The maxillary ostium is narrower when the subject is recumbent than when sitting. The oxygen content in the sinus is related to the patency of the ostium and to some extent to its size. A 90% gas exchange in the sinus normally requires only 5 minutes. The exchange is faster during nasal than during oral breathing. The mucosa of the maxillary sinus has a relatively high blood flow. Oxygen absorption by the mucosa is normally perfusion limited. A considerable part of the absorbed oxygen is utilized directly by the mucosa and not taken up by the blood flow.