Air cell

The lung alveoli are small sac-like structures filled with air. They are found at the end of the smallest airways in the lungs, the bronchioles. Each alveolus measures 200 microns in diameter and is bounded by a wall made up of very thin cells called pneumocytes. Gaseous exchange between inspired air and blood takes place through the wall of the alveoli. The two lungs of an adult human have more than 500 million alveoli, if fully stretched they would occupy an area of 80 square meters.

Description

A. Pulmonary alveoli.
AS. Interalveolar partition.
DA. Ductus alveolaris (alveolar pipeline).
M. Muscle.
D. Mucous gland.
BR. Respiratory bronchial.
N. Nerve.
PA. Pulmonary artery branch.
PV. Pulmonary vein branch.
BT. Terminal bronchiole

Alveoli are microscopic sac-shaped structures surrounded by very thin cell walls.
Gas exchange between blood and inspired air occurs through the alveolar wall, in such a way that carbon dioxide (CO₂) leaves the blood and is expelled to the outside and oxygen (O₂) enters the blood to be transported to different tissues throughout the body.

Inspired air reaches the lungs through the bronchi, which are subdivided into smaller and smaller ducts that eventually give rise to the respiratory bronchioles, each of which branches into several alveolar ducts. Various rounded, diverticulum-shaped structures called alveoli originate from the alveolar ducts; through its wall the exchange of gases between the blood in the capillaries and the inspired air is carried out. In some alveoli there are pores that communicate with the lumen of the adjacent alveolus, called Kohn's pores. Each alveolus is separated from the next by the interalveolar septa, which contain elastic and collagen fibers.

Alveolar cells

Reveal it in 3D

The wall of the alveolus is made up of cells called pneumocytes, which can be of two types: I and II. In the intraalveolar space there are also macrophages that engulf harmful particles that enter through the respiratory tract.

• Type I Pneumocytes: Type I alveolar cells (AEI in English) occupy 95% of the surface of the alveole. They are flat cells that constitute a very thin monoestratified flat epithelium. The core of the cell makes protusion towards alveolar light. It contains few orgánulos and is joined by narrow unions with neighboring pneumocytes.
• Type II Pneumocytes: Type II alveolar cells (AEII in English) are cubic cells with apical microvellosities, abundant roaring endoplasmic reticulous and Golgi apparatus. They occupy only 5% of the alveolo surface. They do not intervene in gaseous exchange, but facilitate the strain and recovery of the size of the alveoli by the synthesis and secretion of the pulmonary surfactant, a surfactant that reduces surface tension. The surfactant is inside the cytoplasm in the lamelar bodies and is secreted by exocytosis in the light of the alveoli, where it forms a liquid film that covers the surface of the epithelium.
• Macrophages. They can be found inside the alveolos (alveolar matrophages) or in the intersection. The macrophages capture and fagocite particles that accidentally arrive inside the alveole, for example dust, pollen, bacteria or tar residues from the smoke and detrites of the alveolo itself.

Smokers have a higher number of macrophages than non-smokers, since they are a defense method for the body to try to eliminate some of the toxic substances contained in tobacco smoke.

Cross section of an alveoli with capillaries. Part of the cross section is expanded to show the diffusion of oxygen gas and carbon dioxide through type I neumocytes and capillary cells.

Section of a pulmonary alveoli. Type I and II tyres are visible.

Alveolar-capillary barrier

Gas exchange occurs through the wall that delimits the alveolus. Said alveolar-capillary barrier is 0.5 μm thick and is made up of the following structures:

1. Pulmonary surfer, on the alveolar surface.
2. Type I pneumocytes of the alveolar epithelium.
3. Pneumocyte basal lamina.
4. Basal lamina of the capillary endothelium, often merged with the basal foil of the pneumocyte.
5. Endothelial cell of the capillary wall.

Scheme of a section of the lung seen in the microscope that shows the lung alveoli and capillaries.

Gas exchange

Interstitial space

The interstitial space lies between the basal membrane of the pneumocytes and the basal membrane of the endothelium of the blood capillaries, forming the interalveolar septa. It is made up of connective tissue and contains different types of cells, including macrophages and fibroblasts, having a matrix made up of collagen fibers and elastic fibers.