Respiratory System: Parts, Function, and Diseases

The respiratory system is the set of organs responsible for the expulsion of carbon dioxide and the entry of oxygeninto the body. This process is known as breathing .

Living beings need oxygen to perform their functions, and, at the same time, produce compounds that must be eliminated, such as carbon dioxide. This is why the respiratory system and the circulatory system have a close interaction in the exchange of gases.

Parts of the respiratory system

The respiratory system is formed by the airways, lungs and respiratory muscles.

Air ways

The airways or respiratory tract comprise the organs that allow the passage of air to the lungs. These organs include nasal cavity, pharynx, larynx, trachea and bronchi.

Nasal Cavity

It is the inner area of ​​the nose. Its main function is to heat, moisten and filter the air when breathing . Also in the nasal cavity is the sense of smell, which allows us to distinguish the smells that surround us.


It is the connection between the nasal cavity and the oral cavity. It is located behind the mouth and conducts the air to the larynx. In the part that connects to the nose, it is called the nasopharynx; where it connects with the mouth, it is called oropharynx.


It is located between the pharynx and the trachea. An easy way to learn what comes first, the larynx or pharynx, is in alphabetical order: F comes before L. Therefore, f aringe comes before l aringe.

The main function of the larynx is to prevent the entry of food or liquids into the trachea. It is also important in the production of sounds: that is where the vocal cords are located.


It is located in front of the esophagus and is a rigid cylinder that lets air pass from the larynx to the bronchi . The rigidity of the trachea is due to rings of cartilage, the same material that gives the structure to the ears and the tip of the nose.

This cartilage is not as strong as bone, but it helps to keep the tube of the trachea open and not crush, which would allow the passage of air.

The trachea of ​​humans is between 10 and 12 cm long and 2 cm wide. It is covered with a mucous substance and some hairs or cilia that help to catch the strange particles that escaped the filtrate of the nose.

The trachea is divided into two tubes that each go to a lung : these are the bronchi, which, in turn, continue to divide like the branches of a tree inside the lungs, forming the bronchioles .


They are the two largest organs inside the rib cage, one on each side of the heart. They are different, the right lung separates in three lobes by two fissures and the left in two lobes. They have a spongy and elastic appearance, so they can vary their volume during the processes of inspiration and expiration.

Inside the lungs, the bronchi divide until they reach the terminal bronchioles whose ends end in bunches. These are the alveoli.

Also, the lungs are surrounded by a membrane or cloth, called a pleura .


The alveoli are the functional units of the respiratory system . They are small bubble-like bags that are found at the end of all the bifurcations of the bronchioles. These sacs have the thickness of just one cell, and are bordered by capillaries, allowing direct contact with the blood.

It is in the alveoli where the external oxygen exchange occurs by internal carbon dioxide. In the lungs of humans there are about 300 million alveoli, each with a size of 0.3 mm.

Respiratory muscles

The respiratory muscles are constituted by the diaphragm and the intercostal muscles. Thanks to them, the lungs are filled and empty of air.


It is the muscle that is located on the floor of the thoracic cavity, separating it from the abdomen. The lungs settle on it.

When the diaphragm contracts, it acts like the plunger of a syringe when it is pulled to suck a liquid. In this case, the air is sucked into the lungs.

Intercostal muscles

These are the muscles that are between the ribs, the bones that make up the rib cage. The movement of these muscles allows the ribs to move upwards, so the lungs can expand when the air enters.

Mechanism of respiration

Pulmonary ventilation includes the entry and exit of air from the organism through inspiration and expiration.

The mechanism of lung breathing or ventilation occurs when air enters through the nose and into the nasal cavity. Then it follows through the pharynx and larynx to the trachea and reaches the bronchi. From here it is distributed through the lungs until the end of the branches, where oxygen diffuses into the blood, and carbon dioxide passes into the alveoli. Finally, the air is expelled when the respiratory muscles relax.


Inspiration or inhalation is the active phase of lung breathing. It occurs when the diaphragm and intercostal muscles contract, pushing the chest down and out. This produces an increase in the thoracic capacity and, as a result, the expansion of the lungs and the decrease in pressure within the thorax.

Air enters the lungs when intrapulmonary pressure is lower than atmospheric pressure (760 mmHg). In each breath, about half a liter of air enters, of which 150 ml remain in the airways. As there is no gas exchange in these roads, there is talk of dead anatomical space.


The expiration is a passive process at rest that follows the inspiration, with the reduction of the thoracic capacity and the increase of the intrapulmonary pressure. This causes the expulsion of air from the lungs.

Gas exchange in breathing

Oxygen and carbon dioxide cross the barrier between the blood and the alveolus by diffusion.

The exchange of oxygen and carbon dioxide occurs through the walls of the capillaries and the alveoli. The movement is made by passive diffusion , that is, gases move from where there is greater pressure at a lower pressure. For this, no energy is required.

The oxygen that enters the lungs is at a pressure of 100 mmHg, while in the capillary blood it is at 40 mmHg. Therefore, oxygen flows from the alveolar space to the red blood cell.

On the other hand, carbon dioxide diffuses much faster through tissues because of its greater solubility. When the red blood cell gets loaded with carbon dioxide into the lungs, the carbon dioxide goes into the alveolar space where the pressure of this gas is much lower.

What are the defense mechanisms of the respiratory system?

Inside the nasal cavity, hairs, cilia and mucus trap dust and small particles, filtering the air that enters the lungs.

The particles that are deposited in the bronchi, are swept out by the cilia and mucus of the walls, and go to the throat where they can be swallowed or expectorated.

Regulatory mechanisms of the respiratory system

Breathing is under voluntary and involuntary control in certain conditions. The automatic process is controlled by the respiratory centers in the brainstem and the medulla. However, when we hold our breath or hyperventilate, it is the cerebral cortex that is in charge.

In moments when we feel fear or anger, it is the hypothalamus and the limbic system that alter our breathing pattern.

The partial pressure of carbon dioxide in the blood is the most important factor in the control of respiration. The ventilation response decreases if the carbon dioxide pressure is reduced.

How does air pollution affect the respiratory system?

Air pollution is produced mainly by industries and vehicles, a problem that worsens in large cities. Gases such as oxides of nitrogen and sulfur, carbon monoxide and hydrocarbons reach the lungs, causing serious damage.

Small pollutant particles dispersed in the air are also found in the form of aerosols, such as dust and smoke, that can be inhaled.

Among the effects produced by gases and particulate pollutants in the air are:

  • Inflammation of the upper airways.
  • Inflammation of the bronchi.
  • Rhinitis and allergic problems.
  • Possibility of cancer development.

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