26.7 : Alveoli and Alveolar Ducts

The respiratory zone of the human body, which stands in contrast to the conducting zone, comprises the structures that actively participate in the exchange of gases. The initiation of this zone is marked by the terminal bronchioles converging into respiratory bronchioles, the tiniest bronchiole classification. The respiratory bronchioles give way to the alveolar ducts that opens into a congregation of alveoli. Actively involved in gas exchange, alveoli resemble tiny sacs similar to clusters of grapes, all intricately connected to alveolar ducts.

In addition, an alveolar sac is simply a congregation of several individual alveoli. The diameter of a single alveolus measures around 200 μm, and its elastic nature allows it to expand during inhalation. Neighboring alveoli are interlinked via alveolar pores, which are small openings that ensure uniform air pressure across the alveoli and lungs.

The alveolar wall comprises three dominant cell types: type I, type II, and alveolar macrophages. Type I alveolar cells, also known as type I pneumocytes, are squamous epithelial cells that form up to 97 percent of the alveolar surface, with a thickness of about 25 nm and a high gas permeability. Dispersed among the type I cells are type II alveolar cells (also known as type II pneumocytes or septal cells) that secrete pulmonary surfactant, a phospholipid and protein concoction that decreases the surface tension of water, preventing alveolar collapse. Alveolar macrophages, immune system phagocytic cells, patrol the alveolar wall to remove debris or pathogens that find their way to the alveoli.

Attached to the extraordinarily thin epithelium formed by type I alveolar cells is an equally thin, elastic basement membrane. This membrane forms the border of the endothelial membrane of the capillaries. The combination of the alveolar and capillary membranes results in a respiratory membrane roughly 0.5 μm (micrometers) in thickness. This respiratory membrane is where the actual gas exchange takes place. It facilitates the simple diffusion of gases, allowing oxygen to be absorbed by the blood for transportation and simultaneously releasing CO₂ into the alveolar air.