Lungs: Definition, Function, Anatomy, Structure of the Respiratory System and Breathing

They are a pair of organs of the respiratory system located in the chest and are the main structures responsible for exchanging oxygen.

As well as carbon dioxide between the air we breathe and the blood.

Anatomy of the respiratory system

The anatomy of the respiratory system can be divided into two main fractions, the anatomy of the respiratory tract and the pulmonary anatomy.

The anatomy of the respiratory tract can be subdivided into the following segments:

  • The extrathoracic or superior airway includes the supraglottic, glottic, and supraglottic regions.
  • The intrathoracic or inferior airway: includes the trachea, the main bronchi, and multiple generations of bronchial branches, whose primary function is the air conduction to the alveolar surface.

The anatomy of the lung includes the lung parenchyma, which carries part of the conduction system, but is mainly involved in the exchange of gases at the alveolar level.

The pulmonary parenchyma is subdivided into lobes and segments.

Structure of the respiratory system


The trachea is a cartilaginous and fibromuscular tube that extends from the inferior side of the cricoid cartilage, the cervical vertebra, to the main carina, the fifth level of the thoracic vertebra.


Its length is 3 cm at birth and 10 to 12 cm in adults, of which 2 to 4 cm are extrathoracic and 6 to 9 cm are intrathoracic.

The tracheal diameters vary widely, ranging from 13 to 25 mm measured from the coronal plane; in men and women, the variability is observed, with a range of 10 to 21 mm, measured from the coronal plane.

The shape of the intrathoracic trachea changes during expiration due to the invagination of the posterior wall, causing up to a 30% reduction in its anteroposterior diameter.

The tracheal wall has four layers: mucosa, submucosa, cartilage or muscle, and adventitia.

The posterior tracheal wall lacks cartilage. Instead, it is supported by a thin band of smooth muscle.

The bronchi:

The bronchi are composed of cartilaginous and fibromuscular elements. However, the distinction between these elements is less evident in the bronchi than in the trachea, especially in the more distal airways.

The thickness of the wall is approximately proportional to the airway diameter in the airways distal to the segmental branches.

For airways less than 5 mm in diameter, the wall should measure from one-sixth to one-tenth of the diameter.

The airways are divided by dichotomous branching, with approximately 23 generations of branches, from the trachea to the alveoli.

Different nomenclature systems have been applied to the bronchial tree over the years.

In general use, there are two main bronchi (on the right and left sides) and three lobar bronchi on the right.

With ten segmental bronchi, two lobar bronchi are on the left, with eight segmental bronchi.

There is no accepted terminology for subsegmental bronchi.

In general, the length and diameter of the central airways vary from right to left.

The vascular supply of the trachea and the bronchial tree depends on the branches of the inferior thyroid arteries, the intercostal arteries, and the bronchial arteries (aortic branches).

These arteries form a peribronchial plexus that follows the bronchial tree deep into the lung parenchyma to supply blood, except for the thyroid artery.

Also to the visceral pleura, to the walls of the pulmonary arteries and veins.


There is some symmetry between the right and left lungs. Both lungs are divided into lobes.

The thick functional subunits of each lung are called segments and have a close relationship with the segmental bronchi.

The right lung comprises ten segments: 3 in the right upper lobe (apical, anterior, and medial), 2 in the right middle lobe (medial and lateral), and 5 in the right lower lobe (superior, medial, anterior, lateral and posterior).

The left lung comprises eight segments: 4 in the left upper lobe (apicoposterior, anterior, superior lingular, and inferior lingular) and 4 in the left lower lobe (superior, anteromedial, lateral, and posterior).

The lungs are covered by the visceral pleura, which is contiguous with the parietal pleura, as reflected from the lateral surfaces of the mediastinum.

The visceral pleura forms invaginations in both lungs, which are called fissures.

There are two complete fissures in the right lung and a complete fissure with an incomplete crack in the left; these separate the different pulmonary lobes.

The pleura also forms the pulmonary ligament, a double layer of pleura that extends caudally along the mediastinum from the inferior pulmonary vein to the diaphragm.

Pulmonary vasculature:

There is a close relationship between the bronchial tree and the anatomy of the pulmonary vasculature, composed mainly of the pulmonary arteries and veins.

The main pulmonary artery originates in the right ventricle and is divided into two branches.

The right pulmonary artery passes behind the aorta and the superior vena cava, emerging laterally to the anterior atria and slightly inferior to the right main bronchus.

In contrast, the origin of the left pulmonary artery is located anterior to the left main bronchus.

The arborization of the pulmonary arteries varies from right to left. Still, it mainly divides into the trunk, lobular, segmental, and subsegmental arteries, which generally follow the bronchial tree branches.

The pulmonary veins originate in the alveoli and receive drainage from the bronchial and pleural branches.

After the confluence of the small branches in the larger ones, two pulmonary veins, superior and inferior, are formed on each side.

These four veins usually join with or near the left atrium, and this common area is typically intrapericardial.

