Nasal cavity, larynx.
Trachea, bronchi, lungs
ORGANIZATION OF THE RESPIRATORY SYSTEM
We can divide the respiratory system (Figure 23-1) into an upper respiratory system and a lower respiratory system. The upper respiratory system consists of the nose, nasal cavity, paranasal sinuses, and pharynx. These passageways filter, warm, and humidify the incoming air--protecting the more delicate surfaces of the lower respiratory system--and cool and dehumidify outgoing air. The lower respiratory system includes the larynx (voice box), trachea (windpipe), bronchi, bronchioles, and alveoli of the lungs.
Your respiratory tract consists of the airways that carry air to and from the exchange surfaces of your lungs. The respiratory tract can be divided into a conducting portion and a respiratory portion. The conducting portion begins at the entrance to the nasal cavity and extends through the pharynx and larynx and along the trachea, bronchi, and bronchioles to the terminal bronchioles. The respiratory portion of the tract includes the delicate respiratory bronchioles and the sites of gas exchange, the alveoli.
Filtering, warming, and humidification of the inspired air begin at the entrance to the upper respiratory system and continue throughout the rest of the conducting system. By the time air reaches the alveoli, most foreign particles and pathogens have been removed, and the humidity and temperature are within acceptable limits. The success of this "conditioning process" is due primarily to the properties of the respiratory mucosa.
The Respiratory Mucosa
The respiratory mucosa lines the conducting portion of the respiratory system. A mucosa is a mucous membrane, one of the four types of membranes introduced in Chapter 4. It consists of an epithelium and an underlying layer of loose connective tissue.
A pseudostratified, ciliated, columnar epithelium with numerous goblet cells (Figure 23-2) lines the nasal cavity and the superior portion of the pharynx. The structure of the respiratory epithelium changes as you proceed along the respiratory tract. The epithelium lining inferior portions of the pharynx is a stratified squamous epithelium similar to that of the oral cavity. These portions of the pharynx, which conduct air to the lower respiratory tract, also convey food to the esophagus. The pharyngeal epithelium must therefore provide protection from abrasion and chemical attack.
At the beginning of the lower respiratory tract is a pseudostratified ciliated columnar epithelium comparable to that of the nasal cavity. In the smaller bronchioles, this pseudostratified epithelium is replaced by a cuboidal epithelium with scattered cilia. The exchange surfaces of the alveoli are lined by a very delicate simple squamous epithelium. Other, more specialized cells are scattered within the alveolar epithelium.
The Lamina Propria
The lamina propria is the underlying layer of loose connective tissue that supports the respiratory epithelium. In the upper respiratory system and in the trachea and bronchi, the lamina propria contains mucous glands that discharge their secretions onto the epithelial surface. The lamina propria in the conducting portions of the lower respiratory system contains bundles of smooth muscle cells. At the level of the bronchioles, the smooth muscles form relatively thick bands that encircle or spiral around the lumen.
The Respiratory Defense System
The delicate exchange surfaces of the respiratory system can be severely damaged if the inspired air becomes contaminated with debris or pathogens. Such contamination is prevented by a series of filtration mechanisms that together make up the respiratory defense system.
Along much of the length of the respiratory tract, goblet cells in the epithelium and mucous glands in the lamina propria produce a sticky mucus that bathes exposed surfaces. In the nasal cavity, cilia sweep that mucus and any trapped debris or microorganisms toward the pharynx, where it will be swallowed and exposed to the acids and enzymes of the stomach. In the lower respiratory system, the cilia also beat toward the pharynx, moving a carpet of mucus toward the pharynx and cleaning the respiratory surfaces. This process is often described as a mucus escalator (Figure 23-2b).
Filtration in the nasal cavity removes virtually all particles larger than about 10 µm from the inspired air. Smaller particles may be trapped by the mucus of the nasopharynx or secretions of the pharynx before proceeding farther along the conducting system. Exposure to unpleasant stimuli, such as noxious vapors, large quantities of dust and debris, allergens, or pathogens, generally causes a rapid increase in the rate of mucus production in the nasal cavity and paranasal sinuses. (The familiar symptoms of the "common cold" result from the invasion of this respiratory epithelium by any of more than 200 viruses.)
Most particles 1-5 µm in diameter are trapped in the mucus coating the respiratory bronchioles or in the liquid covering the alveolar surfaces. These areas are outside the boundaries of the mucus escalator, but the foreign particles can be engulfed by alveolar macrophages. Most particles smaller than about 0.5 µm remain suspended in the air.
Large quantities of airborne particles may overload the respiratory defenses and produce a variety of illnesses. For example, the presence of irritants in the lining of the conducting passageways can provoke the formation of abscesses that block airflow and reduce pulmonary function, and damage to the epithelium in the affected area may allow irritants to enter the surrounding tissues of the lung. The irritants then produce local inflammation, and there is a strong link between airborne irritants and the development of lung cancer.
THE UPPER RESPIRATORY SYSTEM
The nose is the primary passageway for air entering the respiratory system. Air normally enters the respiratory system through the paired external nares, or nostrils (Figure 23-3a), which open into the nasal cavity. The vestibule is the space contained within the flexible tissues of the nose (Figure 23-3c). The epithelium of the vestibule contains coarse hairs that extend across the external nares. Large airborne particles, such as sand, sawdust, or even insects, are trapped in these hairs and are thereby prevented from entering the nasal cavity.
The nasal septum divides the nasal cavity into left and right portions (Figure 23-3b). The bony portion of the nasal septum is formed by the fusion of the perpendicular plate of the ethmoid bone and the plate of the vomer (Figure 7-3d). The anterior portion of the nasal septum is formed of hyaline cartilage. This cartilaginous plate supports the bridge, or dorsum nasi, and apex (tip) of the nose.
The maxillary, nasal, frontal, ethmoid, and sphenoid bones form the lateral and superior walls of the nasal cavity. The mucous secretions produced in the associated paranasal sinuses, aided by the tears draining through the nasolacrimal ducts, help keep the surfaces of the nasal cavity moist and clean. The olfactory region, or superior portion of the nasal cavity, includes the areas lined by olfactory epithelium: (1) the inferior surface of the cribriform plate, (2) the superior portion of the nasal septum, and (3) the superior nasal conchae. Receptors in the olfactory epithelium provide your sense of smell.
The superior, middle, and inferior nasal conchae project toward the nasal septum from the lateral walls of the nasal cavity. To pass from the vestibule to the internal nares, air tends to flow between adjacent conchae, through the superior, middle, and inferior meatuses (meatus, a passage) (Figure 23-3b). These are narrow grooves rather than open passageways, and the incoming air bounces off the conchal surfaces and churns around like a stream flowing over rapids. This turbulence serves a purpose: As the air eddies and swirls, small airborne particles are likely to come into contact with the mucus that coats the lining of the nasal cavity. In addition to promoting filtration, the turbulence allows extra time for warming and humidifying the incoming air. It also creates eddy currents that bring olfactory stimuli to the olfactory receptors.
A bony hard palate, formed by portions of the maxillary and palatine bones, forms the floor of the nasal cavity and separates the oral and nasal cavities. A fleshy soft palate extends posterior to the hard palate, marking the boundary between the superior nasopharynx and the rest of the pharynx. The nasal cavity opens into the nasopharynx at the internal nares.
