The Eustachian tube (or auditory tube) is a tube that links the pharynx to the middle ear. In adults the Eustachian tube is approximately 35 mm long. It is named after the sixteenth century anatomist Eustachius. Some modern medical books call this the pharyngotympanic tube.
In the equids (horses) and some rodent-like species such as the desert hyrax, an evagination of the eustachian tube is known as the guttural pouch and is divided into medial and lateral compartments by the stylohyoid bone of the hyoid apparatus. This is of great importance in equine medicine as the pouches are prone to infections, and due to their intimate relationship to the cranial nerves (VII,IX,X,XI) and the internal and external carotid artery, various syndromes may arise relating to which is damaged. Epistaxis (nosebleed) is a very common presentation to veterinary surgeons and this may often be fatal unless a balloon catheter can be placed in time to suppress bleeding.
Normally the Eustachian tube is closed, but it can open to let a small amount of air through to equalize the pressure between the middle ear and the atmosphere. When this happens we hear a small pop, an event familiar to aircraft passengers or drivers in mountainous regions. Yawning or swallowing can pull on muscles in the neck, causing the tube to open. Without this airway, air would be unable to escape from one's ear, the middle ear would be isolated from the atmosphere, and could be easily damaged by pressure changes.
Some people are born with the ability to contract just these muscles voluntarily, similar to people who can wiggle their ears. Those who have this ability can hear "pop" or "click" sound in the middle ear when actuating these muscles, and are able to hold the muscle contraction (some refer to this as 'clicking your ears to equalize the pressure'). Doing so will make one's voice sound louder to oneself. This ability allows such people to voluntarily equalize pressures at will when making rapid ascents or descents, typically in aircraft flights or large elevation changes in either tall buildings or mountainous treks. When the breath (inhale or exhale) is controlled, air pressure can be intentionally increased or decreased in the middle ear (breathing through the nose only or mouth), where the feeling of a cool air breeze can be felt inside the eustachian tube.
Occasionally, if the voluntary contraction timing is missed during a rapid pressure change, a slight yawning (opening of the jaw) action combines to assist in pressure equalization.
See also: Valsalva maneuver
Some people suffer from the rare patulous Eustachian tube condition, in which the Eustachian tube remains intermittently open, causing an echoing sound of their own heartbeat, breathing and speech, which may be temporarily relieved by flipping the head upside down.
Smoking can also cause damage to the cilia that protect the Eustachian tube from mucus which can result in the clogging of the tube and a buildup of bacteria in the ear leading to an inner ear infection in some cases.
Eustachian tube dysfunction could be caused by recurring and chronic cases of sinus infection. This results from the excessive mucus production which causes obstruction to the openings of the eustachian tubes.