels:

• Arteries
• Veins
• Capillaries

Arteries

These blood vessels carry blood from the heart to different parts of the body. These have thick muscular walls and narrow lumen. The blood flows in arteries with high pressure as compared to the veins. They are pinkish or bright red in colour because they usually carry oxygenated blood. The exception includes the pulmonary artery that carries deoxygenated blood to the lungs.

Fig: Arteries

Veins

The veins carry from different parts of the body to the heart. They are usually deep red or bluish in colour due to the flow of deoxygenated blood. The exception includes the pulmonary veins that carry oxygen-rich blood from the lungs to the heart. Blood flow through veins is uniform. The veins have wide lumen and comparatively thinner muscular walls. They have valves to prevent the backflow of blood.

Fig: Veins

Capillaries

The arteries and veins branch out into smaller blood vessels called arterioles and venules, respectively. The arterioles further branch out into even thinner blood vessels called capillaries when they reach the tissues and these capillaries unite to form the veins. They have a very thin wall made up of a single layer of epithelial cells and small diameter. The exchange of nutrients, respiratory gases, wastes, etc., between the blood in the capillaries and the tissue they surround takes place through capillaries.

Fig: Blood flow in capillaries

Cardiac conduction system

The rate at which the heart transmits electrical impulses is known as cardiac conduction. Due to these impulses, contraction, and relaxation of the cardiac muscles occurs. Blood is continuously pumped throughout the body because of the constant cardiac cycle that includes muscle contraction followed by relaxation. Exercise, temperature, and hormones produced by the endocrine system are a few factors that might affect cardiac conduction. The cardiac conduction includes four steps:

• Impulse generation by SA node
• Impulse conduction by AV node
• Impulse conduction by AV bundle
• Impulse conduction by Purkinje fibres

Impulse generation by SA node

The sinoatrial node (SA) is also known as the pacemaker of the heart which is responsible for the contraction and generating nerve impulses. These impulses travel through the heart wall allowing both atria to contract. The SA node is situated in the upper wall of the right atrium. It is made up of nodal tissue, which possesses both muscular and nervous tissue characteristics.

Impulse conduction by AV node

The atrioventricular node (AV) is situated close to the bottom of the right atrium, on the right side of the wall dividing the atria. The AV node receives impulses from the SA node after a delay of a tenth of a second. This delay permits the atria to close and release their contents into the ventricles before the contraction of the ventricles.

Impulse conduction by AV bundle

The atrioventricular bundle is then used to transmit the impulses. The impulses are sent from the AV node to the AV bundle of fibres, which splits into two bundles, to the left and right ventricles of the heart.

Impulse conduction by Purkinje fibres

The atrioventricular bundles begin to further separate into Purkinje fibres at the bottom of the heart. The muscle fibres in the ventricles contract when the impulses get to these fibres.

Cardiac cycle

Cardiac cycle is described as the sequence of events that occur during the heartbeats. The cardiac conduction propels the cardiac cycle. The cardiac cycle includes two phases: diastole and systole. Diastole is the phase of relaxation of heart muscle and systole is the phase of contraction of the heart muscles.

It starts with the atrial systole or ventricular diastole when the ventricles remain relaxed and empty and the auricles contract and pump the blood that they have received into the ventricles. The tricuspid and bicuspid valves remain open during this phase and the semilunar valves remain closed. This phase lasts for 0.1 seconds.

This is followed by the atrial diastole or ventricular systole phase which lasts for 0.3 seconds. During this phase the auricles/atria relax and the ventricles contract to pump blood into the pulmonary artery and aorta. The bicuspid and tricuspid valves remain closed and the semilunar valves remain open during this phase. The auricles receive blood from the vena cavas and the pulmonary vein during their diastolic phase.

This is followed by relaxation of the ventricles which relax along with the already relaxed auricles. As all the four chambers of the heart relax at this point, this phase is known as joint diastole and lasts for 0.4 seconds.

Thus, the duration of one cardiac cycle is 0.8 seconds. In a minute, the heart of a healthy adult beats around 72 times.

GIF: Cardiac cycle

Functions of the human heart

Any organism's heart's responsibility is to maintain the blood flowing through the body at a steady rate. By doing so, oxygen is replenished and nutrients are distributed across the cells and tissues. Some of the main functions of the human heart are listed below:

• The primary function of the human heart is to pump blood throughout the body.
• Blood carries nutrients, hormones, glucose, and other substances to different regions of the body, including the human heart.
• The heart is responsible for maintaining adequate blood pressure in the human body.
• The heart regulates the speed and rhythm of the heart rate.