The Nasal Mucosa
The mucosa of the nasal cavity prepares the air you breathe for arrival at your lower respiratory system. Throughout much of the nasal cavity, the lamina propria contains an abundance of arteries, veins, and capillaries that bring nutrients and water to the secretory cells. The lamina propria of the nasal conchae also contains an extensive network of large and highly expandable veins. This extensive vascularization provides a mechanism for warming and humidifying the incoming air (as well as for cooling and dehumidifying the outgoing air). As cool, dry air passes inward over the exposed surfaces of the nasal cavity, the warm epithelium radiates heat and the water in the mucus evaporates. Air moving from your nasal cavity to your lungs has been heated almost to body temperature, and it is nearly saturated with water vapor. This mechanism protects more delicate respiratory surfaces from chilling or drying out—two potentially disastrous events. Breathing through your mouth eliminates much of the preliminary filtration, heating, and humidifying of the inspired air. To avoid alveolar damage, patients breathing on a respirator, which utilizes a tube to provide air directly into the trachea, must receive air that has been externally filtered and humidified.
As air moves out of the respiratory tract, it again passes across the epithelium of the nasal cavity. This air is warmer and more humid than the air that enters; it warms the nasal mucosa, and moisture condenses on the epithelial surfaces. Thus breathing through your nose also helps prevent heat loss and water loss to your environment.
The pharynx is a chamber shared by the digestive and respiratory systems. It extends between the internal nares and the entrances to the larynx and esophagus. The curving superior and posterior walls of the pharynx are closely bound to the axial skeleton, but the lateral walls are flexible and muscular.
The Larynx is situated in anterior neck area on level IV-VI cervical vertebrae. At the front infrahyoid muscles of neck cover it. Vessels and nervous bundles and lobes of thyroid gland lie from sides of larynx. Laryngeal part of pharynx adjoins behind it.
Larynx skeleton consists of pair and odd cartilages.
• Thyroid cartilage, which consists of right and left plates (lamina dextra et sinistra), and also has superior horns and inferior horns; the plates converge forming laryngeal prominence (Adam’s apple);
• Cricoid cartilage which has anteriorly arch behind - plate of cricoid cartilage;
• Epiglottis cartilage.
The cartilages of the larynx. Posterior view.
• Arytenoid cartilage, which has a base and apex, muscular process and vocal process. These cartilage lie on plate of cricoid cartilage;
• Corniculate cartilage lies in aryepiglottic fold on top of arytenoid cartilages;
• Cuneiform cartilage lies in aryepiglottic fold front of corniculate cartilages.
In larynx they distinguish such articulations:
• Cricoid-thyroid joint is between inferior cornu of thyroid cartilage and arch of cricoid cartilage; in this joint movement is possible around transversal axis;
• Cricoid-arytenoid joint is situated between base of arytenoid cartilages and plate of cricoid cartilage. Arytenoid cartilage can rotate slide to meet one another.
Ligaments of the larynx:
• Thyro-hyoid membrane, which hangs larynx to hyoid bone;
• Crico-thyroid ligament;
• Thyro-epiglottic ligament;
• Hyoepiglottic ligament;
• Vestibular ligaments, which are situated over vocal ligaments.
The ligaments of the larynx. Antero-lateral view.
Fibroelastic membrane the larynx:
· Elastic cone contains in its superior margin vocal ligament;
· Quadrangular membrane, which is situated over elastic cone and in its inferior margin contains vestibular ligament.
Fibroelastic membranes together with laryngeal cartilages form a laryngeal skeleton.
The laryngeal Muscles subdivide on muscles that narrow/broaden the glottis, muscles that change tension of vocal ligament.
Constrictors of the glottis:
· lateral cricoarytenoid muscle;
· thyroarytenoid muscle;
· transverse arytenoid muscle;
· oblique arytenoid muscles.
Muscles-dilators of the glottis
• thyro-arytenoid muscle has thyro-epiglottic part. Action: it raises the epiglottis and broadens an entrance into larynx and vestibule.
• posterior cricoid-arytenoid muscle.
Muscles changing tension of vocal ligament:
• crico-thyroid muscle stretches a vocal ligament.
• vocal muscle is situated in thickness of vocal fold and changes an tension degree of vocal cords.
Laryngeal cavity has aditus laryngis [entrance], vestibule, interventricular space, glottis and infraglottic cavity.
Larynx has true vocal folds and glottis. Larynx begins by entrance into larynx, which is limited at the front, by epiglottis, behind – by arytenoid cartilages, and laterally - by arytenoepiglottic folds, where cuneiform and corniculate tubercles are situated (places of the same name cartilages). Glottis is a most narrow place in laryngeal cavity; it is situated between right and left vocal plicae. Laryngeal ventricle is fissure disposed between vocal and vestibular plicae.
Infraglottic cavity is inferior broadened part of larynx, which continues into trachea.
The larynx or organ of voice is placed at the upper part of the air passage. It is situated between the trachea and the root of the tongue, at the upper and forepart of the neck, where it presents a considerable projection in the middle line. It forms the lower part of the anterior wall of the pharynx, and is covered behind by the mucous lining of that cavity; on either side of it lie the great vessels of the neck. Its vertical extent corresponds to the fourth, fifth, and sixth cervical vertebræ, but it is placed somewhat higher in the female and also during childhood. Symington found that in infants between six and twelve months of age the tip of the epiglottis was a little above the level of the fibrocartilage between the odontoid process and body of the axis, and that between infancy and adult life the larynx descends for a distance equal to two vertebral bodies and two intervertebral fibrocartilages. According to Sappey the average measurements of the adult larynx are as follows:
Until puberty the larynx of the male differs little in size from that of the female. In the female its increase after puberty is only slight; in the male it undergoes considerable increase; all the cartilages are enlarged and the thyroid cartilage becomes prominent in the middle line of the neck, while the length of the rima glottidis is nearly doubled.
The larynx is broad above, where it presents the form of a triangular box flattened behind and at the sides, and bounded in front by a prominent vertical ridge. Below, it is narrow and cylindrical. It is composed of cartilages, which are connected together by ligaments and moved by numerous muscles. It is lined by mucous membrane continuous above with that of the pharynx and below with that of the trachea.
The Cartilages of the Larynx (cartilagines laryngis) are nine in number, three single and three paired, as follows:
Two Cuneiform. Epiglottis
The Thyroid Cartilage (cartilago thyreoidea) is the largest cartilage of the larynx. It consists of two laminæ the anterior borders of which are fused with each other at an acute angle in the middle line of the neck, and form a subcutaneous projection named the laryngeal prominence (pomum Adami). This prominence is most distinct at its upper part, and is larger in the male than in the female. Immediately above it the laminæ are separated by a V-shaped notch, the superior thyroid notch. The laminæ are irregularly quadrilateral in shape, and their posterior angles are prolonged into processes termed the superior and inferior cornua.
The outer surface of each lamina presents an oblique line which runs downward and forward from the superior thyroid tubercle situated near the root of the superior cornu, to the inferior thyroid tubercle on the lower border. This line gives attachment to the Sternothyreoideus, Thyreohyoideus, and Constrictor pharyngis inferior.
