ALVEOLI

Function: terminal air spaces were gas exchange takes place.

Composition: thin-walled polyhedral chambers that are surrounded and separated from one another by a thin connective tissue layer with numerous blood capillaries. Each alveolus is confluent with a respiratory bronchiole, by means of:

Alveolar ducts -elongate airways that have almost no walls, only alveoli, as their peripheral boundary with rings of smooth muscle in the knob-like interalveolar septa
Alveolar sacs - spaces surrounded by clusters of alveoli

ALVEOLAR EPITHELIUM

Composition:

Type I alveolar cells (pneumocytes) - extremely thin squamous cells that line about 95% of the alveolar surface area
Type II alveolar cells (pneumocytes) or septal cells - cuboidal cells cover about 5% of the alveolar air surface

Function: release of lamellar bodies into the alveolar space. They are rich in phospholipids among which is the surface active agent surfactant. It forms a monomolecular layer over the alveolar epithelium

Occasional brush cells

Function: receptors that monitor air quality in the lung

Basement membrane

ALVEOLAR SEPTUM

Location: the tissue between adjacent alveolar air spaces. The openings in the septa - alveolar pores of Kohn, allow circulation of air from one alveolus to another

Function: site of air-blood barrier

Composition:

Alveolar epithelial cells
Basement membrane of the alveolar epithelium
Basement membrane of the capillary endothelium
Endothelial cells of the rich capillary network
Other connective tissue elements

ALVEOLAR MACROPHAGES

Location: in connective tissue of the septum and in the air space of the alveoli

Function: remove inhaled particulate matter from the air spaces and red blood cells from the septum