ABSTRACT
The conducting airways consist of the trachea, bronchi (contain cartilage), and bronchioles (lack cartilage). The composition of the conducting airways includes an epithelial lining, basement membrane (basal lamina; 80-90 nm, rich in laminin and type-IV collagen), subepithelial connective tissue (submucosa or lamina propria), smooth muscle, and adventitia (Kuhn, 1988). The epithelium of the conducting airways varies from a pseudo-stratified columnar type, mainly consisting of three cell types (ciliated, basal, and secretory cells), interconnected by tight junctions in the proximal bronchi, to a progressively more cuboidal, non-ciliated epithelium in the distal bronchioles. The thickness of the epithelium decreases from approximately 60 µm in the trachea to 10 µm in the bronchioles. The total surface area in humans averages at about 2.5m2, and during breathing the airways contain merely 4 per cent of the total volume of air inhaled.