The inner surface is smooth; above and behind, it is slightly concave and covered by mucous membrane. In front, in the angle formed by the junction of the laminæ, are attached the stem of the epiglottis, the ventricular and vocal ligaments, the Thyreoarytænoidei, Thyreoepiglottici and Vocales muscles, and the thyroepiglottic ligament.
The upper border is concave behind and convex in front; it gives attachment to the corresponding half of the hyothyroid membrane.
The lower border is concave behind, and nearly straight in front, the two parts being separated by the inferior thyroid tubercle. A small part of it in and near the middle line is connected to the cricoid cartilage by the middle cricothyroid ligament.
The posterior border, thick and rounded, receives the insertions of the Stylopharyngeus and Pharyngopalatinus. It ends above, in the superior cornu, and below, in the inferior cornu. The superior cornu is long and narrow, directed upward, backward, and medialward, and ends in a conical extremity, which gives attachment to the lateral hyothyroid ligament. The inferior cornu is short and thick; it is directed downward, with a slight inclination forward and medialward, and presents, on the medial side of its tip, a small oval articular facet for articulation with the side of the cricoid cartilage.
During infancy the laminæ of the thyroid cartilage are joined to each other by a narrow, lozenge-shaped strip, named the intrathyroid cartilage. This strip extends from the upper to the lower border of the cartilage in the middle line, and is distinguished from the laminæ by being more transparent and more flexible.
The Cricoid Cartilage (cartilago cricoidea) is smaller, but thicker and stronger than the thyroid, and forms the lower and posterior parts of the wall of the larynx. It consists of two parts: a posterior quadrate lamina, and a narrow anterior arch, one-fourth or one-fifth of the depth of the lamina.
The lamina (lamina cartilaginis cricoideæ; posterior
portion) is deep and broad, and measures from above downward about 2 or
The arch (arcus cartilaginis cricoideæ; anterior portion)
is narrow and convex, and measures vertically from 5 to
On either side, at the junction of the lamina with the arch, is a small round articular surface, for articulation with the inferior cornu of the thyroid cartilage.
The lower border of the cricoid cartilage is horizontal, and connected to the highest ring of the trachea by the cricotracheal ligament.
The upper border runs obliquely upward and backward, owing to the great depth of the lamina. It gives attachment, in front, to the middle cricothyroid ligament; at the side, to the conus elasticus and the Cricoarytænoidei laterales; behind, it presents, in the middle, a shallow notch, and on either side of this is a smooth, oval, convex surface, directed upward and lateralward, for articulation with the base of an arytenoid cartilage.
The inner surface of the cricoid cartilage is smooth, and lined by mucous membrane.
The Arytenoid Cartilages (cartilagines arytænoideæ) are two in number, and situated at the upper border of the lamina of the cricoid cartilage, at the back of the larynx. Each is pyramidal in form, and has three surfaces, a base, and an apex.
The posterior surface is a triangular, smooth, concave, and gives attachment to the Arytænoidei obliquus and transversus.
The antero-lateral surface is somewhat convex and rough. On it, near the apex of the cartilage, is a rounded elevation (colliculus) from which a ridge (crista arcuata) curves at first backward and then downward and forward to the vocal process. The lower part of this crest intervenes between two depressions or foveæ, an upper, triangular, and a lower oblong in shape; the latter gives attachment to the Vocalis muscle.
The medial surface is narrow, smooth, and flattened, covered by mucous membrane, and forms the lateral boundary of the intercartilaginous part of the rima glottidis.
The base of each cartilage is broad, and on it is a concave smooth surface, for articulation with the cricoid cartilage. Its lateral angle is short, rounded, and prominent; it projects backward and lateralward, and is termed the muscular process; it gives insertion to the Cricoarytænoideus posterior behind, and to the Cricoarytænoideus lateralis in front. Its anterior angle, also prominent, but more pointed, projects horizontally forward; it gives attachment to the vocal ligament, and is called the vocal process.
The apex of each cartilage is pointed, curved backward and medialward, and surmounted by a small conical, cartilaginous nodule, the corniculate cartilage.
The Corniculate Cartilages (cartilagines corniculatæ; cartilages of Santorini) are two small conical nodules consisting of yellow elastic cartilage, which articulate with the summits of the arytenoid cartilages and serve to prolong them backward and medialward. They are situated in the posterior parts of the aryepiglottic folds of mucous membrane, and are sometimes fused with the arytenoid cartilages.
The Cuneiform Cartilages (cartilagines cuneiformes; cartilages of Wrisberg) are two small, elongated pieces of yellow elastic cartilage, placed one on either side, in the aryepiglottic fold, where they give rise to small whitish elevations on the surface of the mucous membrane, just in front of the arytenoid cartilages.
The Epiglottis (cartilago epiglottica) is a thin lamella of fibrocartilage of a yellowish color, shaped like a leaf, and projecting obliquely upward behind the root of the tongue, in front of the entrance to the larynx. The free extremity is broad and rounded; the attached part or stem is long, narrow, and connected by the thyroepiglottic ligament to the angle formed by the two laminæ of the thyroid cartilage, a short distance below the superior thyroid notch. The lower part of its anterior surface is connected to the upper border of the body of the hyoid bone by an elastic ligamentous band, the hyoepiglottic ligament.
The anterior or lingual surface is curved forward, and covered on its upper, free part by mucous membrane which is reflected on to the sides and root of the tongue, forming a median and two lateral glossoepiglottic folds; the lateral folds are partly attached to the wall of the pharynx. The depressions between the epiglottis and the root of the tongue, on either side of the median fold, are named the valleculæ. The lower part of the anterior surface lies behind the hyoid bone, the hyothyroid membrane, and upper part of the thyroid cartilage, but is separated from these structures by a mass of fatty tissue.
The posterior or laryngeal surface is smooth, concave from side to side, concavo-convex from above downward; its lower part projects backward as an elevation, the tubercle or cushion. When the mucous membrane is removed, the surface of the cartilage is seen to be indented by a number of small pits, in which mucous glands are lodged. To its sides the aryepiglottic folds are attached.
Structure.—The corniculate and cuneiform cartilages, the epiglottis, and the apices of the arytenoids at first consist of hyaline cartilage, but later elastic fibers are deposited in the matrix, converting them into yellow fibrocartilage, which shows little tendency to calcification. The thyroid, cricoid, and the greater part of the arytenoids consist of hyaline cartilage, and become more or less ossified as age advances. Ossification commences about the twenty-fifth year in the thyroid cartilage, and somewhat later in the cricoid and arytenoids; by the sixty-fifth year these cartilages may be completely converted into bone.
Ligaments.—The ligaments of the larynxare extrinsic, i. e., those connecting the thyroid cartilage and epiglottis with the hyoid bone, and the cricoid cartilage with the trachea; and intrinsic, those which connect the several cartilages of the larynx to each other.
Extrinsic Ligaments.—The ligaments connecting the thyroid cartilage with the hyoid bone are the hyothyroid membrane, and a middle and two lateral hyothyroid ligaments.
