OUTLINE

25.1 General Organization and Functions of the 748 25.1a Respiratory System Functions 748 25 25.2 Upper 750 25.2a Nose and 750 25.2b 750 25.2c 750 25.3 Lower Respiratory Tract 753 Respiratory 25.3a 753 25.3b 757 25.3c Bronchial Tree 758 25.3d Respiratory , Alveolar Ducts, and Alveoli 760 System 25.4 762 25.4a Pleura and Pleural Cavities 762 25.4b Gross of the Lungs 762 25.4c Supply To and From the Lungs 763 25.4d Lymphatic Drainage 765 25.5 Pulmonary Ventilation 766 25.6 Dimensional Changes During External Respiration 767 25.7 Innervation of the Respiratory System 769 25.7a Ventilation Control by Respiratory Centers of the Brain 770 25.8 Aging and the Respiratory System 771 25.9 Development of the Respiratory System 774

MODULE 11: RESPIRATORY SYSTEM

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he respiratory (res pi-r ́ ă-tō r ē ́ ; respiro = to breathe) system T provides the means for required by living cells. must be supplied without interruption, and , a waste product generated by the cells, must be continuously expelled. Sphenoidal sinus The respiratory and cardiovascular systems are inseparable partners. While the respiratory system exchanges gases between the atmo- Nasal cavity sphere and the blood, the cardiovascular system transports those Upper gases between the lungs and the body cells. This chapter examines respiratory the cells, tissues, and organs involved in the complex and vital tract Pharynx process of respiration.

Larynx 25.1 General Organization and Trachea Functions of the Respiratory System Lower Learning Objectives: respiratory tract Bronchi 1. Identify the components of the conducting and respiratory Lungs portions of the respiratory system. 2. Describe and compare external and internal respiration. 3. Identify and describe the other functions of the respiratory Pleura system.

Anatomically, the respiratory system consists of an upper Diaphragm respiratory tract and a lower respiratory tract (figure 25.1). Functionally, it can be divided into a conducting portion, which transports air, and a respiratory portion, where gas exchange with the blood occurs. The conducting portion includes the nose, Figure 25.1 nasal cavity, and pharynx of the upper respiratory tract and the Gross Anatomy of the Respiratory System. The major components larynx, trachea, and progressively smaller airways (from the pri- of the respiratory system are organized into the upper and lower mary bronchi to the terminal bronchioles) of the lower respiratory respiratory tracts. tract. The respiratory portion is composed of small airways called respiratory bronchioles and alveolar ducts as well as air sacs called alveoli in the lower respiratory tract.

25.1a Respiratory System Functions In addition to gas exchange, the respiratory system also func- The primary function most of us associate with the respiratory tions in gas conditioning, sound production, olfaction, and defense. system is , also termed pulmonary ventilation. Breathing consists of two cyclic phases: inhalation (in-hă-lā sh ́ ŭn), also Gas Conditioning called inspiration, and exhalation, also called expiration (eks-pi- As inhaled gases pass through conducting airways, the gases are rā sh ́ ŭ n). Inhalation draws gases into the lungs, and exhalation “conditioned” prior to reaching the gas exchange surfaces of the forces gases out of the lungs. lungs. Specifically, the gases are warmed to body temperature, humidified (moistened), and cleansed of particulate matter through Gas Exchange contact with the respiratory and its sticky mucous cov- The continuous movement of gases into and out of the lungs is ering. Conditioning is facilitated by the twisted pathways through necessary for the process of gas exchange. There are two types the nasal cavity and paranasal sinuses, which cause the inhaled of gas exchange: external respiration and internal respiration. air to become very turbulent during inhalation. This swirling External respiration involves the exchange of gases between the of inhaled gases means the air remains in the nasal cavity and atmosphere and the blood. Oxygen in the atmosphere is inhaled paranasal sinuses for a relatively longer time, providing greater into the lungs. It diffuses from the lungs into the blood within the opportunity for conditioning. cardiovascular system at the same time carbon dioxide diffuses from the blood into the lungs in order to be exhaled. Internal Sound Production respiration involves the exchange of gases between the blood and As air is forced out of the lungs and moves through the larynx, the cells of the body. Blood transports oxygen from the lungs to sound may be produced, such as speech or . Other ana- the body cells and transports carbon dioxide produced by the body tomic structures aid sound production, including the nasal cavity, cells to the lungs. paranasal sinuses, teeth, , and .

