Anatomy of the Respiratory System
I Boundaries and Functions of the Upper Airway
A Boundaries: From the anterior nares to the true vocal cords.
1. Heating or cooling inspired gases to body temperature (37° C).
3. Humidifying inspired gases to a relative humidity of approximately 100% at body temperature.
4. Olfaction: Act of smelling.
1. The nose is a rigid structure of cartilage and bone; the superior one third is made up of the nasal and maxilla bones, and the inferior two thirds is made up of five large pieces of cartilage (Figure 4-1).
2. The two external openings are called the nostrils, external nares, or anterior nares. Their lateral borders are termed the alae.
3. The nasal cavity is divided into two nasal fossae by the septal cartilage.
4. Each nasal fossa is divided into three regions: vestibular, olfactory, and respiratory.
a. Vestibular region: An area of slight dilation inside the nostril, bordered laterally by the alae and medially by the nasal septal cartilage.
(1) The vestibular region is lined with stratified squamous epithelium (Table 4-1). Layers of relatively flat cells continuously are being produced, replacing those lost at the surface.
TABLE 4-1
Anatomical Comparison of Epithelium in Upper Respiratory Tract
| Structure | Epithelium |
| Vestibular region/nose | Stratified squamous |
| Olfactory region/nose | Pseudostratified columnar |
| Respiratory region/nose | Pseudostratified ciliated columnar |
| Paranasal sinuses | Pseudostratified ciliated columnar |
| Nasopharynx | Pseudostratified ciliated columnar |
| Oropharynx | Stratified squamous |
| Laryngopharynx | Stratified squamous |
| Larynx/above true cords | Stratified squamous |
(2) Coarse nasal hairs (vibrissae) project anteriorly and inferiorly.
(3) Sebaceous glands secrete sebum, a greasy substance that keeps the nasal hairs soft and pliable.
(4) The nasal hairs are the first line of defense for the upper airway, acting as very gross filters of inspired air.
b. Olfactory region: An area in each nasal cavity defined by the superior concha laterally, nasal septal cartilage medially, and roof of the nasal cavity superiorly.
(1) Contained is the olfactory epithelium responsible for the sense of smell.
(2) The olfactory epithelium is yellowish brown and appears as pseudostratified columnar epithelial cells. These cells are interspersed with more deeply placed olfactory cells, whose sensory filament, the olfactory hairs, protrudes to the epithelial surface. Similar in structure to the ciliated cells lining the respiratory region but without the cilia (Figure 4-2).
(3) Largely because of the architecture of the nasal cavity, sniffing causes inspired gases to be drawn to the olfactory region and not much farther into the respiratory tract. This provides a protective mechanism for sampling potentially noxious environmental gases.
c. Respiratory region: An area in each nasal cavity inferior to the olfactory region and posterior to the vestibular region. The respiratory region comprises most of the surface area of the nasal fossa.
(1) Contained in the respiratory region of each nasal fossa are three bony plates called turbinates or conchae. The turbinates extend in a medial and inferior direction from the lateral walls of the nasal fossa.
(2) The three turbinates or conchae (superior, middle, and inferior) overhang and define the three corresponding passageways through each nasal cavity, respectively, the superior, middle, and inferior meati (see Figure 4-1).
(3) Because of the arrangement of the turbinates and folded mucous membrane covering the turbinates in the nose, the respiratory region has a volume of approximately 20 ml and a remarkably large surface area of approximately 160 cm2.
(4) Turbulent flow is created through this region.
(5) Heating, humidifying, and filtering of inspired gases are accomplished by the turbulent flow through this region of the nose. The turbulent flow produces a greater probability that each gas molecule will come in contact with the large surface area of the vascular nasal mucous membrane. This large gas-to-nasal surface interface allows the following:
(a) The abundant underlying vasculature to heat inspired gases to body temperature.
(b) The moist nasal mucous membrane to give up 650 to 1000 ml of H2O per day in bringing inspired gases to a relative humidity of 80% on leaving the nose and entering the nasopharynx.