Lung lymphatic system:

The lymphatic drainage of the lungs begins with the lymphatic vessels that first drain into the intraparenchymal lymph nodes.

They then move to the peribronchial (hilar) lymph nodes and transfer to subcarinal, tracheobronchial, and paratracheal lymph nodes.

The lymph nodes eventually communicate with the venous system through the bronchial-media lymphatic trunk and the thoracic duct or the deep lymph nodes of the cervix (scalene).

However, some variants of lymphatic drainage are significant to consider, generally in the dissemination of pulmonary neoplasms.

Microscopic anatomy:

The trachea has multiple layers.

The mucosa comprises a ciliated pseudostratified columnar epithelium and numerous mucus-secreting goblet cells that rest on a basement membrane with a thin lamina, mainly collagen.

The submucosa contains seromucosa glands.

The adventitia contains cartilaginous rings interconnected by the connective tissue.

The rings of hyaline cartilage have the shape of the letter C and are open later.

The open ends are connected by fibroelastic tissue and a band of smooth muscle (the trachea).

The epithelium of the bronchus is the pseudostratified columnar epithelium, also with numerous goblet cells.

This epithelium passes first to a simple cylindrical epithelium and then to a cuboidal epithelium as it continues branching into smaller bronchioles.

The cartilage support is eventually lost at the bronchiolar level (0.5 to 1.0 mm in diameter).

The muscular layer becomes the dominant structure and consists of smooth muscle and elastic fibers.

At this level, the mucosa can be very folded due to the loss of the support structure.

The terminal bronchioles are considered the respiratory zone of the lungs (i.e., the area where gas exchange occurs).

They are divided into respiratory bronchioles, which continue downstream as alveolar ducts, wholly lined with alveoli and alveolar sacs.

More than 300 million alveoli in the human lung are covered by an extensive network of capillaries (branches of the pulmonary arteries).

The respiratory zone constitutes most of the lungs.

The epithelium of the respiratory bronchiole is primary cubic and may be ciliated; the goblet cells are absent.

The thin support layer is formed by collagenous and smooth muscle.

The alveoli appear as small pockets that interrupt the main wall.

The terminal portion of the respiratory canal gives rise to the alveolar sacs (composed of a variable number of alveoli).

The alveoli are the most minor and most numerous subdivisions of the respiratory system.

The interalveolar septum often contains openings of 10 to 15 μm between the neighboring alveoli that help to equalize the air pressures between them.

The alveolar wall is fragile (25 μm). It is formed by squamous epithelium (type I cells) covered by a thin film of surfactant fluid rich in hydrophilic phospholipids produced by type II cells (septal cells).

This surfactant fluid keeps the alveoli open by reducing the surface tension of the interface between the opposing alveolar surfaces, which is reflected in reduced inspiratory work.

The basal lamina is in intimate contact with the capillaries of the pulmonary vascular system, which favors the transfer of oxygen to the red blood cells and the release and transfer of carbon dioxide to the alveolar airway.

Natural variants:

Several changes occur as a human being develops from a fetus to a fully grown adult.

Some of these changes follow regular patterns, and others compensate for specific conditions or occur for unknown reasons.

The congenital anatomical variants of the lungs are present in the following forms:

  • Agenesis: Complete congenital absence of one or both lungs, the latter being incompatible with life (the condition is associated with other congenital anomalies and is rare).
  • Aplasia or hypoplasia is the presence of a rudimentary bronchus that ends in a blind pouch without evidence of pulmonary vasculature or pulmonary parenchyma.
  • Accessory lobes, and lobe fusion: Variations on the pulmonary lobes that are caused mainly by incomplete obliteration of the visceral pleural folds, result from the presence of abnormal vessels (creating extra lobes).

The congenital anatomical variants of the airway are present in the following forms:

Bronchial variations: The variations in the patterns of the bronchial tree are mainly due to the displacement of the segmental and subsegmental bronchi (reduction migration and selection theories).

The anatomical anomalies of the bronchi may be the preferred locations for deformities, chronic inflammations, and bronchial neoplasms.

The congenital anatomical variants of the diaphragm are present in the following forms:

Normal variations in the diaphragm are consistent mainly with different muscle insertion sites that form the diaphragm or due to a congenital disability that leads to communication from the abdominal cavity to the thorax, such as:

  • Hernia Bochdalek.
  • Hernia Morgagni.
  • Diaphragmatic eventration.

Pathophysiological variants:

The pathological variants are related to changes in the structure of the respiratory tract, the pulmonary parenchyma, or adjacent structures that lead to the alteration of the normal anatomy of the respiratory system.

The most common of these variants are presented below:

Emphysema and chronic obstructive pulmonary disease:

Emphysema and chronic obstructive pulmonary disease are caused by an accumulation of inflammatory mucus, causing loss of elasticity due to the destruction of lung tissue recoil.

An increase in the resistance of the conductive airways causes an abnormal permanent dilatation of the air spaces distal to the terminal bronchioles.

