As gas flows in and out of the nasopharynx, the pressure in that region fluctuates. It drops below or rises above atmospheric pressure, which is itself not constant but is subject to changes in altitude and weather. Such pressure changes in the nasopharynx produce a pumping of gas into and out of the middle ear. The net amount of middle ear gas transferred from or to the nasopharynx will, component for component, in steady state exactly equal the amount of middle ear gas transferred to or from the microcirculation by means of diffusional absorption by (or release from) the mucosa. In the case of a permanently patulous eustachian tube, a single parameter, characteristic of the rate of ventilation through the open eustachian tube, is found to determine the gas composition in the middle ear, whereas in the case of a middle ear ventilated by tympanostomy, two rate-of-ventilation parameters, one for gas flow through the ventilation tube and one for flow through a periodically open eustachian tube, determine the steady state gas composition. A high rate of ventilation favors absorption of oxygen and venting of carbon dioxide from the middle ear in both cases.