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What Are Sinuses?

Diagram of sinusesSinuses, also called paranasal sinuses, are air-filled spaces within the facial bones surrounding the nose. All the sinuses are connected through hollow cavities in the skull—known as sinus cavities.

The paranasal sinuses include a maxillary sinus in each cheek, six to 12 ethmoid sinuses on each side of the nose between the eyes, a sphenoid sinus close to the center of the head behind the ethmoid sinuses, and a frontal sinus on each side in the forehead.

The ethmoid, frontal and maxillary sinuses surround the eye. All the sinuses are in the vicinity of the nose, hence “para.”

Each sinus has an opening—called the ostium—that connects it to the nose.

The paranasal sinuses have several theorized purposes. They lighten the skull and allow our voices to resonate better. Functionally, they produce mucus, which moisturizes the inside of the nose. This layer of mucus protects the nose and sinuses from dust, dirt, pollutants and micro-organisms (bacteria and viruses). In normally-functioning sinuses, the layer of mucus is swept slowly out of each sinus into the nose via the ostium and backward into the throat, where it is swallowed.

The Nose
The nose is divided into two cavities, with the nasal septum, ideally in the midline, separating them. Inside the nose are turbinates, rounded projections that run the length of the nasal cavity along the side of the nose.

There are three turbinates on each side, called the superior, middle and inferior (upper, middle and lower) turbinates. The area between each turbinate is called a meatus, which means opening or passage. The ethmoid, maxillary and frontal sinuses drain via their ostia into the middle meatus. The function of the turbinates is to increase the surface area inside the nasal cavity to heat, humidify and filter the air for passage into the lungs.

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