Nose

The nose is mainly an air-conditioning and filtering device. Although the resistance to airflow through the nose is greater than resistance through the mouth, most adults breathe through the nose at times of rest. High nasal resistance from swollen mucous membranes and rapid breathing from exercise usually cause people to switch to mouth breathing. Specialized structures in the nose increase its airflow resistance, but these structures are necessary for the nose to accomplish its filtering, warming, and humidifying functions.

The cartilaginous anterior portion of the nasal septum divides the nasal cavity into two channels called nasal fossae. The vomer and ethmoid bones form the posterior septum (Figure 1-2). The two nasal fossae lead posteriorly into a common chamber (the nasopharynx) through openings called choanae. The nasal septum is often deflected to one side or the other, more often to the left than the right,¹ possibly making the passage of a catheter or artificial airway through this side difficult. Three downward-sloping, scroll-shaped bones called conchae project from the lateral walls of the nasal cavity toward the nasal septum. The conchae create three irregular passages—the superior meatus, middle meatus, and inferior meatus (Figure 1-3). Because they create turbulence, the conchae are also called turbinates. The convoluted design of the turbinates greatly increases the surface area of the nasal cavity. The maxillary bone forms the anterior three fourths of the nasal cavity floor, called the hard palate (see Figure 1-3). Cartilaginous structures form the posterior fourth, called the soft palate. Palatal muscles close the posterior openings of the nasal cavities during swallowing or coughing, isolating the nasal cavities from the oral cavity.

Squamous, nonciliated epithelium lines the anterior third of the nose; pseudostratified, ciliated columnar epithelium interspersed with many mucus-secreting glands covers the posterior two thirds, including the turbinates. This mucus-secreting epithelium is called the respiratory mucosa. Immediately under the mucosa is an extensive capillary network adjoining a system of still deeper, high-capacity vessels. These deep vessels can dilate or constrict and change the volume of blood that flows into the capillaries, altering the mucosa’s thickness. The capillaries have tiny openings or fenestrations that allow water transport to the epithelial surface. These fenestrations are not present in the capillaries of lower airways. Countercurrent blood flow and connections between arterial and venous vessels (arteriovenous anastomoses) improve the ability of the nasal mucosa to adjust the temperature and water content of inspired air. Warm arteriolar blood flows parallel with but in the opposite (countercurrent) direction of cooler blood flowing in the venules, lessening the mucosa’s heat and water-vapor loss. Arteriovenous anastomoses and countercurrent blood flow are not present in airways below the larynx.²

The main functions of the nose are the humidification, heating, and filtering of inspired air. As inspired air passes over the richly vascular epithelial surface (made larger by the presence of the turbinates), its temperature and water content increase rapidly. The turbinates disrupt the incoming airstream and create swirling, chaotic flow, increasing the chances that tiny airborne particles will collide with and adhere to the sticky mucous layer covering the nasal epithelium. Nasal secretions contain immunoglobulins and inflammatory cells, which are the first defense against inspired microorganisms. The nose is so efficient as a filter that most particles larger than 5 µm in diameter do not gain entry to the lower airways.³

By the time inspired air reaches the nasopharynx, it gains considerable water vapor and heat. Exhaled air cools as it leaves the nose, causing water vapor to condense on its structures, which humidifies the subsequent inspired air.

The process of intubation involves the insertion of an artificial airway or endotracheal tube through the nose or mouth and into the trachea (Figure 1-4), which means the air-conditioning function of the nose is lost, and unmodified cool, dry gas directly enters the trachea. This places a heavy burden on the tracheal mucosa, which is not designed to accommodate cool, dry gases.

Connected to the nasal cavities are several empty airspaces within the skull and facial bones called paranasal sinuses. These sinuses are lined with a mucus-secreting epithelium, continuous with the epithelium of the nasal cavities. Mucus from the sinuses drains into the nasal cavity through openings located beneath the conchae. The sinuses are symmetrically paired and located in the frontal, ethmoid, sphenoid, and maxillary bones. Inflammation and infection may swell the membranes lining the sinuses, impairing drainage and increasing sinus cavity pressure. Chronic sinus infections provide a source of bacteria-laden secretions that are sometimes aspirated into the lower respiratory tract, potentially causing lower respiratory infections.

Pharynx

The pharynx is the space behind the nasal cavities that extends down to the larynx (see Figure 1-1). The term pharynx stems from the Greek word meaning “throat.” The nasopharynx is the portion behind the nasal cavities that extends down to the soft palate. The oropharynx, the space behind the oral cavity, is bounded by the soft palate above and the base of the tongue below. The laryngopharynx is the space below the base of the tongue and above the larynx.

