Alveoli

Derived from Latin word “alveus” (“a tray, trough, basin”), literally alveoli (singular: alveolus) means simply sacs.

Alveoli are tiny air sacs (only one cell thick) in the lungs at the end of smallest airways, where the oxygen and carbon dioxide exchange takes place. The average human has nearly 300-480 million in each lung or 700 million in a pair of lungs, providing total surface area of 100m2.

ANATOMY

These are simple air sacs, one cell thick, having a network of capillaries on them for extensive gas exchange.

CELLS PRESENT and FUNCTION

Alveoli are the ultimate organs for gaseous exchange for all oxygen requirements of our body. Oxygen come inside the air sacs where blood vessels network is present which exchange oxygen with that of carbon dioxide. These functions are facilitated by following type of cells

Type 1 also known as Squamous Alveolar cells that form structure outside the walls.
Type 2 also called as Great Alveolar cells. They secrete pulmonary surfactant by exocytosis. These cells secrete this pulmonary surfactant, a surface active lipoprotein complex, to lower the surface tension of water and allows the membrane to separate, and therefore increase capability to exchange gases. Re-inflation followed by exhalation is made easy by these type of cells.
Macrophages, they destroy any invading organisms such as bacteria.

CLINICAL POINTS

In Emphysema elastic tissue of lungs is broken down and total surface area decreases.
In Asthma, bronchioles with bottle like neck are constricted and hence air flow to lungs is greatly reduced.

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