PULMONARY ALVEOLI: CHARACTERISTICS, FUNCTIONS, ANATOMY - ANATOMY AND PHYSIOLOGY - 2024

The pulmonary alveoli are small sacs located in the lungs of mammals, surrounded by a network of blood capillaries. Under a microscope, in an alveolus, the lumen of the alveolus and its wall, made up of epithelial cells, can be distinguished.

They also contain connective tissue fibers that give them their characteristic elasticity. Type I flat cells and type II cube-shaped cells can be distinguished in the alveolar epithelium. Its main function is to mediate gas exchange between air and blood.

As the breathing process occurs, air enters the body through the windpipe, where it travels to a series of tunnels within the lung. At the end of this intricate network of tubes are the alveolar sacs, where air enters and is taken up by blood vessels.

Already in the blood, the oxygen in the air is separated from the rest of the components, such as carbon dioxide. This last compound is eliminated from the body through the exhalation process.

General characteristics

Inside the lungs there is a spongy textured tissue formed by a fairly high number of pulmonary alveoli: from 400 to 700 million in the two lungs of a healthy adult human. The alveoli are sac-like structures covered internally by a sticky substance.

In mammals, each lung contains millions of alveoli, closely associated with the vascular network. In humans, the area of ​​the lungs is between 50 and 90 m ² and contains 1000 km of blood capillaries.

This high number is essential to ensure the required oxygen uptake and thus be able to comply with the high metabolism of mammals, mainly due to the endothermy of the group.

Respiratory system in mammals

Air enters through the nose, specifically through the "nostrils"; This passes into the nasal cavity and from there to the internal nostrils connected to the pharynx. Here two routes converge: the respiratory and the digestive.

The glottis opens to the larynx and then the trachea. This is divided into two bronchi, one in each lung; in turn, the bronchi divide into bronchioles, which are smaller tubes and lead to the alveolar ducts and alveoli.

Features

The main function of the alveoli is to allow gas exchange, vital for respiratory processes, allowing oxygen to enter the bloodstream to be transported to the tissues of the body.

In the same way, the pulmonary alveoli participate in the removal of carbon dioxide from the blood during the processes of inhalation and exhalation.

Anatomy

The alveoli and alveolar ducts consist of a very thin single-layer endothelium that facilitates the exchange of gases between the air and the blood capillaries. They have an approximate diameter of 0.05 and 0.25 mm, surrounded by capillary loops. They are rounded or polyhedral in shape.

Between each consecutive alveolus is the interalveolar septum, which is the common wall between the two. The border of these septa forms the basal rings, formed by smooth muscle cells and covered by the simple cuboidal epithelium.

In the outer part of an alveolus are the blood capillaries that, together with the alveolar membrane, form the alveolus-capillary membrane, the region where gas exchange takes place between the air that enters the lungs and the blood in the capillaries.

Due to their peculiar organization, the pulmonary alveoli are reminiscent of a honeycomb. They are constituted on the outside by a wall of epithelial cells called pneumocytes.

Accompanying the alveolar membrane are cells responsible for the defense and cleaning of the alveoli, called alveolar macrophages.

Cell types in the alveoli

The structure of the alveoli has been widely described in the literature and includes the following cell types: type I that mediate gas exchange, type II with secretory and immune functions, endothelial cells, alveolar macrophages that participate in defense and interstitial fibroblasts.

Type I cells

Type I cells are characterized by being incredibly thin and flat, presumably to facilitate gas exchange. They are found in approximately 96% of the surface of the alveoli.

These cells express a significant number of proteins, including T1-α, aquaporin 5, ion channels, adenosine receptors, and genes for resistance to various drugs.

The difficulty of isolating and culturing these cells has impeded their in-depth study. However, a possible function of homoshesis in the lungs is raised, such as the transport of ions, water and participation in the control of cell proliferation.

The way to overcome these technical difficulties is by studying the cells by alternative molecular methods, called DNA microarrays. Using this methodology, it was possible to conclude that type I cells are also involved in protection against oxidative damage.

Type II cells

Type II cells are cuboidal in shape and are usually located in the corners of the alveoli in mammals, being found in only 4% of the remaining alveolar surface.

Its functions include the production and secretion of biomolecules such as proteins and lipids that constitute lung surfactants.

Pulmonary surfactants are substances composed mainly of lipids and a small portion of protein, which help reduce the surface tension in the alveoli. The most important is dipalmitoylphosphatidylcholine (DPPC).

Type II cells are involved in the immune defense of the alveoli, secreting various types of substances such as cytokines, whose role is the recruitment of inflammatory cells within the lungs.

Furthermore, in several animal models it has been shown that type II cells are responsible for keeping the alveolar space free of fluids and are also involved in sodium transport.

Interstitial fibroblasts

These cells are spindle-shaped and characterized by long actin extensions. Its function is the secretion of the cellular matrix in the alveolus to maintain its structure.

