PULMONARY PARENCHYMA: DESCRIPTION, HISTOLOGY, DISEASES - ANATOMY AND PHYSIOLOGY - 2025

The lung parenchyma is the functional tissue of the lung. It is composed of an air conduction system and a gaseous exchange system. It has different structural components in the tubes and ducts that constitute it from the nose to the pulmonary alveoli.

Around the pipe system, the lung parenchyma has elastic and collagen fibers arranged in the form of a mesh or network that has elastic properties. Some elements of the pipe system have smooth muscle in their structure, which allows the diameter of each tube to be regulated.

Basic diagram of the human respiratory system (Source: UNSHAW via Wikimedia Commons)

The lung does not have muscles that allow its expansion or retraction, this function is fulfilled by the muscles of the rib cage, which are called "respiratory muscles". The lungs, from this point of view, are organs that passively follow the movements of the "box" that surrounds them.

There is also no ligament or structure that fixes the lungs to the rib cage, both hang from their respective main bronchi, the right bronchus and the left bronchus, and both the rib cage and the lung are covered with a membrane called the pleura.

Diseases of the lung parenchyma can be simply classified as infectious diseases, tumor diseases, restrictive diseases, and obstructive diseases.

An environment free of toxic substances and smoke or particles in suspension and not consuming drugs by inhalation or cigarettes prevent many of the main diseases that affect the lung parenchyma and, therefore, respiratory function.

Anatomo-functional description

The lungs are two organs located within the rib cage. They are composed of a system of pipes that undergoes 22 divisions called “bronchial generations”, which are found before reaching the alveolar sacs (23), which are the gas exchange sites where the respiratory function is carried out.

From the main bronchi to the 16th bronchial generation, the airways fulfill exclusively conduction functions. As the routes are subdivided, the diameter of each particular tube becomes smaller and smaller and its wall increasingly thinner.

Pulmonary gas exchange and conduction system, the bronchi (Source: Arcadian, via Wikimedia Commons)

When the walls of the piping system lose cartilage, its name changes from bronchus to bronchiole, and the last generation of bronchial tubes with exclusive conduction function is called the terminal bronchiole.

From the terminal bronchiole, the following bronchial generations are called respiratory bronchioles, until they give rise to the alveolar ducts and end in the alveolar sacs or alveoli.

Gaseous exchange system

The only function of the alveoli is the exchange of gases (O2 and CO2) between the alveolar air and the blood that circulates through the alveolar capillaries and forms a capillary network or mesh around each alveolus.

This structural subdivision of the airways makes it possible to increase the surface available for gas exchange. If each of the alveoli is removed from one lung, stretched and placed side by side, the surface area reaches between 80 and 100 m2, which is roughly the size of an apartment.

The blood volume in contact with this enormous surface area is approximately 400 ml, which allows the red blood cells, which are the ones that carry O2, pass one after the other through the pulmonary capillaries.

This huge surface area and an extremely thin barrier between the two gas exchange territories provide the ideal conditions for this exchange to take place quickly and efficiently.

Pleura

The lung and rib cage are attached to each other through the pleura. The pleura is made up of a double membrane made up of:

- A leaf that receives the name of leaf or parietal pleura, which is strongly adhered to the internal surface of the rib cage covering its entire surface.

- A sheet called the visceral pleura, strongly attached to the outer surface of both lungs.

Representative diagram of the lung pleura (Source: OpenStax College via Wikimedia Commons)

Between the visceral and parietal leaf there is a thin layer of liquid that allows the two leaves to slide against each other, but which generates great resistance for the separation of both leaves. For this reason, the visceral and parietal leaves of the pleura are held together and thus the chest wall and lung are joined.

When the chest wall expands as a result of the respiratory muscles, the lung follows, through its pleural junction, the movements of the cage and, therefore, is distended, increasing its volume. When the anterior muscles relax, the cage retracts, reducing the size of each lung.

From the first breaths that occur at birth, both lungs expand and acquire the size of the rib cage, establishing the pleural relationship. If the rib cage opens or air, blood, or fluid enters the pleural cavity in a significant way, the pleurae separate.