As inspired gas abruptly changes its direction of flow at the posterior nasopharynx, inhaled foreign particles collide with and adhere to the sticky mucous membrane. Lymphatic tissues in the nasopharynx and oropharynx provide an immunological defense against infectious agents. These tissues include the pharyngeal (adenoid), palatine, and lingual tonsils (see Figure 1-3). These tissues may become inflamed and swollen and may interfere with nasal breathing especially in children owing to their smaller airways; chronic inflammation of the tonsils may warrant surgical removal.

The eustachian tubes, also called auditory tubes, connect the middle ear with the nasopharynx (see Figure 1-3). These tubes allow pressure equalization between the middle ear and atmosphere. Inflammation and excessive mucus production in the nasopharynx may block the eustachian tubes and hinder the pressure-equalizing process; this can momentarily impair hearing and cause pain, especially during abrupt changes in atmospheric pressure. Children younger than 3 years of age are especially susceptible to this condition because their eustachian tubes are small and easily occluded; artificial pressure-equalizing tubes, also known as myringotomy tubes, are sometimes placed through the ear’s tympanic membrane (eardrum) to create an alternate route for pressure equalization.

The oropharynx and laryngopharynx accommodate food and air and are lined with nonciliated, stratified squamous epithelium. The laryngopharynx, also called the hypopharynx, separates the digestive and respiratory tracts. Proper function of sensory and motor nerves innervating the pharyngeal musculature is crucial in preventing food and liquid aspiration into the respiratory tract. The pharyngeal reflex has its sensory component in the ninth cranial (glossopharyngeal) nerve and its motor component in the tenth cranial (vagus) nerve. This reflex arc is responsible for the gag and swallowing reflexes.

Deeply unconscious persons sometimes lose the pharyngeal and laryngeal reflexes and aspirate foreign material into their lungs. In such individuals, an artificial airway (endotracheal tube) with an inflatable cuff may be inserted orally or nasally through the larynx and into the trachea. After it is in place, the cuff is inflated to form a seal between the tracheal wall and tube to minimize aspiration of pharyngeal contents (see Figure 1-4). However, even if the cuff is properly inflated, pharyngeal secretions eventually migrate past the cuff into the lower airway, For this reason, mechanically ventilated patients, in whom endotracheal intubation is required, are susceptible to the development of lung infections, or ventilator-associated pneumonia (VAP); the longer the duration of mechanical ventilation, the greater the risk of VAP. Normal pharyngeal muscle tone prevents the base of the tongue from falling back and occluding the laryngopharynx, even in a person who is supine and asleep. Deep unconsciousness may relax pharyngeal muscles enough to allow the base of the tongue to rest against the posterior wall of the pharynx, occluding the upper airway; this is called soft tissue obstruction and is the most common threat to upper airway patency. If the head droops forward, the oral cavity and pharynx-larynx axis form an acute angle that may partially or completely obstruct the upper airway (Figure 1-5). Partial upper airway obstruction produces a low-pitched snoring sound as inspired air vibrates the base of the tongue against the posterior wall of the pharynx. Complete obstruction causes strong inspiratory efforts without sound or air movement. Soft tissues between the ribs and above the sternum may be sucked inward (intercostal and suprasternal retractions) as the person struggles to inhale.

Both forms of soft tissue upper airway obstruction can be easily removed by extending the neck and pulling the chin anteriorly (see Figure 1-5, C). This maneuver pulls the tongue forward out of the airway and aligns the oral and nasal cavities with the pharynx-larynx axis. This is sometimes called the sniffing position.

Pharyngeal anatomy plays a role in the incidence of obstructive sleep apnea (OSA),⁴ as do pharyngeal reflexes and muscle tone. The normal pharynx narrows during sleep, greatly increasing upper airway resistance. Abnormal enlargement of soft tissues can further narrow or occlude the airway, and repetitive cessations of breathing (apnea) may occur during sleep.

Larynx

The larynx is a cartilaginous, cylindrical structure that acts as a valve on top of the trachea. The larynx is sometimes called the voice box because it contains the vocal cords that control the size of the opening into the trachea (glottis [rima glottidis]). The main cartilage of the larynx in the middle of the neck is the thyroid cartilage, sometimes called the Adam’s apple, which is easily palpable in men.