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Olfaction columnar epithelium lining much of the upper respiratory tract. The superior region of the nasal cavity is covered with olfac- Mucous housed within the deep to the epi- tory epithelium, which contains receptors for the sense of smell thelium contribute to the layer of covering the epithelium (see chapter 19). These receptors are stimulated when airborne and keep it from drying out. Mucous glands also secrete lysozyme, molecules are inhaled and dissolved in the mucus covering this an enzyme that helps defend against inhaled bacteria. The layer olfactory epithelium. of sticky mucus traps inhaled dust, dirt particles, microorganisms, and pollen. If we are exposed to airborne allergens, large quanti- Defense ties of small particulate material, irritating gases, or pathogens, the Finally, both the structure of the respiratory system and some of rate of production increases. the cells within the protect the body against infection by airborne molecules. The entrance to the respiratory WHATW DID YOU LEARN? system (the nose) is inferiorly directed, is lined with coarse hairs, and has twisted passageways to prevent large particles, micro- ●1 Explain the functions carried out by the respiratory system in addition to gas exchange. organisms, and from entering. Additionally, numerous goblet cells are dispersed throughout the pseudostratified ciliated ●2 How does mucus help with respiratory system functions?

CLINICAL VIEW pulmonary infections are required chronically, because prevention and early treatment of infection are vital to reducing long-term Cystic fibrosis (sis tiḱ f ¯-broı ¯ sis) ́ is the most common serious genetic complications. Absorption problems caused by pancreatic damage disease in Caucasians, occurring with a frequency of 1 in 3200 births. are treated with orally administered digestive enzymes, vitamins, The condition is inherited as an autosomal recessive trait, and is rare and caloric supplements. Since the gene responsible for cystic among people of Asian and African descent. The name cystic fibrosis fibrosis has been identified, scientists have been investigating refers to the characteristic scarring and cyst formation within the pan- ways to insert copies of the healthy gene into the epithelial cells creas, first recognized in the 1930s. Cystic fibrosis affects the organs of the respiratory tracts of cystic fibrosis patients. In the most that secrete mucin, tears, sweat, digestive juices, and saliva. A defec- promising method found thus far, the healthy gene is transmitted tive gene produces an abnormal plasma membrane protein involved in via a modified adenovirus. chloride ion transport, so individuals with cystic f ibrosis cannot secrete chloride. This lack of chloride secretion causes sodium and water to Mucus builds up and move from the mucus back into the secretory cell itself, thus dehydrat- blocks the bronchial ing the mucus covering the epithelial surface. The mucus becomes thick tree, leading to chronic and sticky, obstructing the airways of the lungs and the ducts of the respiratory infections. and salivary glands. In the lungs, the mucus becomes so thick it results in . Pulmonary infections, secondary to airway obstruction, are common and can be life-threatening. In the case of the pancreas, the obstructed ducts lead to a backup of digestive enzymes that eventually destroy the pancreas itself. Interestingly, the normal chloride transport protein works in the opposite direction in the sweat glands of the skin. Chloride and sodium are not reabsorbed from the sweat, and so they become concentrated on the skin in individuals with cystic fibrosis. Mothers of babies with Mucus buildup blocks cystic fibrosis often find that the baby tastes “salty” when kissed. the pancreatic ducts and prevents digestive Thus, clinically elevated chloride levels in sweat are one method of enzymes from entering diagnosing the disease. the .

