Upper Airway Anatomy and Gas Flow

1. Oropharynx and Nasopharynx

   1. The air passages extending from the nares and lips through the nasopharynx and oropharynx, through the larynx to the cricoid cartilage make up the functional upper airway. The upper airway serves a host of functions: warming and humidifying passing air, filtering particulate matter, and preventing aspiration.

   2. The upper airway mucosa is highly vascular and well innervated (must be appreciated when performing nasopharyngeal intubation with endotracheal tubes, nasogastric sumps or feeding tubes, or fiberoptic bronchoscopes).

   3. The pharynx is 12- to 15-cm long and is divided into the nasopharynx, the oropharynx, and the laryngopharynx (lying posterior to the larynx). The supine position, sleep, and general anesthesia may promote obstruction of the oropharynx by the tongue, soft palate, and pharyngeal musculature as their tone decreases.

2. Larynx

   1. The larynx is a complex structure that lies anterior to the 4th to the 6th cervical vertebrae and consists of several muscles, their ligaments, and associated cartilaginous structures (Fig. 24-1).

   2. The larynx serves as the organ of phonation, plays an important role in coughing, and in airway protection from aspiration.

   3. The paired vocal cords attach posteriorly to the vocal process of each arytenoid and anteriorly meet at the junction of the thyroepiglottic ligament of the anterior portion of the thyroid cartilage. The triangular opening formed by the vocal ligaments is the glottis with its apex anteriorly (Fig. 24-2).

Pharyngeal Innervation. Innervation of the pharynx is supplied via sensory and motor branches of the glossopharyngeal nerve (CN IX) and vagus nerve (CN X) (external and internal branches of the superior laryngeal nerves, recurrent laryngeal nerves).

Tracheal and Bronchial Structure

1. The trachea originates at the cricoid cartilage (at the level of vertebra C6) and extends approximately 10 to 12 cm (females) and 12 to 14 cm (males) to terminate in a bifurcation (carina) at the T4/5 vertebral level (2nd intercostal space, the angle of Louis) (Fig. 24-3).

2. The right main bronchus is wider (16 vs. 13 mm), shorter (1.5 to 2.5 vs. 4.5 to 5 cm) and more vertical than the left (Fig. 24-4).

Respiratory Airways and Alveoli. The airways continue to divide into smaller diameter conduits until one arrives at the bronchioles with diameters less than 0.8 mm. The immune defenses of the lung are extremely important because of the direct exposure of this organ to the external environment via the airways. Exaggerated inflammatory response and the activity of these cells and others may be harmful to the lung; acute respiratory distress syndrome (ARDS) and emphysema are examples.