The nose and air passages produce a slippery liquid called mucus , which helps to warm and moisten the air and allows it to travel the airways more easily. Mucus also helps to trap dirt and dust particles and some bacteria. Tiny hairs called cilia , located on the inner walls of the air passages, slowly move the mucus up toward the throat and nose, carrying with it any foreign particles. If particles irritate the nose, a sneeze may occur. If the particles are located deeper in the air passages a hearty cough will move them out.
Much more mucus is produced when the body is fighting a cold.
There are special epithelial cells that line the larger bronchi. Each cell has about 250 cilia growing from it. Goblet cells located between the epithelial cells produce almost half a cup of mucus a day. When the mucus gets into the upper airways, it can be coughed out. A cough is triggered when mucus or foreign matter irritates nerve endings in the larynx, trachea, or larger bronchi.