The primary treatment for cystic fibrosis involves agents that Cystic fibrosis results in thickened mucus that obstructs both the break up the thick mucus in the lungs. In addition, for respiratory passageways and ducts of glands such as the pancreatic ducts.

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located immediately inferior to their corresponding nasal conchae. 25.2 Upper Respiratory Tract As inhaled air passes over constricted, narrow grooves in each meatus, the inhaled air becomes turbulent. Increased turbulence Learning Objectives: ensures that the air remains in the nasal cavity for a longer time, 1. Identify the structures and describe the organization and so that the air becomes warmed and humidified. Because the con- functions of upper respiratory tract organs. chae help produce this turbulence, they are sometimes called the 2. Identify and compare the regions of the pharynx. “turbinate” . Besides functioning in filtration, conditioning, and olfaction, The upper respiratory tract is composed of the nose and the nasal cavity is a resonating chamber that contributes to sound nasal cavity, paranasal sinuses, pharynx, and associated struc- production, discussed later in this chapter. tures. These structures are all part of the conducting portion of the respiratory system. When an individual has an upper respiratory WHAT DO YOU THINK? tract infection, some or all of these structures are involved. ●1 What does it mean if someone has a “deviated septum”? What 25.2a Nose and Nasal Cavity kinds of problems can arise with a deviated septum? The nose is the main conducting airway for inhaled air. The nose is supported superiorly by paired nasal bones that form the bridge 25.2b Paranasal Sinuses of the nose. Anteroinferiorly from the bridge is the fleshy, cartilagi- Four bones of the contain paired air spaces called the para- nous dorsum nasi. The dorsum nasi is supported by one pair of lat- nasal (par-ă-nā s ́ ăl; para = alongside) sinuses, which together eral and two pairs of alar cartilages. Paired , or decrease skull weight. These spaces are named for the bones nares (nā res; ́ sing., nā ris), ́ open on the inferior surface of the nose. in which they are housed; thus, from a superior to inferior direction, The internal surface of the nose leads to the nasal cavity they are the frontal, ethmoidal, sphenoidal, and maxillary sinuses (figure 25.2). The nasal cavity is continuous posteriorly with the (figure 25.3; see also chapter 7). All sinuses communicate with nasopharynx via paired openings called choanae (kō an- ́ ē; sing., the nasal cavity by ducts and are lined with the same pseudostrati- ), or internal nares. The frontal bone, nasal bones, cribri- fied ciliated columnar epithelium as the nasal cavity. form plate of the ethmoid, and sphenoid bone form the roof of the nasal cavity. The palatine process of the maxillae and the horizon- tal plate of the palatine bones form the hard palate, which is the nasal cavity floor. The anterior region of the nasal cavity, near the Study Tip! nostrils, is called the vestibule. The nasal cavity and the paranasal sinuses are the primary struc- The nasal cavity is lined with pseudostratified ciliated tures that warm and humidify the air we inhale. To illustrate this, columnar epithelium. Within this epithelium are numerous goblet breathe through your instead of your nose on a cold day. Your cells that produce mucin, and immediately deep to this epithelium and trachea may feel “raw,” and you may because the air is an extensive vascular network. Near the vestibule are coarse entering the lungs from your mouth is not being properly conditioned. hairs called vibrissae (vı̄-bris ́ē; sing., vibrissa; vibro = to quiver) Repeat this experiment by breathing through your nose. Once the air is that help trap larger particles before they pass through the nasal warmed and humidified in the nasal cavity, it is much easier to breathe. cavity. The most superior part of the nasal cavity contains the olfactory epithelium, which is composed of both a pseudostrati- fied ciliated columnar epithelium and olfactory receptor cells. The divides the nasal cavity into left and right 25.2c Pharynx portions. It is formed anteriorly by septal nasal . A thin, The common space used by both the respiratory and digestive bony sheet formed by the perpendicular plate of the systems is the pharynx (far ́ingks), commonly called the throat (superiorly) and the bone (inferiorly) forms the posterior (figure 25.2). The pharynx is funnel-shaped, meaning that it is part of the nasal septum. slightly wider superiorly and narrower inferiorly. The pharynx Along the lateral walls of the nasal cavity are three paired, originates posterior to the nasal and oral cavities and extends bony projections: the superior, middle, and inferior nasal con- inferiorly to the level of the bifurcation of the larynx and chae (kon ́kē; sing., concha; a shell). These conchae subdivide . For most of its length, the pharynx is the common the nasal cavity into separate air passages, each called a nasal pathway for both inhaled and exhaled air (the respiratory system) meatus (mē -ā t ́ ŭs). The superior, middle, and inferior meatuses are and ingested food (the digestive system).