Disorders of the human heart

One of the most prevalent disorders that people suffer from is heart disease. The common conditions that affect the heart are:

• Heart attack
• Heart failure
• Cardiac arrest
• Congenital heart disease

Heart attack

This condition is also known as myocardial infarction. This occurs due to blockage when plaque formed due to deposition of fatty substances ruptures in the blood vessels and restricts the blood flow. As a result of such blockage, damage or death of cardiac muscle tissue happens.

Fig: Condition of heart attack

Heart failure

It is a condition in which the heart is unable to adequately pump blood to meet the body's needs. It is also known as congestive heart failure because congestion of the lungs is the main symptom.

Fig: Heart failure

Cardiac arrest

It is a condition when the heart stops beating. Due to this, the blood stops flowing to the brain and other vital organs of the body.

Fig: Cardiac arrest

Congenital heart disease

It is a condition of defects or diseases of the heart that exist from birth as a result of a heart development error. These defects occur due to a number of reasons, such as infection like Rubella in the mother, administration of harmful drugs during the 1^st trimester of pregnancy, and chromosomal abnormalities.

Fig: Congenital heart disease

Practice Problems

Q1. Select the right path for blood flow from the heart to the body and back again.

a. Arteries→Capillaries→Arterioles→Venules→Veins
b. Arteries→Arterioles→Capillaries→Venules→Veins
c. Arteries→Arterioles→Capillaries→Veins→Venules
d. Arteries→Venules→Capillaries→Arterioles→Veins

Solution: Arteries are types of blood vessels that carry oxygenated blood to all parts of the body from the heart. It is further branched into arterioles which are further divided into the smallest and thin blood vessels called capillaries. Through capillaries, the exchange of nutrients and waste material takes place between the blood vessels and the tissues. Venules gather and collect the blood carrying waste and carbon dioxide. The veins that carry deoxygenated blood back to the heart are connected to these venules. Hence, the correct option is b.

Q2. Identify whether the given statement is true or false.

The term "heart failure" means the heart has stopped working completely.

Answer: The given statement is false. This is because heart failure is a condition in which the heart is unable to adequately pump blood to meet the body's needs. This may happen as a result of inadequate blood flow into the heart chambers or weakened heart muscle.

Q3. The right side of the human heart carries __________ and the left side of the human heart carries ___________.

A. oxygenated blood, deoxygenated blood
B. deoxygenated blood, oxygenated blood
C. oxygenated blood, oxygenated blood
D. deoxygenated blood, deoxygenated blood

Solution: Through the superior and inferior vena cava, the right atrium of the heart gets deoxygenated blood from all areas of the body. The right atrium subsequently sends this deoxygenated blood to the right ventricle. The pulmonary artery transports blood from the right ventricle to the lungs where it gets oxygenated. After being oxygenated, the blood travels back to the left atrium through the pulmonary vein, where it is then transferred to the left ventricle which pumps it to the whole body through the aorta. Hence, the correct option is b.

Q4. Identify the structure which is known as the pacemaker of the heart.

A. Bundle of His
B. AV node
C. SA node
D. Purkinje Fibres

Solution: The sinoatrial node (SA) is known as the natural pacemaker of the heart which is responsible for generating nerve impulses which initiate contraction of the heart muscles. Hence, the correct option is c.

FAQs

Q1. What is the heart rate in a child?
Answer: In a healthy child, the heart beats 90-100 times per minute. On the other hand, in a healthy adult, the heart beats 70-72 times per minute. Smaller the body of an organism, faster is the circulation and more is the number of times the heart beats in a minute.

Q2. Which part of the brain regulates the heartbeat?
Answer: The medulla oblongata is a part of the hindbrain that controls the heartbeat which is an involuntary activity.

Q3. What is the weight of a healthy heart?
Answer: The healthy heart of an adult weighs around 5-5.25 ounces.

Q4. How many times does the heart recycle blood in a day?
Answer: The heart recycles blood roughly around 1000 times every day.

YOUTUBE LINK: https://www.youtube.com/watch?v=sKFfgnlKSJw&t=1843s