The Hyothyroid Membrane (membrana hyothyreoidea; thyrohyoid membrane) is a broad, fibro-elastic layer, attached below to the upper border of the thyroid cartilage and to the front of its superior cornu, and above to the upper margin of the posterior surface of the body and greater cornua of the hyoid bone, thus passing behind the posterior surface of the body of the hyoid, and being separated from it by a mucous bursa, which facilitates the upward movement of the larynx during deglutition. Its middle thicker part is termed the middle hyothyroid ligament (ligamentum hyothyreoideum medium; middle thyrohyoid ligament), its lateral thinner portions are pierced by the superior laryngeal vessels and the internal branch of the superior laryngeal nerve. Its anterior surface is in relation with the Thyreohyoideus, Sternohyoideus, and Omohyoideus, and with the body of the hyoid bone.
The Lateral Hyothyroid Ligament (ligamentum hyothyreoideum laterale; lateral thyrohyoid ligament) is a round elastic cord, which forms the posterior border of the hyothyroid membrane and passes between the tip of the superior cornu of the thyroid cartilage and the extremity of the greater cornu of the hyoid bone. A small cartilaginous nodule (cartilago triticea), sometimes bony, is frequently found in it.
Ligaments of the larynx. Posterior view.
The Epiglottis is connected with the hyoid bone by an elastic band, the hyoepiglottic ligament (ligamentum hyoepiglotticum), which extends from the anterior surface of the epiglottis to the upper border of the body of the hyoid bone. The glossoepiglottic folds of mucous membrane (page 1075) may also be considered as extrinsic ligaments of the epiglottis.
The Cricotracheal Ligament (ligamentum cricotracheale) connects the cricoid cartilage with the first ring of the trachea. It resembles the fibrous membrane which connects the cartilaginous rings of the trachea to each other.
Intrinsic Ligaments.—Beneath the mucous membrane of the larynx is a broad sheet of fibrous tissue containing many elastic fibers, and termed the elastic membrane of the larynx. It is subdivided on either side by the interval between the ventricular and vocal ligaments, the upper portion extends between the arytenoid cartilage and the epiglottis and is often poorly defined; the lower part is a well-marked membrane forming, with its fellow of the opposite side, the conus elasticus which connects the thyroid, cricoid, and arytenoid cartilages to one another. In addition the joints between the individual cartilages are provided with ligaments.
The Conus Elasticus (cricothyroid membrane) is composed mainly of yellow elastic tissue. It consists of an anterior and two lateral portions. The anterior part or middle cricothyroid ligament (ligamentum cricothyreoideum medium; central part of cricothyroid membrane) is thick and strong, narrow above and broad below. It connects together the front parts of the contiguous margins of the thyroid and cricoid cartilages. It is overlapped on either side by the Cricothyreoideus, but between these is subcutaneous; it is crossed horizontally by a small anastomotic arterial arch, formed by the junction of the two cricothyroid arteries, branches of which pierce it. The lateral portions are thinner and lie close under the mucous membrane of the larynx; they extend from the superior border of the cricoid cartilage to the inferior margin of the vocal ligaments, with which they are continuous. These ligaments may therefore be regarded as the free borders of the lateral portions of the conus elasticus, and extend from the vocal processes of the arytenoid cartilages to the angle of the thyroid cartilage about midway between its upper and lower borders.
An articular capsule, strengthened posteriorly by a well-marked fibrous band, encloses the articulation of the inferior cornu of the thyroid with the cricoid cartilage on either side.
Each arytenoid cartilage is connected to the cricoid by a capsule and a posterior cricoarytenoid ligament. The capsule (capsula articularis cricoarytenoidea) is thin and loose, and is attached to the margins of the articular surfaces. The posterior cricoarytenoid ligament (ligamentum cricoarytenoideum posterius) extends from the cricoid to the medial and back part of the base of the arytenoid.
The thyroepiglottic ligament (ligamentum thyreoepiglotticum) is a long, slender, elastic cord which connects the stem of the epiglottis with the angle of the thyroid cartilage, immediately beneath the superior thyroid notch, above the attachment of the ventricular ligaments.
Movements.—The articulation between the inferior cornu of the thyroid cartilage and the cricoid cartilage on either side is a diarthrodial one, and permits of rotatory and gliding movements. The rotatory movement is one in which the cricoid cartilage rotates upon the inferior cornua of the thyroid cartilage around an axis passing transversely through both joints. The gliding movement consists in a limited shifting of the cricoid on the thyroid in different directions.
The articulation between the arytenoid cartilages and the cricoid is also a diarthrodial one, and permits of two varieties of movement: one is a rotation of the arytenoid on a vertical axis, whereby the vocal process is moved lateralward or medialward, and the rima glottidis increased or diminished; the other is a gliding movement, and allows the arytenoid cartilages to approach or recede from each other; from the direction and slope of the articular surfaces lateral gliding is accompanied by a forward and downward movement. The two movements of gliding and rotation are associated, the medial gliding being connected with medialward rotation, and the lateral gliding with lateralward rotation. The posterior cricoarytenoid ligaments limit the forward movement of the arytenoid cartilages on the cricoid.
Interior of the Larynx—The cavity of the larynx (cavum laryngis) extends from the laryngeal entrance to the lower border of the cricoid cartilage where it is continuous with that of the trachea. It is divided into two parts by the projection of the vocal folds, between which is a narrow triangular fissure or chink, the rima glottidis. The portion of the cavity of the larynx above the vocal folds is called the vestibule; it is wide and triangular in shape, its base or anterior wall presenting, however, about its center the backward projection of the tubercle of the epiglottis. It contains the ventricular folds, and between these and the vocal folds are the ventricles of the larynx. The portion below the vocal folds is at first of an elliptical form, but lower down it widens out, assumes a circular form, and is continuous with the tube of the trachea.
The entrance of the larynx is a triangular opening, wide in front, narrow behind, and sloping obliquely downward and backward. It is bounded, in front, by the epiglottis; behind, by the apices of the arytenoid cartilages, the corniculate cartilages, and the interarytenoid notch; and on either side, by a fold of mucous membrane, enclosing ligamentous and muscular fibers, stretched between the side of the epiglottis and the apex of the arytenoid cartilage; this is the aryepiglottic fold, on the posterior part of the margin of which the cuneiform cartilage forms a more or less distinct whitish prominence, the cuneiform tubercle.
Sagittal section of the larynx and upper part of the trachea
The Ventricular Folds (plicœ ventriculares; superior or false vocal cords) are two thick folds of mucous membrane, each enclosing a narrow band of fibrous tissue, the ventricular ligament which is attached in front to the angle of the thyroid cartilage immediately below the attachment of the epiglottis, and behind to the antero-lateral surface of the arytenoid cartilage, a short distance above the vocal process. The lower border of this ligament, enclosed in mucous membrane, forms a free crescentic margin, which constitutes the upper boundary of the ventricle of the larynx.
The Vocal Folds (plicœ vocales; inferior or true vocal cords) are concerned in the production of sound, and enclose two strong bands, named the vocal ligaments (ligamenta vocales; inferior thyroarytenoid). Each ligament consists of a band of yellow elastic tissue, attached in front to the angle of the thyroid cartilage, and behind to the vocal process of the arytenoid. Its lower border is continuous with the thin lateral part of the conus elasticus. Its upper border forms the lower boundary of the ventricle of the larynx. Laterally, the Vocalis muscle lies parallel with it. It is covered medially by mucous membrane, which is extremely thin and closely adherent to its surface.
