Functions of respiratory tract :

1. The cartilages in the trachea keep the respiratory tract extended, allowing for the unobstructed passage of air between the outside atmosphere and the alveoli of the lungs.

2. The mucus lining keeps the respiratory passage moistened and traps particles as dust in the air preventing them from reaching the alveoli.

3. The diameter of the respiratory passages may be altered by muscles in their walls, thus regulating the volume of air entering the lungs.

4. Cells in connective tissue protect against infection and inhaled foreign particles not trapped by mucus. Lymphocytes and plasma cells produce antibodies in the presence of antigens, and macrophages and polymorphonuclear leukocytes are phagocytic. 

5. The respiratory tracts maintains the temperature of the air.