Functional anatomy of the upper airway

Anatomically, the upper airway consists of the pharynx and nasal cavities. However, functionally, the larynx and trachea may be included, and the oral cavity provides an alternate entrance to the respiratory passages. The nose is a pyramidal structure composed of bone and cartilage attached to the facial skeleton, and is divided by a midline septum into the two nasal cavities. The nose functions as a heater and humidifier of inspired gas, a voice resonator, and houses the olfactory receptors. The paranasal sinuses drain into the nasal cavities. An endotracheal tube may be passed through the nose into the trachea when necessary to protect the airway and achieve positive-pressure ventilation. The mouth opens posteriorly into the oropharynx and forms the entrance to the digestive tract as well as an alternate pathway for respiration. It is also involved in phonation. Orotracheal intubation can be used as an alternative to nasal intubation to achieve airway protection and ventilation when necessary; however, variations in upper airway anatomy may make this technique difficult. In supine unconscious persons, backward movement of the tongue and lower jaw may cause airway obstruction. The pharynx is a U-shaped fibromuscular tube extending from the base of the skull to the cricoid cartilage at the entrance to the esophagus. Anteriorly it opens into the nasal cavity, the mouth, and the larynx, which divide it into the naso-, oro-, and laryngopharynx, respectively. The pharynx thus forms a common aerodigestive tract and is intimately involved with the act of swallowing. The larynx consists of a framework of cartilages and fibroelastic membranes covered by a sheet of muscles and lined with mucous membrane. It evolved as a protective valve mechanism at the upper end of the lower airway necessitated by an unusual crossover between the airway and alimentary canal. It functions as an open valve in respiration, a partially closed valve in phonation, and as a closed valve protecting against aspiration during swallowing. The larynx extends from its oblique entrance formed by the aryepiglottic folds, the tip of the epiglottis, and the posterior commissure to the lower border of the cricoid cartilage and bulges posteriorly into the laryngopharynx. The trachea extends from the lower edge of the cricoid cartilage to the carina where it divides into the mainstem bronchi. It is formed by U-shaped cartilaginous rings anteriorly and is closed posteriorly by the trachealis muscle. A properly placed endotracheal tube should have its tip at about midtracheal level.(ABSTRACT TRUNCATED AT 400 WORDS)