Coronal section of larynx and upper part of trachea.
The Ventricle of the Larynx (ventriculus laryngis [Morgagnii]; laryngeal sinus) is a fusiform fossa, situated between the ventricular and vocal folds on either side, and extending nearly their entire length. The fossa is bounded, above, by the free crescentic edge of the ventricular fold; below, by the straight margin of the vocal fold; laterally, by the mucous membrane covering the corresponding Thyreoarytænoideus. The anterior part of the ventricle leads up by a narrow opening into a cecal pouch of mucous membrane of variable size called the appendix.
The appendix of the laryngeal ventricle (appendix ventriculi laryngis; laryngeal saccule) is a membranous sac, placed between the ventricular fold and the inner surface of the thyroid cartilage, occasionally extending as far as its upper border or even higher; it is conical in form, and curved slightly backward. On the surface of its mucous membrane are the openings of sixty or seventy mucous glands, which are lodged in the submucous areolar tissue. This sac is enclosed in a fibrous capsule, continuous below with the ventricular ligament. Its medial surface is covered by a few delicate muscular fasciculi, which arise from the apex of the arytenoid cartilage and become lost in the aryepiglottic fold of mucous membrane; laterally it is separated from the thyroid cartilage by the Thyreoepiglotticus. These muscles compress the sac, and express the secretion it contains upon the vocal folds to lubricate their surfaces.
The entrance to the larynx, viewed from behind.
The Rima Glottidis is the elongated fissure or chink between the
vocal folds in front, and the bases and vocal processes of the arytenoid
cartilages behind. It is therefore subdivided into a larger anterior
intramembranous part (glottis vocalis), which measures about
three-fifths of the length of the entire aperture, and a posterior
intercartilaginous part (glottis respiratoria). Posteriorly it is
limited by the mucous membrane passing between the arytenoid cartilages. The
rima glottidis is the narrowest part of the cavity of the larynx, and its level
corresponds with the bases of the arytenoid cartilages. Its length, in the
male, is about
Laryngoscopic view of interior of larynx.)
Muscles.—The muscles of the larynx are extrinsic, passing between the larynx and parts around—these have been described in the section on Myology; and intrinsic, confined entirely to the larynx.
The intrinsic muscles are:
The Cricothyreoideus (Cricothyroid) Triangular in form, arises from the front and lateral part of the cricoid cartilage; its fibers diverge, and are arranged in two groups. The lower fibers constitute a pars obliqua and slant backward and lateralward to the anterior border of the inferior cornu; the anterior fibers, forming a pars recta, run upward, backward, and lateralward to the posterior part of the lower border of the lamina of the thyroid cartilage.
The medial borders of the two muscles are separated by a triangular interval, occupied by the middle cricothyroid ligament.
The Cricoarytænoideus posterior (posterior cricoarytenoid) (Fig. 958) arises from the broad depression on the corresponding half of the posterior surface of the lamina of the cricoid cartilage; its fibers run upward and lateralward, and converge to be inserted into the back of the muscular process of the arytenoid cartilage. The uppermost fibers are nearly horizontal, the middle oblique, and the lowest almost vertical.
The Cricoarytænoideus lateralis (lateral cricoarytenoid) (Fig. 959) is smaller than the preceding, and of an oblong form. It arises from the upper border of the arch of the cricoid cartilage, and, passing obliquely upward and backward, is inserted into the front of the muscular process of the arytenoid cartilage.
Side view of the larynx, showing muscular attachments.
Muscles of larynx. Posterior view.
Muscles of larynx. Side view. Right lamina of thyroid cartilage removed.
The Arytænoideus is a single muscle, filling up the posterior concave surfaces of the arytenoid cartilages. It arises from the posterior surface and lateral border of one arytenoid cartilage, and is inserted into the corresponding parts of the opposite cartilage. It consists of oblique and transverse parts. The Arytænoideus obliquus, the more superficial, forms two fasciculi, which pass from the base of one cartilage to the apex of the opposite one, and therefore cross each other like the limbs of the letter X; a few fibers are continued around the lateral margin of the cartilage, and are prolonged into the aryepiglottic fold; they are sometimes described as a separate muscle, the Aryepiglotticus. The Arytænoideus transversus crosses transversely between the two cartilages.
The Thyreoarytænoideus (Thyroarytenoid) is a broad, thin, muscle which lies parallel with and lateral to the vocal fold, and supports the wall of the ventricle and its appendix. It arises in front from the lower half of the angle of the thyroid cartilage, and from the middle cricothyroid ligament. Its fibers pass backward and lateralward, to be inserted into the base and anterior surface of the arytenoid cartilage. The lower and deeper fibers of the muscle can be differentiated as a triangular band which is inserted into the vocal process of the arytenoid cartilage, and into the adjacent portion of its anterior surface; it is termed the Vocalis, and lies parallel with the vocal ligament, to which it is adherent.
Muscles of the larynx, seen from above.
A considerable number of the fibers of the Thyreoarytænoideus are prolonged into the aryepiglottic fold, where some of them become lost, while others are continued to the margin of the epiglottis. They have received a distinctive name, Thyreoepiglotticus, and are sometimes described as a separate muscle. A few fibers extend along the wall of the ventricle from the lateral wall of the arytenoid cartilage to the side of the epiglottis and constitute the Ventricularis muscle.
Actions.—In considering the actions of the muscles of the larynx, they may be conveniently divided into two groups, vix.: 1. Those which open and close the glottis. 2. Those which regulate the degree of tension of the vocal folds.
The Cricoarytœnoidei posteriores separate the vocal folds, and, consequently, open the glottis, by rotating the arytenoid cartilages outward around a vertical axis passing through the cricoarytenoid joints; so that their vocal processes and the vocal folds attached to them become widely separated.
The Cricoarytœnoidei laterales close the glottis by rotating the arytenoid cartilages inward, so as to approximate their vocal processes.
The Arytœnoideus approximates the arytenoid cartilages, and thus closes the opening of the glottis, especially at its back part.
The Cricothyreoidei produce tension and elongation of the vocal folds by drawing up the arch of the cricoid cartilage and tilting back the upper border of its lamina; the distance between the vocal processes and the angle of the thyroid is thus increased, and the folds are consequently elongated.
The Thyreoarytœnoidei, consisting of two parts having different attachments and different directions, are rather complicated as regards their action. Their main use is to draw the arytenoid cartilages forward toward the thyroid, and thus shorten and relax the vocal folds. But, owing to the connection of the deeper portion with the vocal fold, this part, if acting separately, is supposed to modify its elasticity and tension, while the lateral portion rotates the arytenoid cartilage inward, and thus narrows the rima glottidis by bringing the two vocal folds together.
