Abstract
Respiratory activity (gas metabolism) of the middle ear cavity (including the mastoid cells) plays an important role in ventilation by opening and closing the eustachian tube in a normobaric environment. Electron microscopy shows that the submucosal capillaries of the human mastoid cells are structures which facilitate the intra and extravascular transport of substances, suggesting that these capillaries are involved in gas metabolism. The PO2 (53.7±6.5mmHg: 22 ears) of the human tympanic cavity is lower than the atmospheric O2 (150mmHg) pressure. This phenomenon suggests that a low PO2 in the middle ear cavity is useful in preventing infection, and that the gas metabolic function of the mastoid cells, with their characteristic submucosal capillary structure and their larger volume than that of the tympanic cavity, plays an important role in maintaining the ventilatory function of the middle ear cavity. The results indicate that tubal ventilation in a normobaric environment keeps the internal pressure of the middle ear cavity similar to or slightly higher than the ambient pressure by the production of gas tension, which inhibits gas flow from the pharynx to the middle ear cavity.
