The other end is in the nasopharynx, the place behind the nose where the nose and throat join. That's the very same place where you get inflamed tissues and mucus when you have a cold. As a result, the Eustachian tube gets congested when you have a cold and doesn't allow air into the middle ear freely. This results is the stuffed ears and popping most people experience with colds. The adenoids, which are lymphoid tissue just like the tonsils, are also in the nasopharynx.
Large adenoids can press on the Eustachian tubes interfering with their proper function.
And adenoids tend to get larger when they become inflamed during upper respiratory infections. Since one end of the Eustachian tube is in the nasopharynx, and swallowing opens the Eustachian tube, one thing to keep in mind is that infants who drink milk flat on their backs are at higher risk for developing middle ear infections because milk may get into the entrance of the Eustachian tube and cause it to become inflamed. For little children a congested Eustachian tube is a big problem. A child's Eustachian tube is smaller than an adult's, and, because of differences in facial shape and structure, a child's Eustachian tube is much more prone to obstruction than an adult's. When the Eustachian tube doesn't allow air into the middle ear for a long time, since the porous bone surrounding the middle ear absorbs air from the middle ear, a vacuum develops.