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Frontal sinus Paranasal sinuses Sphenoidal sinus

Superior Superior meatus Middle meatus Nasal Nasal Inferior meatus cavity cavity Choanae Vestibule Pharyngeal tonsil Opening of auditory tube Hard palate Soft palate Uvula

Oral cavity Nasopharynx Oropharynx Pharynx

Tongue Laryngopharynx

Palatine tonsil Lingual tonsil

Epiglottis Esophagus

Larynx cartilage Trachea Pharynx: Nasopharynx

Oropharynx

Laryngopharynx Ethmoidal sinuses

Sphenoidal sinus Superior meatus (b) Regions of pharynx Middle nasal concha Middle meatus Inferior nasal concha Inferior meatus Vestibule Nasopharynx Hard palate

Soft palate Uvula Tongue Oral cavity Oropharynx Dentures Lingual tonsil

Laryngopharynx

Epiglottis Esophagus

Cricoid cartilage

(a) Sagittal section Figure 25.2 Anatomy of the Upper Respiratory Tract. The upper respiratory tract includes the nose, nasal cavity, paranasal sinuses, and pharynx. (a) A diagrammatic sagittal section and cadaver photo of the show the upper respiratory tract structures and their relationship to the larynx, trachea, and esophagus. (b) The three specific regions of the pharynx (nasopharynx, oropharynx, and laryngopharynx) are highlighted in a diagrammatic sagittal section.

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drink enters the nasopharynx and the nasal cavity. For example, if a person is and laughing at the same time, the soft palate cannot form as good a seal for the nasopharynx. The force Frontal from the laughing and the lack of a good seal may propel some of Ethmoidal the material into the nasal cavity. In other instances, severe vom- Sphenoidal iting can propel material so forcibly that the vomitus is expelled through both the oral cavity and the nasal cavity. In the lateral walls of the nasopharynx, paired auditory Maxillary tubes (eustachian tubes or pharyngotympanic tubes) connect the nasopharynx to the middle . Recall that the auditory tubes equalize air pressure between the middle ear and the atmosphere by allowing excess air pressure to be released into the naso- pharynx. The posterior nasopharynx wall also houses a single pharyngeal tonsil (commonly called the [ad ́ĕ-noydz; aden = , eidos = resemblance]). Oropharynx The middle pharyngeal region, the oropharynx (ō r ́ō -far ́ingks), is immediately posterior to the oral cavity. The oropharynx is bounded by the edge of the soft palate superiorly and by the inferi- Figure 25.3 orly. It is a common respiratory and digestive pathway through which Paranasal Sinuses. The paranasal sinuses are air-filled cavities both air and swallowed food and drink pass. Nonkeratinized strati- named for the bones in which they are found: frontal, ethmoidal, fied squamous epithelium lines the oropharynx because this epithe- sphenoidal, and maxillary. lium is strong enough to withstand the abrasion of swallowed food. The fauces (faw s ́ ē z; throat) is the opening that represents the threshold for entry into the oropharynx from the oral cavity. The pharynx is lined by a mucosa and contains skeletal mus- Two pairs of muscular arches, the anterior palatoglossal arches cles that are primarily used for swallowing. Its flexible lateral walls and the posterior palatopharyngeal arches, form the entrance into are distensible in order to force swallowed food into the esophagus. the oropharynx from the oral cavity. The pharynx is partitioned into three adjoining regions: the naso- Lymphatic organs in the oropharynx provide the “first line pharynx, oropharynx, and laryngopharynx (table 25.1). Figure of defense” against ingested or inhaled foreign materials. The 25.2b color-codes these regions of the pharynx for your reference. palatine tonsils are on the lateral wall between the arches, and the lingual tonsils are at the base of the tongue. Nasopharynx The nasopharynx (nā z ́ ō -far ́ingks) is the superiormost region of Laryngopharynx the pharynx. The nasopharynx is located directly posterior to the The inferior, narrowed region of the pharynx is the laryngopharynx nasal cavity and superior to the soft palate, which separates it from (lă-ring g ́ ō -far ́ingks). It extends inferiorly from the hyoid bone and the posterior part of the oral cavity. It is lined with a pseudostrati- is continuous with the larynx and esophagus. The laryngopharynx fied ciliated columnar epithelium. terminates at the superior border of the esophagus, which is equiva- Normally, only air passes through the nasopharynx. Material lent to the inferior border of the cricoid cartilage in the larynx. In from the oral cavity and oropharynx is typically blocked from fact, the larynx forms the anterior wall of this part of the pharynx. entering the nasopharynx by the soft palate, which elevates when The laryngopharynx is lined with a nonkeratinized stratified squa- we swallow. However, sometimes an accident occurs, and food or mous epithelium since it permits passage of both food and air.

