Human Respiratory System: Definition, Parts, Functions and Associated Diseases

Respiration is the process that carries oxygen into the lungs and moves oxygen through the body.

The act of breathing includes: inhaling and exhaling air into the body, the absorption of oxygen from the air to produce energy, and the discharge of carbon dioxide, which is the by-product of the process.

The lungs extract oxygen and pass it through the bloodstream, from where it is transferred to the tissues and organs, which allows the vital functions of the body to be carried out.

The lungs also take carbon dioxide from the blood and release it into the air when we exhale.

Breathing begins in the nose and mouth, air is inhaled through the nose or mouth, and travels down the back of the throat to the windpipe.

The windpipe then divides into air passages called bronchial tubes .

For the lungs to function at their best, these airways must be open during inhalation and exhalation and free of inflammation or swelling and excessive or abnormal amounts of mucus.

As the bronchial tubes pass through the lungs, they divide into smaller air passages called bronchioles, these are made up of small air sacs called alveoli.

The respiratory system has more than 300 million alveoli and has a layer of small blood vessels called capillaries, where oxygen from the inhaled air passes through the walls of the alveoli into the blood.

After absorbing oxygen, the blood leaves the lungs and is sent to the heart. The heart then pumps it through the body to provide oxygen to cells in tissues and organs.

As cells use oxygen, carbon dioxide is produced and absorbed into the blood.

In the exhalation process, the carbon dioxide carried by the blood and returning to the lungs is eliminated.

Parts of the respiratory system

The respiratory system is divided into two parts:

Upper respiratory tract

When you breathe in, air enters your body through your nose or mouth. It passes down the throat through the larynx to the windpipe, and from there it enters the lungs .

All of these structures work to channel fresh air from the outside world into the body.

The upper respiratory tract must always be clear so that the individual can carry out the respiratory process.

The upper respiratory tract also contributes to heating the air before reaching the lungs.

The nasal cavity (nose)

It is the best entry for outside air into the respiratory system. The hairs that line the inner wall are part of the air cleaning system.

The paranasal sinuses

They are small openings in the bones of the face that connect to the nasal cavity. These contribute to the regulation of the temperature and humidity of the air that is inhaled.

The oral cavity (mouth)

Air can also enter this way, especially if you have a habit of breathing through your mouth or when your nostrils are temporarily blocked.


They are a lymphatic tissue located in the upper part of the throat. When adenoids interfere with breathing, they are sometimes removed.

The lymphatic system, which consists of connecting nodes (knots of cells) and vessels, carries fluid throughout the body.

This system helps the body resist infection by filtering out foreign matter, including germs and producing cells (lymphocytes) to fight them.

The tonsils

They are lymph nodes located in the wall of the pharynx.

The tonsils are not an important part of the body’s germ-fighting system. If they become infected very frequently, they are sometimes usually eliminated.


This collects the incoming air from the nose and passes it down into the windpipe.

La epiglottis

It is a flap of tissue that protects the entrance to the windpipe. It closes when you swallow food that should go into the esophagus and stomach.


It is the passage that goes from the mouth and throat to the stomach.

Lower respiratory tract

This tract includes the trachea, bronchi, bronchioles and lungs, the act of breathing occurs in this part of the system.

The organs of the lower respiratory tract are located in the thoracic cavity.

They are outlined and protected by the rib cage, the thoracic bone or sternum, the muscles between the ribs and the diaphragm, which constitute a muscular partition between the thorax and the abdominal cavity.


It is the passage that goes from the pharynx to the lungs.


The trachea is divided into two main tubes called bronchi, which go one for each lung where they divide into bronchioles.

The bronchial tubes are lined by very small hairs that have a wave motion.

Mucus traps and holds much of the dust, germs, and other unwanted matter that invades the lungs.

This movement carries a sticky or liquid phlegm up and into the throat, where it is coughed up.

This is how your lungs get rid of mucus when you cough.


The lungs are two pairs of organs, which are cone-shaped, and which occupy a large part of the chest area, together with the heart.

The function of the lungs is to transport oxygen to the bloodstream, which is needed for the vital functions of cells and their proper functioning, and it also helps to eliminate carbon dioxide, as a waste product.

These are divided into “lobes” or large sections of tissue separated by “fissures” or dividers.

Three lobes are observed in the right lung, while the left lung has only two lobes, because the heart is occupying an important space on the left side of the chest.

There are smaller portions, into which the lungs are divided, called “bronchopulmonary segments.”

These are pyramidal shaped areas that are also separated from each other by membranes, there are about 10 of them in each lung and they receive blood and air supply.

Cried her

The pleurae are smooth membranes that line the lungs.

The pleurae have two layers, a “visceral” layer that adheres closely to the outer surface of the lungs and a “parietal” layer that lines the inside of the chest wall (rib cage).

The pleura contribute to inhalation and exhalation avoiding friction.

