External nares, or nostrils, perform the basic function of being the passageway through which oxygen enters the body. Nostrils have many different parts within them that perform their own individual functions.
Nostrils of mammals and birds have cartilages called turbinates. These turbinates warm the oxygen brought in through the nostrils to help the lungs function more efficiently and humidify the air to prevent dryness of the lungs.
In between the nostrils is the septum. It consists of four bones and cartilage and serves the functions of separating the right and left nasal cavities, providing support to the nose and regulating airflow into the nasal passages.
At the top of the septum, humans have two additional, internal nares. These are called posterior nares, or choanae. Posterior nares contain approximately 1,000 strands of hair, called cilia. The cilia serve the important function of filtering out dust and other particles before they reach the lungs. This helps to protect the nasal passage as well as other portions of the respiratory tract.
Beyond the cilia, in the extreme rear of the nasal cavity, lies the olfactory epithelium. The epithelium is what make the nose capable of smelling. It consists of receptor cells that send signals to the brain when hit they are hit by a molecule of a substance. The signal is then read by the brain, which decodes it using memory to determine various scents.