Table 25.1 Regions of the Pharynx Function Epithelial Lining Characteristics Nasopharynx Conducts air Pseudostratifi ed ciliated columnar Posterior to nasal cavity epithelium Pharyngeal tonsil on posterior wall Auditory tubes open into nasopharynx to equalize air pressure in the middle ear

Oropharynx Conducts air; serves as passageway Nonkeratinized stratifi ed squamous Posterior to oral cavity for food and drink epithelium Paired palatine tonsils on lateral walls Lingual tonsils on base of tongue (and thus in anterior region of oropharynx) Extends between soft palate and level of hyoid bone

Laryngopharynx Conducts air; serves as passageway Nonkeratinized stratifi ed squamous Extends from level of hyoid bone to beginning of for food and drink epithelium esophagus (posterior to level of cricoid cartilage in larynx)

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WHATW DID YOU LEARN? with a nonkeratinized stratified squamous epithelium. Inferior to the , the larynx lining becomes a pseudostratified cili- ●3 What changes occur in inhaled gases as they travel through the respiratory system? ated columnar epithelium. The larynx conducts air into the lower respiratory tract, and produces sounds. ●4 What is the function of the nasal conchae? The larynx is supported by a framework of nine pieces of ●5 How is swallowed food prevented from entering the nasopharynx? cartilage (three individual pieces and three cartilage pairs) that are held in place by ligaments and muscles. The largest cartilage is the thyroid cartilage, which forms only the anterior and lat- eral walls of the larynx. It has no posterior component and it is 25.3 Lower Respiratory Tract formed from . A dense band Learning Objectives: called the thyrohyoid membrane attaches the superior border of the thyroid cartilage to the hyoid bone. The V-shaped ante- Identify the structures and describe the organization and 1. rior projection of the thyroid cartilage is called the laryngeal functions of lower respiratory tract organs and regions. (lă-rin ́jē -ăl) prominence (commonly referred to as the “Adam’s Describe the characteristics of the respiratory membrane. 2. apple” in males). The overall growth of the thyroid cartilage is The lower respiratory tract is made up of conducting air- stimulated by testosterone; thus, the Adam’s apple is usually ways (larynx, trachea, bronchi, bronchioles, and their associated prominent and larger in males following puberty. structures) as well as the respiratory portion of the respiratory The ring-shaped cricoid (krı̄ ́koyd; kridos = a ring) cartilage system (respiratory bronchioles, alveolar ducts, and alveoli) (table forms the inferior base of the larynx and connects to the trachea 25.2; see figure 25.1). A lower respiratory tract infection affects inferiorly. The cricoid cartilage is composed of hyaline cartilage. It some or all of these structures. has a narrow anterior region, but its posterior region is wide to sup- port the posterior larynx. A dense regular connective tissue band 25.3a Larynx called the cricothyroid ligament attaches the cricoid cartilage to The larynx (lar ́ingks), also called the voice box, is a short, some- the inferior edge of the thyroid cartilage. what cylindrical airway (figure 25.4). It is continuous superiorly The large, spoon- or leaf-shaped epiglottis (ep-i-glot ́is; epi = with the laryngopharynx, and inferiorly with the trachea; it is ante- on, glottis = mouth of windpipe) is formed primarily of elastic car- rior to the esophagus. The superior aspect of the larynx is lined tilage. The epiglottis projects superiorly into the pharynx from its