(c) Particles suspended in the inspired gas to contact the sticky mucous membrane, thus filtering out particles ≥5 μm by inertial impaction to an efficiency of approximately 100%.
(6) The epithelial lining of the respiratory region of the nasal cavity is pseudostratified ciliated columnar epithelium (see Figure 4-2).
(a) The cells are cylindrical and appear to be two cell layers thick because of the high lateral pressures compressing the cells. Actually, the epithelium is only one cell layer thick, with each columnar cell making contact with the basement membrane.
(b) Each columnar cell has 200 to 250 cilia on its surface. Each of the cilia contains two central and nine paired peripheral fibrils. It is the sliding interaction of these fibrils that is thought to cause the beating of the cilia.
(c) Goblet cells and submucosal glands are interspersed throughout the epithelium and, along with capillary seepage, are responsible for production of mucus (100 ml/day in health).
(d) Mucus exists in two layers:
(e) On the forward stroke, the cilia become rigid. Their tips touch the undersurface of the gel layer and propel it toward the oropharynx. On the backward stroke, the cilia become flaccid, fold on themselves, and slide entirely through the sol layer to their resting position without producing a retrograde motion of the gel layer.
(f) The cilia of a particular cell and adjacent cells beat in a coordinated and sequential fashion that produces a motion similar to a wave. This allows a unidirectional flow of mucus. The cilia beat approximately 1000 to 1500 times/min and move the mucous layer at a rate of 2 cm/min.
(g) Functions of the mucus and pseudostratified ciliated columnar epithelium (mucociliary blanket) are:
5. The nose is responsible for one half to two thirds of the total airway resistance during nasal breathing. Therefore, it is not surprising that mouth breathing predominates during stress (e.g., exercise or disease).
6. The nose ends with the outlet of the nasal cavity into the nasopharynx through the internal nares (posterior nares or choanae).
D The oral cavity (see Figure 4-1)
1. The oral cavity extends from the lips to the palatine folds, a double web on each side of the oral cavity where the palatine tonsils reside.
2. The oral cavity is separated from the nasal cavity by the palate. The anterior two thirds is the bony hard palate, and the posterior one third, without bone, is the soft palate.
3. The oral cavity is considered an accessory respiratory passage because air usually only flows through it during stress and exercise.
4. The tongue attached to the floor of the cavity is involved in the mechanical aspects of swallowing, taste, and phonation.
a. The posterior surface is supplied by a nerve ending producing the vagal gag reflex.
b. The lingual tonsils are located at the base of the tongue.
5. The mucosal surface provides modest humidification and warming of inspired air.
E Paranasal sinuses (see Figures 4-1 and 4-3)
1. Sinuses are cavities of air in the bones of the cranium.
2. The function of the sinuses is not clearly understood, but it may be twofold:
a. To give the voice resonance (prolongation and intensification of sound).
b. To lighten the head to some extent because the space occupied by the sinuses is filled with air rather than bone.
3. The sinuses are absent or rudimentary at birth and grow almost simultaneously with the development of the permanent teeth. Formation of the sinuses is responsible for the alteration in facial shape that occurs at this time.
4. All of the air sinuses are lined with pseudostratified ciliated columnar epithelium and produce mucus, which drains into the nasal meati.
5. If nasogastric tubes or nasotracheal intubation blocks sinus drainage, sinusitis and sinus infection often result.
6. Groups of paranasal sinuses: Frontal, maxillary, sphenoidal, and ethmoidal.
a. The frontal sinuses appear as paired sinuses medial to the orbits of the eye and superior to the roof of the nasal cavity between the external and internal surfaces of the frontal bone. They drain into the anterior portion of the middle meati (middle passageway [cavity] formed by the turbinates).
b. The maxillary sinuses appear as paired sinuses lateral to each nasal cavity and inferior to the orbits of the eye in the body of the maxilla. These sinuses, the largest of all the air sinuses, drain into the middle meati.
c. The sphenoidal sinuses appear as paired sinuses posterior and inferior to the roof of the nasal cavity and superior to the internal nares (choanae) in the body of the sphenoid bone. They drain into the superior meati.
d. The ethmoidal sinuses are paired sinuses that exist in three groups: anterior, medial, and posterior ethmoidal. They exist just lateral to the superior and middle conchae, medial to the orbits of the eyes, inferior to the frontal sinuses, and superior to the maxillary sinuses in the ethmoid bone. The ethmoidal sinuses drain into the superior and middle meati.
