The physiology of mucus and sputum production in the respiratory system.

The main function of the respiratory system is to draw air into the lungs to allow the exchange of gases with blood circulating to the lungs. This blood supplies the cells of the body with oxygen and removes the waste products of metabolism. Tissues of the respiratory tract are thin and delicate, and become thinnest at the surfaces of the aveoli, where gaseous exchange occurs. The body has a number of mechanisms which protect these tissues and ensure that debris and bacteria do not reach them.