Table 25.2 Structures of the Lower Respiratory Tract Structure1 Anatomic Description Wall Support Epithelial Lining Function Larynx Connects to pharynx and Nine pieces of cartilage; Nonkeratinized stratifi ed Conducting: Air trachea; composed of supported by ligaments and squamous epithelium Produces sound cartilage, , skeletal muscle superior to vocal folds; and laryngeal ligaments; also pseudostratifi ed ciliated called the voice box columnar epithelium inferior to vocal folds Trachea Flexible, but semirigid C-shaped cartilage rings Pseudostratifi ed ciliated Conducting: Air tubular connecting columnar epithelium larynx to primary bronchi; incomplete, C-shaped cartilages keep trachea patent (open) Bronchi Largest airways of the Incomplete rings and Larger bronchi lined by Conducting: Air bronchial tree; consist of irregular plates of cartilage; pseudostratifi ed ciliated primary, secondary, tertiary, some columnar epithelium; and smaller bronchi smaller bronchi lined by simple columnar epithelium Bronchioles Smaller conducting airways No cartilage; proportionately Epithelium ranges from Conducting: Air of bronchial tree; larger greater amounts of smooth simple columnar (for the Smooth muscle in bronchioles branch into muscle in walls of terminal largest bronchioles) to walls allows for smaller bronchioles; terminal bronchioles simple cuboidal (for smaller and bronchioles are the last part bronchioles) bronchodilation of the conducting portion Respiratory bronchioles Smallest conducting airways; No cartilage; smooth muscle Simple cuboidal epithelium Respiratory: Gas exchange begin the respiratory portion is scarce in their walls Alveolar ducts Tiny airways that branch No cartilage, no smooth Simple squamous epithelium Respiratory: Gas exchange off respiratory bronchioles; muscle multiple alveoli found along walls of Alveoli Tiny microscopic air sacs No cartilage, no smooth Simple squamous epithelium Respiratory: Gas exchange muscle

1 Structures are listed in the order that air passes through them during inhalation.

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Epiglottis Aryepiglottic fold Epiglottis

Hyoid bone Hyoid bone Thyrohyoid Thyrohyoid membrane membrane Thyrohyoid membrane Cuneiform Adipose Larynx cartilage connective tissue Thyroid cartilage Thyroid Corniculate cartilage cartilage Vestibular ligament Laryngeal Vocal ligament prominence Cricothyroid ligament Cricothyroid ligament Cricoid cartilage Cricoid cartilage

Trachea Tracheal Tracheal cartilage cartilage

(a) Anterior (b) Posterior (c) Midsagittal Figure 25.4 Larynx. The larynx functions primarily to prevent food and fluid from entering the lower respiratory tract. Its secondary function is sound production. Laryngeal anatomy and its relationship to the hyoid bone and trachea are compared in (a) anterior, (b) posterior, and (c) midsagittal views.