The larynx lies at the level of the fourth through sixth cervical vertebrae in men and is located higher in women and children. The top portion of the larynx is a complex triangular box that is flat posteriorly and composed of an intricate network of cartilages, ligaments, and muscle (Figure 1-6). A mucous membrane continuous with the mucous membrane of the pharynx and trachea lines the interior of the larynx. Nine cartilages (three paired and three unpaired) and many muscles and ligaments form the larynx. The unpaired epiglottis is a thin, flat, leaf-shaped cartilage above the glottis. The lower end of the epiglottis (a long, narrow stem) is attached to the thyroid cartilage. From this attachment, it slants upward and posteriorly to the base of the tongue, where its upper free end is broad and rounded (see Figures 1-3 and 1-6). A vascular mucous membrane covers the epiglottis. The lower base of the tongue is attached to the upper epiglottis by folds of mucous membrane, forming a small space (the vallecula) between the epiglottis and tongue (Figure 1-7). The vallecula is an important landmark used during the insertion of a tube into the trachea (intubation).

Besides speech, the major function of the larynx is preventing the lower airway from aspirating solids and liquids during swallowing and breathing. The epiglottis does not seal the airway during swallowing.⁵ Instead, the upward movement of the larynx toward the base of the tongue pushes the epiglottis downward, which causes it to divert food away from the glottis and into the esophagus. The free portion of the upper epiglottis in an adult lies at the base of the tongue, but in a newborn it lies much higher, behind the soft palate. This position of the upper epiglottis helps

to account for preferential nose breathing in newborns and why it is difficult to achieve effective deposition of inhaled aerosolized medications in the lower airways of a newborn.⁶

The vascular mucous membrane covering the epiglottis may become swollen and enlarged if inflamed by infectious agents or mechanical trauma. A swollen epiglottis may severely obstruct or occlude the laryngeal air passage of a small newborn. Inflammation of the epiglottis (epiglottitis) is a life-threatening emergency in infants and requires immediate placement of an artificial airway by skilled medical personnel.

The thyroid cartilage is the largest of all laryngeal cartilages, enclosing the main cavity of the larynx anteriorly (see Figure 1-6). The lower epiglottis attaches just below the notch on its inside upper anterior surface.

The cricoid cartilage, just below the thyroid, is the only complete ring of cartilage that encircles the airway in the larynx or trachea. The cricothyroid ligament connects the cricoid and thyroid cartilages (see Figure 1-6). The cricoid limits the endotracheal tube size that can pass through the larynx. The cricoid ring is the narrowest portion of the upper airway in an infant. Inside the larynx, the vocal cords lie just above the cricoid cartilage.

The membranous space between the thyroid and cricoid cartilages, the cricothyroid membrane (see Figure 1-6), is sometimes the puncture site for an emergency airway opening when structures above it are occluded. A longer term surgical opening into the airway (tracheostomy) is generally located 1 to 3 cm below the cricoid cartilage.

The remaining cartilages (arytenoid, corniculate, and cuneiform) are paired. These cartilages are in the lumen of the larynx and serve as attachments for ligaments and muscles (see Figure 1-6). The arytenoids are attachment points for vocal ligaments that stretch across the lumen of the larynx and attach to the thyroid cartilage.

The vocal folds consist of two pairs of membranes that protrude into the lumen (inner cavity) of the larynx from the lateral walls (see Figures 1-3 and 1-7). The upper pair is called the false vocal cords; the lower pair is called the true vocal cords because only these folds play a part in vocalization. The true vocal folds form a triangular opening between them that leads into the trachea below. This opening is called the rima glottidis, or glottis (see Figure 1-7).

The vocal cords’ ability to open and close the airway is essential for generating and releasing high pressure in the lung during a cough (an extremely important lung defense mechanism). Artificial airways such as endotracheal and tracheostomy tubes render a cough ineffective because they prevent the vocal cords from sealing the airway.

The vocal cords are wider apart during quiet inspiration than expiration. During deep inspiration, the vocal cords offer little airflow resistance. The glottis is the narrowest part of the adult larynx. Swelling (edema) of the vocal cords increases resistance to airflow, especially in an infant. Glottic and subglottic edema secondary to viral infections, also known as croup, is a common cause of upper airway obstruction in infants and young children. During inspiration, croup causes a characteristic high-pitched crowing sound called stridor. This sound is created by high-velocity air flowing through a narrowed glottis.

The laryngeal mucous membrane is composed of stratified squamous epithelium above the vocal cords and pseudostratified columnar epithelium continuous with the mucosa of the trachea below the vocal cords. Branches of the tenth cranial (vagus) nerve provide motor innervation for all intrinsic muscles of the larynx through the recurrent laryngeal nerve.