The term alveolus derives from the Latin alveolus → small cavity.
In spite of the small size, the pulmonary alveoli have a very important function: the exchange of respiratory gases between the blood and the atmosphere.
Most pulmonary alveoli gather in groups located at the end of each respiratory bronchiolo. Through the latter they receive atmospheric air from the upper contiguous tracts of the airways (terminal bronchioles, bronchioles, tertiary, secondary and primary bronchi, trachea, larynx, pharynx, nasopharynx and nasal cavities).
Along the wall of the respiratory bronchioles hemispherical extroflexions, called pulmonary alveoli, begin to be recognized.
The respiratory bronchioles preserve the branched structure of the bronchial tree, increasing the number of alveoli hosted as they originate lower caliber ducts.
The pulmonary alveoli appear as small air chambers of spherical or hexagonal dimension, with an average diameter of 250-300 micrometers in the phase of maximum insufflation. The primary role of the alveoli is to enrich the blood with oxygen and to clean it of carbon dioxide. The high density of these alveoli characterizes the spongy morphological aspect of the lung; moreover, it significantly increases the gaseous exchange surface, which on the whole reaches 70 - 140 square meters in relation to sex, age, height and physical training (we are talking about an area equal to an apartment with two rooms or a court tennis).
The wall of the alveoli is very thin and consists of a single layer of epithelial cells. Unlike bronchols, the thin alveolar walls are devoid of muscle tissue (because it would hinder gas exchange). Despite the impossibility of contracting, the abundant presence of elastic fibers gives the alveoli a certain ease to the extension, during the inspiratory process, and to the elastic return during the expiratory phase.
The region between two adjacent alveoli is known as the interalveolar septum and consists of alveolar epithelium (with its 1st and 2nd type cells), alveolar capillaries and often a layer of connective tissue. The intralveolar septa reinforce the alveolar ducts and somehow stabilize them.
The pulmonary alveoli can be connected to other adjacent alveoli through very small holes, known as pores of Khor. The physiological significance of these pores is probably that of balancing the air pressure within the lung segments.
Structure of the alveoli
Each pulmonary alveolus consists of a single, thin layer of exchange epithelium, in which two types of epithelial cells are known, called pneumocytes:
- Squamous alveolar cells, also known as type I cells or respiratory epitheliocytes;
- Type II cells, also known as septal cells or surfactant cells;
Most of the alveolar epithelium is formed by type I cells, which are arranged to form a continuous cellular layer. The morphology of these cells is very particular, because they are very thin and have a small swelling at the nucleus, where the various organelles are piled up.
The alveolar epithelium is also composed of type II cells, scattered singly or in groups of 2-3 units among the type I cells. The septal cells possess two main functions. The first is to secrete a liquid rich in phospholipids and proteins, called surfactant; the second is to repair the alveolar epithelium when it is seriously damaged.
The surfactant liquid, continually secreted by the septal cells, is able to prevent excessive distension and collapse of the alveoli. Furthermore, it helps to make the gas exchange between the alveolar air and the blood easier.
Without the production of surfactant by type II cells, serious respiratory problems would develop, such as total or partial collapse of the lung (atelectassia). This condition can also be determined by other factors, such as a trauma (pneumothorax), a pleurisy or a chronic obstructive pulmonary disease (COPD).
Type II alveolar cells seem to contribute to minimizing the volume of liquid present in the alveoli, conveying water and solutes outside the air spaces.
The presence of immune cells is recorded in the pulmonary alveoli. In particular, alveolar macrophages are responsible for eliminating all those potentially harmful substances, such as atmospheric dust, bacteria and polluting particles. Not surprisingly, these monocyte derivatives are known as dust or dust cells.
Blood circulation
Each pulmonary alveolus has a high vascularization, guaranteed by numerous capillaries. Inside the pulmonary alveoli, the blood is separated from the air by a very thin membrane.
The oxygen-rich blood from the pulmonary veins reaches the left ventricle of the heart. Then, thanks to the activity of the myocardium, it is pushed in all parts of our body. The blood to "clean up", instead, starts from the right ventricle and through the pulmonary arteries reaches the lungs. It should be noted, therefore, that in the pulmonary blood circulation the veins carry oxygenated blood while the arteries carry the venous blood, the exact opposite of what was seen for the systemic circulation.
In a resting person, the amount of oxygen exchanged between the alveolar air and the blood is around 250-300 ml per minute, while the quantity of carbon dioxide diffused from the blood to the alveolar air is about 200-250 ml . These values can increase about 20 times during an intense sporting activity.
