7.1 Air enters the exterior nares and passes into the nasal cavity, and on through the internal nares into the pharynx (Applegate, 2000, 308-312). It is filtered, warmed and moistened as it passes through the nasal cavity. From the pharynx it passes over the soft palate and uvula, down the oropharynx, over the epiglottis into the laryngopharynx and on into the trachea. The air flows down to the trachea, which divides at the level of the fifth thoracic vertebra into the right and left primary bronchi, and enters the lungs. The air flows down the bronchi, which then further divide into smaller and smaller passages to form the bronchial tree. The primary bronchi divide into secondary (lobar) bronchi, which branch into tertiary (segmental) bronchi. Branching continues until the bronchial tree gives rise to the bronchioles. The air flows through the bronchial trees to the bronchioles.
The air follows the bronchioles as they get smaller and smaller, to the terminal bronchioles, and then the respiratory bronchioles, which lead into the alveolar ducts which terminate in clusters of minute air sacs - the alveoli (Applegate, 2000, 300-312). Here the air exchanges its oxygen for carbon dioxide across the alveolar squamous epithelium, and across the walls of the capillaries in the alveolar ducts and respiratory bronchioles. The alveoli, alveolar ducts, and respiratory bronchioles are structures in the respiratory zone because this is where gaseous exchange takes place. The larger ducts (terminal bronchioles, bronchioles, bronchi) are conducting structures only, and are not involved in gaseous exchange.
7.2 The role of elastic tissue in the lungs is to accommodate expansion and contraction of the lungs on inspiration and exhalation. As air is drawn into the lungs, they expand considerably in size, and this requires elasticity in the tissue. As air is exhaled, the lungs are reduced in size and elastic tissue helps them ret...