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.