1. The pharynx is a hollow muscular structure lined with epithelium (Figure 4-4).
a. To produce the vowel sounds (phonation) by changing its shape.
b. To serve as a common passageway for ventilatory gases, food, and liquid.
3. The pharynx is approximately 5 in. long and extends from the internal nares (choanae) inferiorly to the esophagus.
4. Sections of the pharynx: Nasopharynx, oropharynx, and laryngopharynx.
a. The nasopharynx is located behind the nasal cavity and extends from the internal nares superiorly to the tip of the uvula inferiorly.
(1) The epithelium is continuous with the epithelium of the nasal cavity and is pseudostratified ciliated columnar epithelium.
(2) The eustachian or auditory tubes open into the nasopharynx on each of its lateral walls and communicate with the tympanic cavity or middle ear (see Figure 4-1).
(a) This allows equilibration of pressure on each side of the tympanic membrane (eardrum) with environmental pressure changes.
(b) Nasal intubation may block the eustachian tube openings and may cause otitis media (middle ear infection) or sinus infections.
(3) The pharyngeal tonsil or adenoid is located in the superior and posterior wall of the nasopharynx.
(a) The pharyngeal tonsil consists of a large concentration of lymphoid tissue comprising the superior portion of Waldeyer’s ring. This ring of lymphoid tissue surrounds and guards the entrance to the respiratory and gastrointestinal tracts.
(4) During the process of swallowing, the uvula and soft palate move in a posterior and superior direction to protect the nasopharynx and nasal cavity from the entrance of food, liquid, or both.
b. The oropharynx is located behind the oral or buccal cavity and extends from the tip of the uvula superiorly to the tip of the epiglottis inferiorly.
(1) The epithelial lining is stratified squamous epithelium.
(2) The palatine tonsils are located lateral to the uvula on the lateral and anterior aspects of the oropharynx.
(3) The lingual tonsil is located at the base of the tongue, superior and anterior to the vallecula (the space between the epiglottis and base of the tongue).
(4) The two palatine tonsils, one lingual and one pharyngeal (adenoid), are the major components of Waldeyer’s ring.
c. The laryngopharynx or hypopharynx extends superiorly from the tip of the epiglottis to a point inferiorly where it bifurcates into the larynx and esophagus.
(1) The epithelial lining is stratified squamous epithelium.
(2) Major functions of the laryngopharynx:
(3) The laryngopharynx leads anteriorly into the larynx and posteriorly into the esophagus.
(4) The larynx is considered the connection between the upper and lower airways, the exact division being the true vocal cords.
(5) The larynx is considered part of the upper and lower respiratory tract. A complete description is given below.
II Boundaries and Functions of the Lower Airway
A Boundaries: From the true vocal cords to the terminal air spaces (alveoli).
1. Ventilation: To and fro movement of gas (gas conduction).
2. External respiration: Actual gas exchange between body (pulmonary capillary blood) and external environment (alveolar gas).
C The larynx (Figures 4-5 and 4-5)
1. The larynx is a boxlike structure made of cartilage connected by extrinsic and intrinsic muscles and ligaments. It is lined internally by a mucous membrane.
3. The larynx extends from the third to sixth cervical vertebrae in the anterior portion of the neck.
4. Unpaired cartilages of the larynx: Epiglottis, thyroid, and cricoid (see Figure 4-5).
(1) Leaf-shaped piece of fibrocartilage.
(2) Anteriorly attached to thyroid cartilage just inferior to the thyroid notch.
(3) Laterally attached to folds of mucous membrane called aryepiglottic folds.
