The sinus, or nasal cavity, serves to lighten the skull, to produce mucus, to warm and moisturize air breathed in through the nose and to serve as a chamber in which speech resonates. The sinus cavity is made up of four pairs of sinuses.
Of the four sinuses, the frontal sinuses are located just above each eye, the maxillary sinuses are behind each cheekbone, the sphenoid sinuses are located near the optic nerves and the ethmoid sinuses are a collection of voids that open up into the other sinus cavities.
The sinuses serve as a partially closed system for heat and moisture. When air passes in through the nose, the humidity and warmth of the sinus cavity warms the air to within 1 degree of body temperature and moisturizes it to prevent a shock to the lungs. The air passes down into the lungs where it provides oxygen to the body. When the air is exhaled, the sinuses trap some of the moisture and warmth on the way out, ensuring the cavities remain at the optimal temperature for treating the next incoming breath.
Short thick hairs in the nasal vestibule trap dust and other intrusive particles before they can make their way into the throat or lungs. These intruders are passed into the esophagus, swallowed and digested instead.