The internal nares serve as a filter for the respiration system, keeping particles from making it into the trachea and the lungs. In addition to cleaning out the air, the internal nares also provide warmth and moisture to the air as it makes its way to the lungs.
Each of the internal nares is lined with mucus and small, fine cilia. As air passes through, the mucus and cilia grab many of the particles out of the air, transporting them to the esophagus, so that people swallow those particles instead of inhaling them.
Capillaries line the walls of the internal nares, pumping blood through the layer of mucus. When air comes through the internal nares, heat goes from the blood in the capillaries to the air. The purpose of warming the air is to keep the lungs from experiencing temperature shock and to help the body maintain its core temperature.
Lungs require moisture to function, and when air passes through the internal nares, it gathers up moisture to keep the linings of the lungs from drying out. When exhalation takes place, and the air goes back through the internal nares, moisture condenses as the air goes by, taking some of the moisture out before the air leaves the nose.