The interior nasal cavity is divided into two major components by the nasal septum. These spaces are mostly lined by a complex mucosa, which is responsible for cleaning, warming and humidifying the inhaled air. Thus this part of the nose serves as an air conditioner for the upper airway. Only when nasal breathing is functioning normally can the mucosa optimally prepare the air for the lower airways.
In recent years it has become clear that the upper and lower airways must be regarded as a functional unit; this is important not only for diagnostic considerations but also influences therapeutic choices. Often diseases of the lower airway are caused by abnormalities of the nose and sinuses. This is especially true for allergic diseases of the airways.