The adult trachea is about 1” (2.5 cm) in diameter and 4” (10 cm) long to the bifurcation, which is also called the carina. Anteriorly, the trachea begins just below the larynx and is located within the thoracic cavity, beneath the upper two-thirds of the sternum. Posteriorly, it begins at the level of the sixth cervical or first thoracic vertebra and extends to about the fifth thoracic vertebra.

The trachea divides into a right and left mainstem bronchus. The left mainstem bronchus leaves the trachea at a sharper angle than the right mainstem bronchus and passes under the aortic arch before entering the lung. The mainstem bronchi enter the lungs at the hila, where the lung tissue attaches to the mediastinum. The bronchi course downward and immediately divide into lobar bronchi, which subdivide into segmental bronchi.

The airways continue to systematically branch, narrow, shorten, and increase in number toward the lung periphery. They divide about 25 times before ultimately opening into the terminal, or respiratory, bronchioles.

The terminal bronchioles, which have a few alveoli budding from their walls, branch into alveolar ducts that are completely lined with alveoli. Known as the respiratory zone, this area is where gas exchange occurs. These airways and their alveoli form the respiratory lobules of the lungs. The terminal bronchioles account for the last six or seven airway divisions. The distance from the terminal bronchiole to the most distal alveolus is only about 5 mm; however, most of the lung surface area available for gas exchange is contained in these spaces.

The airways between the nose and the terminal bronchioles— called the conducting airways—lead inspired gas to the gas exchange regions of the lungs. About 150 cc of inspired air remains in the conducting airways during each breath. These airways, called anatomic dead space, aren’t involved in gas exchange.

The large airways below the glottis contain cartilage, smooth muscle, elastic fibers, connective tissue, and mucous glands. Anteriorly, the trachea and mainstem bronchi are composed of 20 rings of U-shaped cartilage. The cartilage that
opens posteriorly is connected by a band of smooth muscle and fibroelastic tissue. About 30% of the wall thickness in the mainstem and lobar bronchi is cartilage. This cartilage becomes irregular and platelike and eventually disappears in the smaller airways, which consist solely of smooth muscle and elastic fibers.

The mucous and bronchial glands located in the bronchial walls secrete a serous mucus blanket onto the inside airway walls. This secreted mucus forms a thin, sticky layer that lies atop ciliated columnar epithelial cells, which line nearly all of the respiratory tract between the larynx and the terminal bronchioles. The mucus layer traps foreign particles in the inspired air before they can enter the lungs. Cilia move in a wavelike motion, propelling the sticky mucus layer toward the pharynx where it can be coughed out or swallowed. Together, the mucus layer and cilia protect the lungs, acting as a major defense mechanism against inhaled particles.

Each lung is divided into lobes. The right lung has three lobes: the upper, middle, and lower. The left lung has only an upper and a lower lobe. The lung’s apex (superior aspect) is located near the clavicle, and its base (inferior aspect) is located near the diaphragm. (See Lung lobes, page 110.)