Under normal conditions, this is prevented by the protection of the rib cage and the chest wall muscles and diaphragm. There, oxygen is absorbed and waste gases are expelled as the lungs retract or deflate to their original size. Within the breathing passages, air is further filtered and inhaled impurities are trapped by mucus a sticky fluid produced by tiny gland cells in the passage linings. Mucus also moistens the air so that the delicate tissues of the lungs will not be dried by the airflow. Cilia filter the mucus of these collected impurities.
Cilia are microscopic, hairlike projections growing out of the passageway walls.
Millions of cilia inhabit every square inch of the respiratory tract's mucous membranes. The cilia's coordinated movement transports the mucus from the bottom of the lungs toward the throat. Impurities are carried up and out of the peripheral parts of the lung to the larger central airways and the throat so that they can be swallowed, expelled from the mouth by coughing, or sneezed out of the nasal passages. Injured cilia cannot efficiently carry out this function, and certain substances including cigarette smoke and various airborne pollutants damage or slow down the cilia, making the respiratory tract more vulnerable to disease. Purified air reaches the innermost portion of the lungs through the bronchial passages.