1. Wall: The wall of alveolus is very thin. 0.2 micrometers thick. It is composed of squamous cells. There are two types of cells in the wall. Type-1 cells and type-2 cells. It contains collagen and elastin fibers. Collagen is elastic and facilitates contraction-expansion. The capillary network arises from the pulmonary (pulmonary) arteries. Capillary networks extend into the walls of the alveoli and later join to form the pulmonary veins. The walls of the alveoli contain phagocytic macrophages. Macrophages destroy microorganisms and foreign particles.
2. Chamber: Alveolus is a special bubble-like air cavity. It stores O2 and releases CO2. The volume of alveoli increases or decreases according to the amount of gas.
3. Surfactant: Type-2 cells in the alveolar wall contain detergent-like chemicals. These chemicals are called surfactants. Detergents are chemicals like phospholipids. It reduces the surfactant tension of the alveolar wall. Prevents alveolus from collapsing. Contracts and expands the lungs with less effort. Facilitates rapid exchange of CO2 and O2. Destroys germs (bacteria). Surfactant secretion begins in the 23-week-old human fetus.