Table 1.

Characteristics of the upper and lower airway

Airway	Function	Cell types	Diameter (mm)
Nasal/Turbinates	Humidification, temperature regulation, filtration, mucociliary clearance	Pseudostratified columnar ciliary epithelia	10
Sinus	Pneumatized airway lumen in the skull, immunologic defense, patterened mucociliary clearance to ostial openings	Pseudostratified columnar ciliary epithelia	Ostial diameter = 2
Olfactory	Specialized neuro-epithelia to detect odor	Olfactory cells, supporting cells, basal cells, brush cells
Trachea	Cartilaginous tube that connects the upper airway to the pulmonary system	Pseudostratified columnar ciliary epithelia	18
Bronchi	Division of the trachea that conducts air in to the lungs	Pseudostratified columnar ciliary epithelia	5–12
Bronchioles	Division of the bronchi that conducts air to the alveoli	Pseudostratified columnar ciliary epithelia, Clara cells	0.5–3.5
Alveoli	Site of gas exchange, terminal division of respiratory tree	Type 1 and 2 alveolar cells	0.4