In the same way, cells can manage blood flow, reducing it as appropriate.

Alveolar macrophages

The alveoli harbor cells with phagocytic properties derived from blood monocytes called alveolar macrophages.

These are responsible for eliminating, through the process of phagocytosis, foreign particles that have entered the alveoli, such as dust or infectious microorganisms such as Mycobacterium tuberculosis. In addition, they engulf blood cells that could enter the alveoli if there is heart failure.

They are characterized by presenting a brown color and a series of varied extensions. Lysosomes are quite abundant in the cytoplasm of these macrophages.

The number of macrophages can increase if the body has a disease related to the heart, if the individual uses amphetamines or by the use of cigarettes.

Kohn pores

They are a series of pores located in the alveoli located in the interalveolar partitions, which communicate one alveolus with another and allow air circulation between them.

How does gas exchange take place?

Gas exchange between oxygen (O ₂) and carbon dioxide (CO ₂) is the primary purpose of the lungs.

This phenomenon occurs in the pulmonary alveoli, where blood and gas meet at a minimum distance of approximately one micron. This process requires two properly pumped conduits or channels.

One of these is the vascular system of the lung, driven by the right region of the heart, which sends mixed venous blood (made up of venous blood from the heart and other tissues through venous return) to the region where it occurs in exchange.

The second channel is the trachebronchial tree, the ventilation of which is driven by the muscles involved in breathing.

In general, the transport of any gas is governed mainly by two mechanisms: convection and diffusion; the first is reversible, while the second is not.

Gas exchange: partial pressures

When air enters the respiratory system, its composition changes, becoming saturated with water vapor. Upon reaching the alveoli, the air mixes with the air that was left over from the previous breathing circle.

Thanks to this combination, the partial pressure of oxygen drops and that of carbon dioxide increases. Since the partial pressure of oxygen is higher in the alveoli than in the blood entering the lung capillaries, oxygen enters the capillaries by diffusion.

Similarly, the partial pressure of carbon dioxide is higher in the capillaries of the lungs, compared to the alveoli. Therefore, carbon dioxide passes into the alveoli through a simple diffusion process.

Transport of gases from tissues to blood

Oxygen and significant amounts of carbon dioxide are transported by "respiratory pigments", among them hemoglobin, which is the most popular among groups of vertebrates.

The blood responsible for transporting oxygen from the tissues to the lungs must also transport carbon dioxide back from the lungs.

However, carbon dioxide can be transported by other routes, it can be transmitted through the blood and dissolve in plasma; in addition, it can diffuse to blood erythrocytes.

In erythrocytes, most of the carbon dioxide is converted to carbonic acid by the enzyme carbonic anhydrase. The reaction occurs as follows:

CO ₂ + H ₂ O ↔ H ₂ CO ₃ ↔ H ⁺ + HCO ₃ ^-

The hydrogen ions from the reaction combine with hemoglobin to form deoxyhemoglobin. This union avoids a sudden decrease in pH in the blood; at the same time the release of oxygen occurs.

The bicarbonate ions (HCO ₃ ^-) leave the erythrocyte by an exchange for chlorine ions. In contrast to carbon dioxide, bicarbonate ions can remain in plasma thanks to their high solubility. The presence of carbon dioxide in the blood would cause an appearance similar to that of a carbonated drink.

Transport of gases from the blood to the alveoli

As indicated by the arrows in both directions, the reactions described above are reversible; that is, the product can become the initial reactants again.

As soon as the blood reaches the lungs, the bicarbonate enters the blood cells again. As in the previous case, for the bicarbonate ion to enter, a chlorine ion must exit the cell.

At this time the reaction occurs in the reverse direction with the catalysis of the carbonic anhydrase enzyme: the bicarbonate reacts with the hydrogen ion and is converted back into carbon dioxide, which diffuses to the plasma and from there to the alveoli.

Disadvantages of gas exchange in the lungs

Gas exchange only occurs in the alveoli and alveolar ducts, which are located at the end of the tube branches.

For this reason we can speak of a "dead space", where air passes into the lungs but gas exchange does not take place.

If we compare it with other animal groups, such as fish, they have a very efficient single-path gas exchange system. Likewise, birds have a system of air sacs and parabronchi where air exchange occurs, increasing the efficiency of the process.

Human ventilation is so inefficient that in a new inspiration only one sixth of the air can be replaced, leaving the rest of the air trapped in the lungs.

Pathologies associated with the alveoli

Pulmonary ephysema

This condition consists of the damage and inflammation of the alveoli; consequently, the body is not able to receive oxygen, causes coughing, and makes it difficult to recover breath, particularly during physical activities. One of the most common causes of this pathology is smoking.

Pneumonia

Pneumonia is caused by a bacterial or viral infection in the respiratory tract and causes an inflammatory process with the presence of pus or fluids inside the alveoli, thus preventing the intake of oxygen, causing severe difficulties in breathing.

References

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