Mucous Membrane.—The mucous membrane of the larynx is continuous above with that lining the mouth and pharynx, and is prolonged through the trachea and bronchi into the lungs. It lines the posterior surface and the upper part of the anterior surface of the epiglottis, to which it is closely adherent, and forms the aryepiglottic folds which bound the entrance of the larynx. It lines the whole of the cavity of the larynx; forms, by its reduplication, the chief part of the ventricular fold, and, from the ventricle, is continued into the ventricular appendix. It is then reflected over the vocal ligament, where it is thin, and very intimately adherent; covers the inner surface of the conus elasticus and cricoid cartilage; and is ultimately continuous with the lining membrane of the trachea. The anterior surface and the upper half of the posterior surface of the epiglottis, the upper part of the aryepiglottic folds and the vocal folds are covered by stratified squamous epithelium; all the rest of the laryngeal mucous membrane is covered by columnar ciliated cells, but patches of stratified squamous epithelium are found in the mucous membrane above the glottis.
Glands.—The mucous membrane of the larynx is furnished with numerous mucous secreting glands, the orifices of which are found in nearly every part; they are very plentiful upon the epiglottis, being lodged in little pits in its substance; they are also found in large numbers along the margin of the aryepiglottic fold, in front of the arytenoid cartilages, where they are termed the arytenoid glands. They exist also in large numbers in the ventricular appendages. None are found on the free edges of the vocal folds.
Vessels and Nerves.—The chief arteries of the larynx are the laryngeal branches derived from the superior and inferior thyroid. The veins accompany the arteries; those accompanying the superior laryngeal artery join the superior thyroid vein which opens into the internal jugular vein; while those accompanying the inferior laryngeal artery join the inferior thyroid vein which opens into the innominate vein. The lymphatic vessels consist of two sets, superior and inferior. The former accompany the superior laryngeal artery and pierce the hyothyroid membrane, to end in the glands situated near the bifurcation of the common carotid artery. Of the latter, some pass through the middle cricothyroid ligament and open into a gland lying in front of that ligament or in front of the upper part of the trachea, while others pass to the deep cervical glands and to the glands accompanying the inferior thyroid artery. The nerves are derived from the internal and external branches of the superior laryngeal nerve, from the recurrent nerve, and from the sympathetic. The internal laryngeal branch is almost entirely sensory, but some motor filaments are said to be carried by it to the Arytænoideus. It enters the larynx by piercing the posterior part of the hyothyroid membrane above the superior laryngeal vessels, and divides into a branch which is distributed to both surfaces of the epiglottis, a second to the aryepiglottic fold, and a third, the largest, which supplies the mucous membrane over the back of the larynx and communicates with the recurrent nerve. The external laryngeal branch supplies the Cricothyreoideus. The recurrent nerve passes upward beneath the lower border of the Constrictor pharyngis inferior immediately behind the cricothyroid joint. It supplies all the muscles of the larynx except the Cricothyreoideus, and perhaps a part of the Arytænoideus. The sensory branches of the laryngeal nerves form subepithelial plexuses, from which fibers pass to end between the cells covering the mucous membrane.
Over the posterior surface of the epiglottis, in the aryepiglottic folds, and less regularly in some other parts, taste-buds, similar to those in the tongue, are found.
The TRACHEA is a tube, which consists of 16-20 semicircular cartilages, joint each other by annular ligaments. Last built by connective tissue with smooth muscular fibres. Behind semi-rings communicate by each other by membranous tracheal wall. Trachea (windpipe) extends from VI cervical to V thoracic vertebra, where it ramifies on two principal bronchi. This place is tracheal bifurcation. Trachea has cervical part and thoracic part. Cervical part at the front covered by infrahyoid muscles and isthmus of thyroid gland that accords to the second-third tracheal ring. Esophagus (gullet) passes behind the trachea. Thoracic part of trachea is situated in superior mediastinum.
Front view of cartilages of larynx, trachea
Transverse section of the trachea, just above its bifurcation, with a bird’s-eye view of the interior.
Inspired (inhaled) air leaves the pharynx by passing through the glottis, a narrow opening. The larynx surrounds and protects the glottis. The larynx begins at the level of vertebra C4 or C5 and ends at the level of vertebra C6. The larynx is essentially a cylinder whose incomplete cartilaginous walls are stabilized by ligaments and skeletal muscles (Figure 23-4a,b,c, d).
Cartilages of the Larynx
The larynx also contains three pairs of smaller hyaline cartilages: the arytenoid, corniculate, and cuneiform cartilages:
Intrinsic ligaments bind all nine cartilages together to form the larynx (Figure 23-4a,b). Extrinsic ligaments attach the thyroid cartilage to the hyoid bone and the cricoid cartilage to the trachea. The ventricular ligaments and the vocal ligaments extend between the thyroid cartilage and the arytenoids.
The ventricular and vocal ligaments are covered by folds of laryngeal epithelium that project into the glottis. The ventricular ligaments lie within the superior pair of folds, known as the ventricular folds (Figure 23-4b,c, d). The ventricular folds, which are relatively inelastic, help prevent foreign objects from entering the glottis and provide protection for the more delicate vocal folds.
The vocal folds guard the entrance to the glottis. They are located inferior to the ventricular folds. The vocal folds are highly elastic, because they contain bands of elastic tissue called the vocal ligaments. The vocal folds are involved with the production of sounds, and for this reason they are known as the true vocal cords. Because the ventricular folds play no part in sound production, they are often called the false vocal cords.
Air passing through the glottis vibrates the vocal folds and produces sound waves. The pitch of the sound produced depends on the diameter, length, and tension in the vocal folds. The diameter and length are directly related to the size of the larynx. The tension is controlled by the contraction of voluntary muscles that change the position of the arytenoid cartilages relative to that of the thyroid cartilage. When the distance increases, the vocal folds tense and the pitch rises; when the distance decreases, the vocal folds relax and the pitch falls.
Anatomically, children of both genders have slender, short vocal folds, and their voices tend to be high-pitched. At puberty, the larynx of a male enlarges considerably more than that of a female. The true vocal cords of an adult male are thicker and longer, and they produce lower tones, than those of an adult female.
Sound production at the larynx is called phonation (phone, voice). Phonation is one component of speech production, but clear speech also requires articulation, the modification of those sounds by other structures. In a stringed instrument, such as a guitar, the quality of the sound produced does not depend solely on the nature of the vibrating string. The entire instrument becomes involved as the walls vibrate and the composite sound echoes within the hollow body. Similar amplification and resonance occur within your pharynx, oral cavity, nasal cavity, and paranasal sinuses. The combination determines the particular and distinctive sound of your voice. The final production of distinct words further depends on voluntary movements of the tongue, lips, and cheeks.
The Laryngeal Musculature
The larynx is associated with two groups of muscles: (1) the extrinsic laryngeal muscles and (2) the intrinsic laryngeal muscles. The extrinsic laryngeal musculature, which includes muscles of the neck and pharynx, positions and stabilizes the larynx. We considered these muscles in Chapter 11. The intrinsic laryngeal muscles have two major functions. One set regulates tension in the vocal folds; a second set opens and closes the glottis. The muscles involved with the vocal folds insert on the thyroid, arytenoid, and corniculate cartilages. The opening or closing of the glottis involves rotational movements of the arytenoids that move the vocal folds apart or together.
