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THE HUMAN RESPIRATORY SYSTEM The Upper Respiratory Tract Air enters the body either by the or by the . In the latter case, it moves through nasal passages coated with that have fine hair-like and lined with cells projecting into the airways. The mucous traps , , and other foreign materials. The constant motion of the cilia then carries this mucous away from the lungs toward the pharynx, where it is . (Sneezing and coughing can also expel some of this foreign material from the body.) The air is and as it goes through the nasal passages. This conditioning helps protect the delicate tissues of the lungs and the ciliated cells. After leaving the nasal passages or the mouth cavity, the air passes into the pharynx which connects with two passages, the tissue, the and the . A flap of , prevents food from entering the trachea. The walls of the trachea are reinforced by rings of which hold it open at all times. Most of the air that is inspired (breathed in) enters the , the enlarged upper portion of the trachea. To get into the larynx, the air passes through the , a slit which can open or close to varying degrees. The larynx contains the formed from folds of mucous membrane. - thin strong fibers of tissue The Lower Respiratory Tract The trachea continues down into the or chest cavity. Like the nasal passages, its walls are lined with cells, some that have cilia and others that secrete mucous. At its lower end, the trachea divides into two . Each then divides into smaller and smaller . The very small bronchioles lack the supporting rings of cartilage present in the trachea. These small bronchioles finally lead to , each of which ends in a group of alveolar ducts and sacs called . The air passages perform two tasks. The bronchi and bronchioles conduct air in and out, and the alveoli are involved in the exchange of and between the air and the blood. ** Each lung is enveloped by a double layer of membrane, membrane. or