(4) On swallowing, the epiglottis is squeezed between the base of the tongue and thyroid cartilage, causing the epiglottis to pivot in a posterior and inferior direction to cover the laryngeal inlet.
(1) The largest laryngeal cartilage.
(2) The anterior aspect is called the laryngeal prominence or Adam’s apple.
(3) Directly superior to the laryngeal prominence is the thyroid notch.
(4) The posterior and lateral aspects of this cartilage have two superior and two inferior projections: the superior and inferior cornua.
(1) Shaped like a signet ring, the smallest opening in the neonatal airway.
(2) Forms the entire inferior aspect and most of the posterior aspect of the larynx.
(3) There are articulating surfaces for the inferior cornu of the thyroid cartilage on the posterolateral surface.
(4) There are articulating surfaces for the paired arytenoid cartilages on the posterosuperior surface.
(5) Lies inferior to the thyroid and superior to the trachea, to which it attaches.
(6) Lies anterior to the esophagus; therefore, external cricoid pressure may facilitate viewing of the glottis during tracheal intubation and may prevent reflux from the stomach by compressing the esophagus.
5. Paired cartilages of the larynx: Arytenoid, corniculate, and cuneiform (see Figure 4-6).
(1) Shaped like upright pyramids.
(2) The base of each cartilage articulates with the posterosuperior surface of the cricoid cartilage.
(3) Each arytenoid cartilage has a ventral-medial projection from its base called the vocal process, to which the vocal ligaments attach.
(4) Arytenoid cartilages, along with the cricoid cartilage, make up the entire posterior surface of the larynx.
(1) Shaped like cones and are the smallest cartilages of the larynx.
(2) Articulate with the arytenoid cartilages on their superior surface, to which the corniculate cartilages are sometimes fused.
(3) When the larynx is viewed from above, the corniculate cartilages appear as two small elevations on the posteromedial aspect of the laryngeal inlet.
(1) Shaped like small, elongated clubs.
(2) Located lateral and anterior to the corniculate cartilages.
(3) When the larynx is viewed from above, the cuneiform cartilages appear as two small elevations just lateral and anterior to the corniculate cartilages.
(4) Housed in the aryepiglottic folds.
(5) The cuneiforms, along with the aryepiglottic folds, form the lateral aspect of the laryngeal inlet. The epiglottis forms the anterior aspect, and the corniculates form the posterior aspect of the laryngeal inlet.
6. Extrinsic ligaments of the larynx
a. Extrinsic ligaments attach cartilages of the larynx to structures outside the larynx.
b. The thyrohyoid membrane is a broad fibroelastic sheet that attaches the anterior and lateral superior aspects of the thyroid cartilage to the inferior surface of the hyoid bone (the posterior portion of the thyroid cartilage is attached to the hyoid bone by the superior cornu of the thyroid cartilage).
c. The hyoepiglottic ligament is an elastic band that attaches the anterior surface of the epiglottis to the hyoid bone.
d. The cricotracheal ligament connects the lower portion of the cricoid cartilage to the trachea by a broad fibrous membrane.
7. Intrinsic ligaments of the larynx
a. Intrinsic ligaments attach cartilages of the larynx to one another.
b. The thyroepiglottic ligament attaches the inferior aspect of the epiglottis to the thyroid cartilage on its internal surface below the thyroid notch.
c. The aryepiglottic ligament attaches the arytenoid cartilages to the epiglottis and acts as a point of attachment for the aryepiglottic folds.
d. The cricothyroid ligament attaches the anterior portion of the thyroid cartilage to the anterior portion of the cricoid cartilage. It is through this ligament that an emergency cricothyroidotomy is performed.
e. The vocal ligament is a thick band that stretches from the vocal process of the arytenoid cartilages across the cavity of the larynx to attach to the thyroid cartilage just inferior to the thyroepiglottic ligament. The lateral borders of the vocal ligament attach to the inverted free borders of the cricothyroid ligament.
f. The ventricular ligament is a thick band that stretches from the arytenoid cartilage across the cavity of the larynx to the thyroid cartilage. It exists superior and lateral to the vocal ligament.
