A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals they contain branched bones or cartilages called turbinates whose function is to warm air on inhalation and remove moisture on exhalation. Fish do not breathe through their noses, but they do have two small holes used for smelling which may be called nostrils.

The Procellariiformes are distinguished from other birds by having tubular extensions of their nostrils.

In humans it is quite normal for just one nostril to be functioning at any one time. During the course of a day they will switch over approximately every four hours or so. This is known as the Nasal Cycle. Head and neck | Respiratory system

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