The middle ear is connected to the outer ear via the Eustachian tube. The Eustachian tube connects the middle ear with the pharynx, and through pharynx makes contact with the outside air. This means that the air pressure outside the body and the air pressure inside the middle ear are equal. Equal air pressure is very important for the normal function of the eardrum. the Eustachian tube opens whenever you make a jaw movement. Sometimes, when the pharynx is inflamed (usually because of infection), the Eustachian tube gets stacked due to the swelling of the surrounding tissue. Then, trapped air in the middle ear starts to be absorbed by the mucosa lining and air pressure drops. Because the Eustachian tube is obstructed, air can’t enter the middle ear to equalize the pressure. Decreased ear pressure in the middle ear causes a vacuum and the secretion of serous liquid in the middle ear from its mucosal lining. That’s why you feel pressure in your ear and decreased hearing on that ear. An Obstructed Eustachian tube due to inflammation is called a "catarrh" of the Eustachian tube.