<P> The lungs which are located in the thoracic cavity, are protected from physical damage by the rib cage . At the base of the lungs is a sheet of skeletal muscle called the diaphragm . The diaphragm separates the lungs from the stomach and intestines . The diaphragm is also the main muscle of respiration involved in breathing, and is controlled by the sympathetic nervous system . </P> <P> The lungs are encased in a serous membrane that folds in on itself to form the pleurae--a two - layered protective barrier . The inner visceral pleura covers the surface of the lungs, and the outer parietal pleura is attached to the inner surface of the thoracic cavity . The pleurae enclose a cavity called the pleural cavity that contains pleural fluid . This fluid is used to decrease the amount of friction that lungs experience during breathing . </P> <P> The respiratory tract is divided into the upper airways and lower airways . The upper airways or upper respiratory tract includes the nose and nasal passages, paranasal sinuses, the pharynx, and the portion of the larynx above the vocal folds (cords). The lower airways or lower respiratory tract includes the portion of the larynx below the vocal folds, trachea, bronchi and bronchioles . The lungs can be included in the lower respiratory tract or as separate entity and include the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli . </P> <P> The respiratory tract can also be divided into a conducting zone and a respiratory zone, based on the distinction of transporting gases or exchanging them . </P>

What is the difference between upper and lower respiratory tract
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