Humans actually have four nostrils — two exterior nostrils, or nares, and two internal nostrils, called choanae, which are part of the posterior nasal aperture. Choanae connect the nose to the throat and help with breathing. Rarely, humans or animals can be born with choanal atresia, or blocked choanae, and require surgery to be able to breathe properly.
More facts about nostrils and choanae:
- Choanae each contain about 1,000 nasal hairs.
- Fish don't have choanae, but they do have two pairs of exterior nostrils — one set near the jaw, and one set near the eyes.
- Several species of aquatic birds, such as pelicans, have very narrow, slit-like nostrils. This keeps them from getting water up their noses when they dive.