When you swallow, both extrinsic and intrinsic muscles cooperate to prevent food or drink from entering the glottis. Before the material is swallowed, it is crushed and chewed into a pasty mass known as a bolus. Extrinsic muscles then elevate the larynx, bending the epiglottis over the entrance to the glottis, so that the bolus can glide across the epiglottis rather than falling into the larynx (Figure 23-5). While this movement is under way, intrinsic muscles close the glottis. Food particles or liquids that touch the surfaces of the ventricular or vocal folds will trigger the coughing reflex. In a cough, the glottis is kept closed while the expiratory muscles contract, elevating intrapulmonary pressure. When the glottis is opened suddenly, the resulting blast of air from the trachea generally ejects any material that blocks the entrance to the glottis.
CONCEPT CHECK QUESTIONS
The epithelium of
the larynx is continuous with that of the trachea, or windpipe. The trachea is a tough, flexible tube
with a diameter of about
The mucosa of the trachea resembles that of the nasal cavity and nasopharynx. The submucosa, a thick layer of connective tissue, surrounds the mucosa. The submucosa contains mucous glands that communicate with the epithelial surface through a number of secretory ducts. The trachea contains 15-20(Figure 23-6a). Each tracheal cartilage is bound to neighboring cartilages by elastic annular ligaments. The tracheal cartilages stiffen the tracheal walls and protect the airway. They also prevent its collapse or overexpansion as pressures change in the respiratory system.
Each tracheal cartilage is C-shaped. The closed portion of the C protects the anterior and lateral surfaces of the trachea. The open portion of the C faces posteriorly, toward the esophagus. Because the tracheal cartilages do not continue around the trachea, the posterior tracheal wall can easily distort when you swallow, permitting the passage of large masses of food through the esophagus.
An elastic ligament and the trachealis, a band of smooth muscle, connect the ends of each tracheal cartilage (Figure 23-6b). Contraction of the trachealis muscle alters the diameter of the trachea, changing the trachea's resistance to airflow. The normal diameter of the trachea changes from moment to moment, primarily under the control of the sympathetic division of the ANS. Sympathetic stimulation increases the diameter of the trachea and makes it easier to move large volumes of air along the respiratory passageways.
THE PRIMARY BRONCHI
The trachea branches within the mediastinum, giving rise to the right and left primary bronchi. A ridge called the carina marks the line of separation between the two bronchi (Figure 23-6a). The histological organization of the primary bronchi is the same as that of the trachea, with cartilaginous C-shaped supporting rings. The right primary bronchus supplies the right lung, and the left supplies the left lung. The right primary bronchus is larger in diameter, and descends toward the lung at a steeper angle, than the left. Thus most foreign objects that enter the trachea find their way into the right bronchus rather than the left.
further, each primary bronchus travels to a groove along the medial surface of
its lung. This groove, the hilus of the lung, also provides access for entry to
pulmonary vessels and nerves (Figure 23-
The left and right lungs (Figure 23-7a, b) are situated in the left and right pleural cavities. Each lung is a blunt cone, with the tip, or apex, pointing superiorly. The apex on each side extends into the base of the neck superior to the first rib. The broad concave inferior portion, or base, of each lung rests on the superior surface of the diaphragm.
Lobes and Surfaces of the Lungs
The lungs have distinct lobes separated by deep fissures (Figures 23-7a,b). The right lung has three lobes: superior, middle, and inferior, separated by the horizontal and oblique fissures. The left lung has only two lobes: superior and inferior, separated by the oblique fissure. The right lung is broader than the left, because most of the heart and great vessels project into the left thoracic cavity. However, the left lung is longer than the right lung, because the diaphragm rises on the right side to accommodate the mass of the liver.
The curving anterior portion of the lung that follows the inner contours of the rib cage is the costal surface. The mediastinal surface, containing the hilus, has a more irregular shape. The mediastinal surfaces of both lungs bear grooves that mark the passage of the great vessels and of the cardiac impressions, concavities that conform to the shape of the pericardium (Figures 23-7a,b and 23-8). The cardiac impression of the left lung is deeper than that of the right lung. In anterior view, the medial edge of the right lung forms a vertical line, whereas the margin of the left lung is indented at the cardiac notch.
The primary bronchi and their branches form the bronchial tree. Because the left and right primary bronchi are outside the lungs, they are also called extrapulmonary bronchi. As the primary bronchi enter the lungs, they divide to form smaller passageways (Figures 23-6 and 23-10a). Those branches are collectively called the intrapulmonary bronchi.
Each primary bronchus divides to form secondary bronchi, also known as lobar bronchi. The right lung has three lobes, and the right primary bronchus divides into three secondary bronchi: (1) a superior lobar bronchus, (2) a middle lobar bronchus, and (3) an inferior lobar bronchus. The left lung has two lobes, and the left primary bronchus divides into two secondary bronchi: (1) a superior lobar bronchus and (2) an inferior lobar bronchus.
Figure 23-10a follows the branching pattern of the left primary bronchus as it enters the lung. (The number of branches have been reduced for clarity.) Within each lung, the secondary bronchi branch to form tertiary bronchi, or segmental bronchi. The branching pattern differs between the two lungs, but each tertiary bronchus ultimately supplies air to a single bronchopulmonary segment, a specific region of one lung. There are 10 bronchopulmonary segments in the right lung. During development, the left lung also has 10 segments, but subsequent fusion of adjacent tertiary bronchi generally reduces that number to eight or nine.
The walls of the primary, secondary, and tertiary bronchi contain progressively lesser amounts of cartilage. In the secondary and tertiary bronchi, the cartilages form plates arranged around the lumen. These cartilages serve the same purpose as the rings of cartilage in the trachea and primary bronchi. As the amount of cartilage decreases, the relative amount of smooth muscle increases. With less cartilaginous support, the amount of tension in those smooth muscles has a greater effect on bronchial diameter and the resistance to airflow.
bronchus branches several times within the bronchopulmonary segment, giving
rise to multiple bronchioles. These branch further into the finest conducting
branches, called terminal bronchioles. Roughly 6500 terminal bronchioles are
supplied by each tertiary bronchus. Terminal bronchioles have a lumenal
diameter of 0.3-
The walls of bronchioles, which lack cartilaginous supports, are dominated by smooth muscle tissue (Figure 23-10b). In functional terms, the bronchioles are to the respiratory system what the arterioles are to the cardiovascular system. Varying the diameter of the bronchioles provides control over the amount of resistance to airflow and the distribution of air within the lungs.
The ANS regulates the activity in this smooth muscle layer and thereby controls the diameter of the bronchioles. Sympathetic activation leads to enlargement of the airway diameter, or bronchodilation. Parasympathetic stimulation leads to bronchoconstriction, a reduction in the diameter of the airways. Bronchoconstriction also occurs during allergic reactions such as anaphylaxis (Chapter 22), in response to histamine released by activated mast cells and basophils.
Bronchodilation and bronchoconstriction alter the resistance to airflow toward or away from the respiratory exchange surfaces. Tension in the smooth muscles commonly throws the bronchiolar mucosa into a series of folds, limiting airflow; excessive stimulation, as in asthma, can almost completely prevent airflow along the terminal bronchioles.
The connective tissues of the root of each lung extend into the lung's parenchyma. The fibrous partitions, or trabeculae, contain elastic fibers, smooth muscles, and lymphatic vessels. The trabeculae branch repeatedly, dividing the lobes into ever smaller compartments. The branches of the conducting passageways, pulmonary vessels, and nerves of the lungs follow these trabeculae. The finest partitions, or interlobular septa (septum, a wall) divide the lung into pulmonary lobules, each supplied by branches of the pulmonary arteries, pulmonary veins, and respiratory passageways (Figure 23-10a, b). The connective tissues of the septa are in turn continuous with those of the visceral pleura, the serous membrane covering the lungs.