Pneumothorax, hemothorax and hydrothorax:

Pneumothorax, hemothorax, and hydrothorax are caused by a decrease in lung volume secondary to the presence of air, blood, or fluid between the visceral and parietal components of the pleura.

Diaphragmatic hernias:

A diaphragmatic hernia occurs when a defect in the diaphragm allows the abdominal contents to move into the thoracic cavity.

This could be congenital, traumatic, iatrogenic, or due to a weakness in the muscles that make up the diaphragm.

Bronchial and tracheal stenosis:

Bronchial and tracheal stenosis occurs due to secondary processes and numerous malignant and benign processes resulting from surgical procedures and trauma.

The primary defect is the obstruction or collapse of the airways at any level, which causes changes in the airflow that produce hypoxemia.

Paralysis and dysfunction of the vocal cords:

In most cases, paralysis or dysfunction of the vocal cords is caused by recurrent laryngeal or vagus nerve dysfunction that innervates the larynx.

Even when there is no actual anatomy alteration, this condition can cause many of the same problems associated with bronchial and tracheal stenosis.

Infectious processes:

Infectious etiologies such as bacterial pneumonia and tuberculosis include viral, fungal, and bacterial infections.

They are characterized by the consolidation of the affected part of the lung and the filling of the alveolar spaces with exudate, inflammatory cells, and fibrin.

This leads to a decrease in oxygen exchange, lack of ventilation coincidence, and, in severe cases, the destruction of the parenchymal lung.

Interstitial lung diseases:

Interstitial lung diseases include conditions caused by drugs, autoimmune processes, fibrotic diseases, exposure to organic and inorganic dust, sarcoidosis, and lymphangioleiomyomatosis.

As well as histiocytosis X, vasculitis, pulmonary alveolar proteinosis and any other process that causes reduced lung volumes due to an alteration in the lung parenchyma that leads to a mismatch ventilation-perfusion.


Malignancy is the uncontrolled cell growth of tissue in the lungs or airways.

Depending on the location and severity of the malignancy, it can lead to any of the anatomical changes described above.

The breathing

In the first part of breathing, air enters the body through the nose or the mouth.

It passes to the central trachea and continues its route to each of the lungs through the right or left bronchus.

The lungs are separated into sections called lobes, two on the left and three on the right.

Air passage divides into smaller tubes, which finally connect with tiny air sacs called alveoli.

This set of gradually branching tubes is known as the tracheobronchial tree because of its remarkable similarity to the branching pattern of a tree.

The other half of the respiratory system involves blood circulation.

The venous blood present in the body is returned to the right side of the heart and then pumped through the pulmonary artery.

This artery is divided into the lungs, the left, and the right and then continues branching, similar to the tracheobronchial tree.

These vessels branch into a thin network of tiny tubes called capillaries.

The capillaries are located next to the alveoli and are so small that only one red globule can pass through their openings simultaneously.

During this passage, gases are exchanged between blood and air in nearby alveoli.

After passing the alveoli, the capillaries come together to begin to form the pulmonary veins, which carry the blood back to the left side of the heart.

The breath is divided into two components, inhalation and exhalation.

Inhalation is active because it requires muscle contraction.

The primary muscle of respiration is a dome-shaped muscle called the diaphragm, which is located under the lungs.

The diaphragm separates the chest and the abdominal cavities.

As the diaphragm contracts, it flattens out, moving into the abdominal cavity.

This action causes an increase in the size of the thoracic cavity, thus creating a vacuum, and the air is sucked through the mouth or nose.

When physical activity increases dramatically, or with some lung conditions, other muscles such as those in the neck and those between the ribs also help increase the size of the chest cavity.

These muscles are known as accessory muscles of respiration.

The exhalation is passive because it does not require muscle contraction.

During this phase, the expanded lung acts like a stretched elastic band and contracts to its resting position.

This contraction causes all the air in the lungs to come out through the mouth.

In the metabolic process, the cells use energy, and one of the waste products is carbon dioxide gas.

Oxygen-enriched red blood cells release oxygen to the body’s cells and absorb the residual carbon dioxide.

This dark blue blood, deprived of oxygen, is sent to the lungs’ blood vessels.

The carbon dioxide is released by the red blood cells, passes quickly through the capillary wall to the space in the air sac of the adjacent alveoli, and is then removed with each breath out of the mouth or exhalation.

The oxygen in the air sac quickly passes into the capillaries and red blood cells.

The capillary network that transports this bright red blood rich in oxygen flows to the larger vessels and finally empties into the left side of the heart, where it is pumped to all the body tissues.

In this respiratory cycle, the outside air needs to be heated and moistened to match the temperature and humidity in the body.

As the air passes through the tracheobronchial tree, it is heated, and water is added.

The contaminants must also be eliminated.

On the other hand, the hairs of the nose and tiny microscopic hairs called cilia, together with the sticky mucus produced by the coating membrane, help clean the air of the impurities.

The cilia beat in a synchronized way brushing accumulated dirt and mucus towards the mouth.

When the air reaches the alveoli, it is practically sterile, and the accumulated material is expelled or swallowed.