Upper respiratory tract. Computer graphic of a side view of a wire-frame head, showing the upper respiratory tract. Air passages (yellow) lead into the nasal cavity and oral (mouth) cavity, and then join at the throat to form the pharynx. Facial sinuses (white, green, pink, blue) are air-filled cavities in the bones surrounding the nose. From the upper respiratory tract, air is inhaled into, and exhaled out of, the body. At the base of the pharynx (not visible) is a division into the oesophagus of the gut, and the trachea where air passes down into the lungs. In the upper respira- tory tract, air is also warmed and smelt.