In this case, the lung whose parenchyma has abundant elastic tissue and which was expanded or stretched as a result of the pleural relationship, now retracts (as a stretched elastic band does) loses all the air and remains hanging from its main bronchus.

When this happens, the rib cage expands, becoming larger than it was when it was attached to the lung. In other words, both organs acquire their independent elastic rest position.

Histology

Histology of the conduction system

The intrapulmonary conduction system is composed of the different bronchial divisions starting from the secondary or lobar bronchi. The bronchi have a respiratory epithelium that is pseudostratified and is made up of basal cells, goblet cells, and ciliated columnar cells.

The bronchial wall is covered with sheets of cartilage that give it a rigid structure that offers resistance to external compression, so the bronchi tend to remain open. Around the tube are elastic and smooth muscle fibers in a helical arrangement.

The bronchioles do not have cartilage, so they are subjected to the traction forces exerted by the elastic tissue that surrounds them when it is stretched. They offer very little resistance to all the external compressive forces that are applied to them, therefore they can easily and passively change their diameter.

The epithelial lining of the bronchioles varies from a simple ciliated epithelium with scattered goblet cells (in the larger ones), to a ciliated cuboid epithelium without goblet cells and clear cells (in the smaller ones).

The cells in clear that are cylindrical cells with the upper part or apex in the shape of a dome and with short microvilli. They secrete glycoproteins that cover and protect the bronchial epithelium.

Histology of the alveoli

The alveoli are about 300,000,000 in total. They are arranged in bags with many partitions; They have two types of cells called type I and type II pneumocytes. These pneumocytes are joined to each other by means of occluding junctions that prevent the passage of liquid.

Normal lung structure (Source: National Heart Lung and Blood Institute via Wikimedia Commons)

Type II pneumocytes are more prominent cuboid cells than type I. In their cytoplasm they contain lamellar bodies and these pneumocytes are responsible for synthesizing the pulmonary tensoactive substance that covers the internal surface of the alveolus and lowers surface tension.

The alveolar and endothelial basal laminae fuse and the thickness of the alveolar-capillary barrier that gases must pass through to pass from one side to the other is minimal.

Histology of the tissue surrounding the tubing

The tissue that surrounds the piping system has a hexagonal arrangement, it is made up of elastic fibers and collagen fibers that are rigid. Its geometric arrangement forms a net, similar to a nylon stocking, which is made up of rigid individual fibers woven into an elastic structure.

This conformation of elastic tissue and elastic interlocking structure gives the lung its own characteristics, which allow it to passively retract and, under certain expansion conditions, offer minimal resistance to distension.

Diseases

Pulmonary diseases can be infectious in origin by bacteria, viruses or parasites that affect the lung tissue.

Tumors of a different nature, benign or malignant, can also form, capable of destroying the lung and causing the death of the patient due to lung or brain problems, which are the most important areas of lung metastasis.

However, many diseases of various origins can cause obstructive or restrictive syndromes. Obstructive syndromes cause difficulty for the entry and / or exit of air from the lung. Restrictive syndromes cause respiratory distress by reducing the lung's ability to expand.

Examples of obstructive diseases include bronchial asthma and pulmonary emphysema.

Bronchial asthma

In bronchial asthma, the obstruction is due to an active, allergic contraction of the bronchial musculature.

Contraction of the bronchial muscle reduces the diameter of the bronchi and makes it difficult for air to pass. Initially the difficulty is greater during expiration (air out of the lung) since all the retraction forces tend to close the airways even more.

Pulmonary emphysema

In the case of pulmonary emphysema, what occurs is a destruction of the alveolar septa with loss of elastic lung tissue or, in the case of physiological emphysema in adults, the interwoven structure of the lung parenchyma is altered.

In emphysema, the decrease in elastic tissue decreases the pulmonary retraction forces. For any lung volume that is examined, the diameter of the pathways is reduced by reducing the external elastic traction. The end effect is respiratory distress and air trapping.

Lung restrictive syndrome is due to the replacement of elastic tissue by fibrous tissue. This reduces the capacity for lung distension and causes shortness of breath. These patients breathe with smaller and smaller volumes and higher and higher respiratory rates.

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