They also make sure that when the rib cage expands when you breathe, the lungs also expand to fill the extra space.


The breathing process is aided by a large dome-shaped muscle under the lungs called the diaphragm.

It is the strong muscular wall that separates the thoracic cavity from the abdominal cavity. As it moves down, it creates suction to draw in air and expand the lungs.

Inhalation and exhalation are the processes by which the body brings in oxygen and expels carbon dioxide.

When inhaled, the diaphragm contracts downward, creating a vacuum that causes a rush of fresh air into the lungs.

The opposite occurs with exhalation, where the diaphragm relaxes upward, pushing on the lungs, allowing them to deflate.

Lung functions

The oxygen contained in the air reaches the lungs through the bronchi.

These tubes start in the lower part of the trachea and divide into the left and right bronchi and then branch many times through the lungs, into bronchioles, until they reach small air sacs or thin-walled bubbles, which are known as alveoli.

It is in the alveoli where the exchange of oxygen gases and carbon dioxide takes place, between the air and the blood.

In each alveolus there is a complete network of small blood vessels covering them, which are called capillaries, and which constitute tiny branches of the pulmonary arteries.

Oxygen and carbon dioxide must move (or diffuse), between the alveoli and the blood in the capillaries, so it is important that they are close together.

Some of the oxygen from the fresh air travels through the walls of the alveoli into the bloodstream.

While in the capillaries, the blood moves carbon dioxide into the alveoli and absorbs oxygen from the air in the alveoli.

Gas exchange takes place through the membrane of the pulmonary alveolus, which always contains air: oxygen (O2) is absorbed from the air into the blood capillaries and the action of the heart circulates it through all the tissues of the body. .

At the same time, carbon dioxide (CO2) is transmitted from the blood capillaries into the alveoli and is then expelled through the bronchi and upper respiratory tract.

The inner surface of the lungs where gas exchange occurs is very large, due to the structure of the air sacs in the alveoli.

In this way, the oxygen required to perform vital functions enters the body and carbon dioxide is discarded as a waste product.

Blood supply

The lungs receive a very large blood supply as they are very vascular organs.

The arteries that supply blood to the lungs are connected directly to the right side of the heart.

These arteries that carry blood to the lungs, contain a high content of carbon dioxide and very little oxygen, so that the carbon dioxide when expelled, can achieve the absorption of more oxygen through the bloodstream.

The new oxygen-rich blood then returns through paired pulmonary veins to the left side of the heart.

This oxygenated blood is pumped by the heart throughout the body to supply oxygen to cells and organs.

The respiratory system and age

At the time of delivery, the newborn goes from a breath through the placenta to a breath through the lungs, to obtain the necessary oxygen.

In this process, many changes take place in the baby’s lungs, both in air pressure and in blood flow.

When a baby is born prematurely, that is before 37 weeks of pregnancy, this transition process becomes even more difficult.

Since the lungs of the newborn are not yet mature enough to cope with this change and lack a substance that is produced in the cells of the lungs, called surfactant.

Surfactant is a substance that prevents the alveoli from collapsing and pressure is maintained in the newborn’s lungs,

Surfactant prevents the area through which oxygen and carbon dioxide can be exchanged from being reduced, and therefore, without surfactant, the newborn will have problems taking the necessary oxygen, and may develop neonatal respiratory distress syndrome.

The fetus in normal conditions begins to produce surfactant from 28 to 32 weeks of gestation, and therefore if the birth occurs around these weeks, it is possible that not enough surfactant has been produced to maintain its lungs open.

Signs of Neonatal Respiratory Distress Syndrome include tachypnea (very rapid breathing) and cyanosis (blue of the lips and tongue).

Sometimes Neonatal Respiratory Distress Syndrome can be treated by giving the baby an artificially manufactured surfactant through a tube that goes down into the baby’s lungs.

On the other hand, the respiratory system is associated with a series of changes that occur throughout the normal aging process of the individual and that affect both its structure and its function.

In this aging process it is observed:

A gradual enlargement of the pulmonary alveoli.

These air spaces and lose elasticity and increase in size, This translates into a decrease in the area where gas exchange takes place.

The condition in its final stage is known as “senile emphysema.”

A decrease in the elasticity of the chest wall.

This causes that to carry out the process of inhalation and exhalation in the respiratory act, a greater effort is required.

A decrease in the strength of the diaphragm and intercostal muscles.

This decrease is related to the general health of the individual.

These changes cause in an elderly person, greater difficulties in the face of increased stress on the respiratory system, in the face of an infection such as pneumonia , than the same condition in a young person.

Diseases of the respiratory system

Diagnosis of respiratory diseases

The tools most commonly used in a diagnosis for respiratory diseases are chest x-rays, pulmonary function tests.

A nasal swab test measures RNA or protein molecules in human cells and can identify a viral infection.