The upper part of the respiratory system includes the nasal passages, the pharynx (throat), the larynx (voice box), and the main airway, which is called the trachea or windpipe. The two lungs and the lower airways (the bronchi and the smaller bronchioles) are encased in a bony cage formed by the ribs. The airways terminate in millions of balloon-like sacs called alveoli. The alveoli provide a vast surface area for gas exchange with the large numbers of tiny blood vessels that surround them.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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