How does the respiratory tract humidify air?
The respiratory tract is coated with mucus to seal the tissues from direct contact with the air. Water is a prime constituent of the mucus. As air crosses these surfaces of the mucous membranes it picks up water. The lungs present a large, wet surface to the air. The sinus cavities have a volume of space for warming incoming air, and they present another modestly sized wet surface to air passing through them. Same with the much smaller nasal cavities. And the back of the throat and the trachea.
