What is the main function of the Eustachian tube?

The main function of the Eustachian tube is to equalize pressure within the middle ear. This tube connects the middle ear to the back of the throat and plays a crucial role in maintaining the balance of air pressure on both sides of the eardrum. When there is a difference in pressure, such as during changes in altitude or when you have a cold, the Eustachian tube opens to allow air to flow into or out of the middle ear, helping to normalize the pressure. This equalization is important for proper hearing, as significant pressure differences can cause discomfort and impair the functioning of the eardrum.

In contrast, while the other options mention important functions of different structures in the ear or respiratory system, they do not pertain to the Eustachian tube. For instance, the drainage of fluid from the inner ear is typically managed by other mechanisms and structures, and sound vibrations are transmitted by the bones of the middle ear, not the Eustachian tube. Filtering air entering the nasal cavity is a function related to the nasal passages and sinuses, further emphasizing the specific role the Eustachian tube plays in pressure regulation.