They do other things too!
A FIRM TITS is the unfortunate acronym.
Acid-base: controling PaCO2
Filtration: of gas and blood
Infection control: by secreting IgA and the mucociliary escalator
Reservoir: of blood (500mL), O2 (\(2.2L \cdot 0.21\text{FiO2} \approx 500mL\)
Metabolism: ACE acts on ATI and bradykinin, \(\alpha 1\text{-antitrypsin}\) acts on neutrophil elastase
Thermoregulation (and humidification, water retention): by the warming of turbulent gas during inspiration
Inhaled route (for drugs)
Talking / phonation
Surfactant production
Isothermic saturation boundary: point where gas hits body temperature and 100% relative humidity. Usually at secondary bronchi.
During inspiration, cool, dry gas flows past warm musosa. Fluid evaporates from the mucosa into the dry gas. At the same time, heat is transferred by convection from musosa to gas, cooling the mucosa.
During expiration, warm, humid air flows past the cooled mucosa. Gas condenses on the cooler mucosal surface, and heat is transferred from gas back to mucosa. The warmer the environment, the less the gas can be cooled as it is exhaled, and the more fluid is lost.
This process relies on turbulent flow, which is promoted by the nasal turbinates.