Each terminal bronchiole delivers air to a single pulmonary lobule. Within the lobule, the terminal bronchiole branches to form several respiratory bronchioles. These are the thinnest and most delicate branches of the bronchial tree. They deliver air to the exchange surfaces of the lungs.
The preliminary filtration and humidification of the incoming air are completed before air leaves the terminal bronchioles. The epithelial cells of the terminal bronchioles and respiratory bronchioles are cuboidal, with only scattered cilia, and there are no goblet cells or underlying mucous glands.
Respiratory bronchioles are connected to individual alveoli and to multiple alveoli along regions called alveolar ducts (Figures 23-10b). These passageways end at alveolar sacs, common chambers connected to multiple individual alveoli. Each lung contains about 150 million alveoli, and their abundance gives the lung an open, spongy appearance. An extensive network of capillaries is associated with each alveolus (Figure 23-12a); the capillaries are surrounded by a network of elastic fibers. This elastic tissue helps maintain the relative positions of the alveoli and respiratory bronchioles. Recoil of these fibers during exhalation reduces the size of the alveoli and helps push air out of the lungs.
The alveolar epithelium consists primarily of simple squamous epithelium (Figure 23-12b). The squamous epithelial cells, called Type I cells, are unusually thin and delicate. Roaming alveolar macrophages (dust cells) patrol the epithelium, phagocytizing any particulate matter that has eluded the respiratory defenses and reached the alveolar surfaces. Septal cells, also called surfactant cells or Type II cells, are scattered among the squamous cells. These large cells produce an oily secretion, or surfactant, containing a mixture of phospholipids and proteins. Surfactant is secreted onto the alveolar surfaces, where it forms a superficial coating over a thin layer of water.
Surfactant is important because it reduces surface tension in the liquid coating the alveolar surface. As we saw in Chapter 2, surface tension results from the attraction between water molecules at an air-water boundary. The alveolar walls are very delicate, and without surfactant the surface tension would be so high that the alveoli would collapse. The surfactant forms a thin surface layer that interacts with the water molecules, reducing the surface tension and keeping the alveoli open.
If surfactant cells produce inadequate amounts of surfactant due to injury or genetic abnormalities, the alveoli will collapse, and respiration will become difficult. On each breath, the inhalation must be forceful enough to pop open the alveoli. A person with this problem, respiratory distress syndrome, is soon exhausted by the effort required to keep inflating and deflating the lungs.
At the respiratory membrane, the total distance separating the alveolar air and the blood can be as little as 0.1 µm. Diffusion across the respiratory membrane proceeds very rapidly, because (1) the distance is small and (2) both oxygen and carbon dioxide are lipid-soluble. The membranes of the epithelial and endothelial cells thus do not pose a barrier to the movement of oxygen and carbon dioxide between the blood and alveolar air spaces.
CONCEPT CHECK QUESTIONS
The Blood Supply to the Lungs
Your respiratory exchange surfaces receive blood from arteries of your pulmonary circuit. The pulmonary arteries enter the lungs at the hilus and branch with the bronchi as they approach the lobules. Each lobule receives an arteriole and a venule, and a network of capillaries surrounds each alveolus directly beneath the respiratory membrane. In addition to providing a mechanism for gas exchange, the endothelial cells of the alveolar capillaries are the primary source of angiotensin-converting enzyme (ACE). This enzyme, which converts circulating angiotensin I to angiotensin II, plays an important role in the regulation of blood volume and blood pressure (as discussed in Chapters 18 and 21).
Blood from the alveolar capillaries passes through the pulmonary venules and then enters the pulmonary veins, which deliver it to the left atrium. The conducting portions of your respiratory tract receive blood from the external carotid arteries (nasal passages and larynx), the thyrocervical arteries (the inferior larynx and trachea), and the bronchial arteries (the bronchi and bronchioles). (See Figures 21-23, 21-24, and 21-26.) The capillaries supplied by the bronchial arteries provide oxygen and nutrients to the conducting passageways of your lungs. The venous blood flows into the pulmonary veins, bypassing the rest of the systemic circuit and diluting the oxygenated blood leaving the alveoli.
THE PLEURAL CAVITIES AND PLEURAL MEMBRANES
The thoracic cavity has the shape of a broad cone. Its walls are the rib cage, and the muscular diaphragm forms the floor. The two pleural cavities are separated by the mediastinum (Figure 23-8). Each lung occupies a single pleural cavity, which is lined by a serous membrane called the pleura (plural, pleurae). The parietal pleura covers the inner surface of the thoracic wall and extends over the diaphragm and mediastinum. The visceral pleura covers the outer surfaces of the lungs, extending into the fissures between the lobes. Each pleural cavity actually represents a potential space rather than an open chamber, for the parietal and visceral pleurae are usually in close contact. A small amount of pleural fluid is secreted by both pleurae. Pleural fluid gives a moist, slippery coating that provides lubrication, thereby reducing friction between the parietal and visceral surfaces as you breathe. Samples of pleural fluid, obtained by means of a long needle inserted between the ribs, are sometimes obtained for diagnostic purposes. This sampling procedure is called thoracentesis. The fluid extracted is then examined for the presence of bacteria, blood cells, or other abnormal components.
In some disease states, the normal coating of pleural fluid is unable to prevent friction between the opposing pleural surfaces. The result is pain and pleural inflammation, a condition called pleurisy. When pleurisy develops, there may be excessive secretion of pleural fluid, or the inflamed pleurae may adhere to one another, limiting relative movement. In either case, breathing becomes difficult, and prompt medical attention is required.
Changes in the Respiratory System at Birth
There are several important differences between the respiratory system of a fetus and that of a newborn infant. Prior to delivery, pulmonary arterial resistance is high, because the pulmonary vessels are collapsed. The rib cage is compressed, and the lungs and conducting passageways contain only small amounts of fluid and no air. At birth, the newborn infant takes a truly heroic first breath through powerful contractions of the diaphragmatic and external intercostal muscles. The inspired air must enter the respiratory passageways with enough force to overcome the surface tension and inflate the bronchial tree and most of the alveoli. The same drop in pressure that pulls air into the lungs pulls blood into the pulmonary circulation; the changes in blood flow that occur lead to the closure of the foramen ovale, an interatrial connection, and the ductus arteriosus, the fetal connection between the pulmonary trunk and the aorta. (We detailed these events in Chapters 20 and 21.)
The exhalation that follows fails to empty the lungs completely, for the rib cage does not return to its former, fully compressed state. Cartilages and connective tissues keep the conducting passageways open, and the surfactant covering the alveolar surfaces prevents their collapse. Subsequent breaths complete the inflation of the alveoli. Pathologists sometimes use these physical changes to determine whether a newborn infant died before delivery or shortly thereafter. Prior to the first breath, the lungs are completely filled with fluid, and they will sink if placed in water. After the infant's first breath, even the collapsed lungs contain enough air to keep them afloat.
CONCEPT CHECK QUESTIONS