Pulmonary alveoli: characteristics, functions and anatomy

At the most distal point of the bronchial tree are small structures grouped in the shape of a bunch of grapes that are crucial for our life: the pulmonary alveoli.

In them, the exchange of breathing gases takes place, allowing the entry of oxygen into our body and the expulsion of toxic carbon dioxide, in addition to complying with other functions.

Next we will see in depth what the pulmonary alveoli are, what their anatomy is like, what cells constitute them and how they carry out gas exchange.

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What are the pulmonary alveoli?

The pulmonary alveoli are microscopic air sac-like structures found in our lungs, at the ends of other structures, the bronchioles. They are often described as shaped like a raspberry or a bunch of grapes. Each alveolus is approximately 0.2 to 0.5 mm in diameter and is bounded by a wall made up of very thin cells called pneumocytes. On average, an adult person has more than 500 million alveoli which, if stretched out, would occupy an area of ​​80 square meters, the equivalent of a tennis court.

The human respiratory system is made up of several structures, each with specific functions. For example, the conduction system is the one that allows the passage of air from the outside to the inside of the body and vice versa, being formed by the nasal cavity and cavities, the paranasal sinuses, the pharynx, the larynx, the trachea, the bronchi and the bronchioles. the alveoli They are located just at the most distal end of the conduction system., specifically at the end of the respiratory bronchioles, grouped in alveolar sacs or acini.

The respiratory functions of the lungs are largely determined by the alveoli, microstructures that represent more than 90% of its total volume and that constitute the lung parenchyma. Gas exchange between inspired air and blood takes place through the wall of the alveoli. that circulates through the blood capillaries that are found in the thin walls that give shape to the bronchioles.

Some respiratory diseases cause the alveoli to be severely affected, as is the case with asthma or tuberculosis, conditions that greatly hinder the quality of life of the affected person if they do not receive adequate treatment.

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alveoli anatomy

The pulmonary alveoli are found in acini or alveolar sacs, groupings or clusters shaped like a raspberry, a bunch of grapes or a honeycomb. They are defined as the units with blind ends located after a transitional bronchiole, that is, where a terminal bronchiole ends and a respiratory one begins. Within each acinus, all air conduction pathways or channels have alveoli attached to their walls, participating in both conduction and gas exchange. Approximately, an adult human lung has 30,000 acini.

We can describe the alveoli as sacs with a polyhedral structure that, as mentioned, are between 0.2 and 0.5 mm in diameter. The alveoli are separated from each other by a septum. Air that enters the alveolus of one acinus can be transferred to the other alveoli of the same sac through through small pores, since the alveoli that make up an alveolar sac are closely related to each other. Yes.

The pulmonary capillary ducts pass through the septa.. These ducts are thin branches of the pulmonary arteries, through which blood rich in carbon dioxide (CO2) and poor in oxygen (O2) circulates. The destination of this blood is gas exchange. These septa or alveolar walls are very thin, barely 0.5 mm thick, made up of a thin layer of connective tissue that contains extracellular matrix components and different types of cells.

The alveolar walls, better called respiratory membranes, serve as a separation barrier between the air in the alveoli and the blood. It is made up of squamous alveolar cells, capillary endothelial squamous cells, and a basement membrane.

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Alveolar cell types

There are three types of cells that we can highlight in the pulmonary alveoli.

Type I pneumocytes

Type I pneumocytes or alveolar squamous cells are the most abundant cells on the surface of the alveoli, covering approximately 95% of their area. They are thin and wide cells, whose thin walls allow rapid diffusion between air and blood, facilitating gas exchange in the alveoli.

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Type II pneumocytes

Type II pneumocytes or granular pneumocytes They are cuboidal cells, with apical microvilli and have abundant rough endoplasmic reticulum and Golgi apparatus.. They occupy about 5% of the surface of the alveolus. They do not intervene in the gas exchange itself, but they contribute to making breathing possible by facilitating distension and the recovery of the size of the alveoli.

Type II pneumocytes fulfill two functions:

• Repair the alveolar epithelium when the squamous cells are damaged.
• Secrete pulmonary surfactant.

Surfactant is composed of phospholipids and proteins that "warm" both the alveoli and the small bronchioles. in order to prevent pressure build-up and alveolar collapse on exhalation. Were it not for the surfactant, the walls of the deflated alveolar sacs could collapse on each other. as if they were sheets of wet paper, making it very difficult to fill during the next inhalation.

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alveolar macrophages

The most numerous lung cells are alveolar macrophages, also known as dust cells.. These cells slide between the alveolar lumen and the connective tissue, cleaning the surface of any foreign agent through phagocytosis. Its function is to eat dust particles, pollen or other foreign agents that may have passed through the upper portions of the respiratory tract. If the lungs are infected or hemorrhaging, macrophages are responsible for phagocytizing bacteria and blood cells.

Every day, 100 million alveolar macrophages die as they move up the alveolar ducts and through the mucociliary ladder, to be swallowed in the esophagus and digested as part of the process of removing dirt from the lungs.

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Its main functions

The alveoli are the most distal structures of the respiratory system, which causes them to carry out functions of vital importance for external respiration. Among them we highlight:

• They increase the surface area for gas exchange.
• They facilitate gaseous exchange between air and blood.
• They expand during inhalation to fill with O2-rich air.
• They contract during exhalation to empty CO2-rich air.
• Its macrophages protect us from harmful substances, particles and microorganisms.

Next we will delve into its main function in the gas exchange process.

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gas exchange

Respiration is an essential process for most living beings and the cells that make them up. Breathing does not only imply introducing enough oxygen into our body to keep it alive and allow various vital functions to continue to be carried out, but also it also involves the removal of waste products produced by metabolism. If they are not eliminated, they can accumulate causing severe damage to the body.

What we know as respiration actually comprises three different but functionally related processes: ventilation, use of oxygen at the cellular level, and gas exchange.

Ventilation is the mechanical process that enables the movement of outside air, rich in oxygen, into the lungs; and the movement of interior air, rich in carbon dioxide, to the exterior, expelling it from the lungs.

With the use of oxygen we refer to all the chemical reactions, typical of cellular metabolism, that occur thanks to the presence of this gas and by means of which the necessary energy is obtained for the maintenance of cellular processes and corporal.

As we have introduced in previous sections, gas exchange is the exchange of oxygen and carbon dioxide between the blood and the air contained in the lungs and between the blood, organs and tissues.

Specific, the pulmonary alveoli are involved in the gaseous exchange of respiration. The air that is drawn into the lungs during inhalation is rich in oxygen, with concentration levels of This gas is higher than that of the blood that circulates through the blood capillaries in the walls alveolar It is thanks to the differences in oxygen concentration between inhaled air and blood that allow O2 to diffuse into our bloodstream.

When the cells of our body receive oxygen from the blood (by diffusion), they use it to obtain energy that can be used to carry out different functions, on which our lifetime. This energy comes in various forms, such as ATP and related molecules.

The problem with cellular metabolism, in which oxygen is used, is that some waste is always produced. It is not a completely clean process since it produces a waste gas: CO2. The accumulation of carbon dioxide in both cells and tissues is very toxic to our body, so it must be eliminated. The cells get rid of the CO2 by throwing it into the blood, from where it will be eliminated from the body during exhalation.

In this way, the cells exchange O2 for CO2 with the blood. As this happens, the concentration of the toxic gas is increasing in the blood, exceeding the concentration level of CO2 in the air. Thus, when the blood reaches the alveoli, it exchanges its CO2 for external O2, also causing a change in concentrations.