In human beings, a nostril, also called a naris, the plural of which is nares, is an opening at either end of the pair of channels leading from the exterior of the nose into the head where they join the airway that leads to the pharynx and the lungs. Each end of these channels, known as nasal passages, has a pair of nostrils, so humans actually have a total of four of these openings, two external and two internal. This anatomical arrangement is duplicated in many types of animals as well.
While it is often mistakenly believed that a nostril is the passageway through the nose, this is incorrect. In fact, a nostril is simply an opening at either end of the nasal cavity. They allow air to move through the nasal cavity, in either direction, during breathing. In some fishes, nostrils may be present, but are part of the olfactory system only and not used for breathing.
The two pairs of nostrils are called the posterior and anterior nares. The anterior nares are the external openings of the nose and are the openings most commonly referred to as nostrils. The posterior nares are the internal pair of openings leading from the back of the nasal cavity to the pharynx and then to the lungs.
The nostrils, besides allowing the entry of air into the nasal cavity and eventually to the lungs, also serve another purpose. They allow air to pass over olfactory sensors in the nasal cavity, which are the main part of the anatomy that produce a sense of smell. The presence of nostrils in pairs can, in some cases, allow an individual to determine the direction from which a scent emanates, through a differentiation of the relative intensity of the scent as perceived in each nostril.
Small hairs in the nasal cavity and the posterior nares act as filters for tiny particles in the air and prevent foreign material from entering the lungs. Each posterior nostril can have approximately 1,000 of these hairs. More of these nasal hairs are distributed throughout the nasal cavity.