attachment to the thyroid cartilage. When a person swallows, the inferior ligaments, called vocal ligaments, are covered by a mucous larynx moves anteriorly and superiorly, causing the epiglottis to close membrane. These ligaments together with their mucosa are called over the laryngeal opening and prevent materials from entering the the vocal folds (figure 25.5). Vocal folds are “true vocal cords” larynx. After swallowing, the larynx returns to its normal position, because they produce sound when air passes between them. The and the epiglottis elevates and returns to its original position. superior ligaments are called vestibular ligaments (see figure The paired arytenoid (ar-i-tē noyd) ́ cartilages have a pyrami- 25.4c). Together with the mucosa covering them, they are called the dal shape, and they rest on the superoposterior border of the cri- vestibular folds (figure 25.6). These folds are “false vocal cords” coid cartilage. The paired corniculate (kō r-nik ́ū -lā t; corniculatus = because they have no function in sound production, but protect the horned) cartilages attach to the superior surface of the arytenoid vocal folds. The vestibular folds attach to the corniculate cartilages. cartilages. The paired cuneiform (kū n ́ ē -i-fō rm; cuneus = wedge) When intrinsic muscles of the larynx make the arytenoid cartilages do not directly attach to any other cartilages. Instead, cartilages pivot, they can abduct or adduct the vocal folds. The they are supported within a mucosa-covered connective tissue opening between the vocal folds is called the rima glottidis (rı̄ m ́ ă; sheet called the aryepiglottic fold. The aryepiglottic fold extends slit; glo-tı̄ -dis). ́ This opening widens if the vocal folds are abducted between the lateral sides of each arytenoid cartilage and the epi- and becomes narrower if the vocal folds are adducted (see fig- glottis to support some of the laryngeal soft tissue structures. ure 25.5). The term glottis (glot ́is) refers to the rima glottidis plus Two groups of laryngeal muscles are located within the the vocal folds. larynx. The intrinsic muscles attach to the arytenoid and cornicu- When air is forced through the rima glottidis, the vocal folds late cartilages. They cause the arytenoid cartilages to pivot, and begin to vibrate, and this vibration produces sound. The non- regulate tension on the vocal folds. The extrinsic muscles are the keratinized stratified squamous epithelium lining the vocal folds (see chapter 11) that attach the hyoid bone to withstands this abrasive contact between the two vocal folds and the thyroid cartilage. They normally stabilize the larynx and help their vibrational activity during sound production. The length, ten- move it during swallowing. sion, and position of the vocal folds determine the characteristics of the sound, as follows: Sound Production Certain structures of the larynx function specifically in sound ■ The range of a voice (be it soprano or bass) is determined production. Two pairs of ligaments extend from the posterior by the length of the vocal folds. Longer vocal folds produce surface of the thyroid cartilage to the arytenoid cartilages. The lower sounds than shorter vocal folds. As we grow, our

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Adducted (closed) vocal folds Abducted (open) vocal folds

Anterior

Thyroid cartilage

Cricoid cartilage

Vocal ligaments

Arytenoid cartilage Corniculate cartilage

Posterior (a) Cartilages and ligaments

Base of tongue

Epiglottis

Vestibular folds Vocal folds

Aryepiglottic fold

Cuneiform cartilage

Corniculate cartilage

Rima glottidis

(b) Laryngoscopic view Figure 25.5 Vocal Folds. The vocal folds (true vocal cords) are epithelium-covered elastic ligaments extending between the thyroid and arytenoid cartilages. These folds surround the rima glottidis and are involved in sound production. Adducted (closed) and abducted (open) vocal folds are shown in (a) a superior view of the cartilages and ligaments only and (b) a diagrammatic laryngoscopic view of the coverings around these cartilages and ligaments.

vocal folds increase in length, which is why our voices become deeper as we mature into adults. Also, both the Study Tip! growth of the thyroid cartilage and the longer and thicker The vocal folds are comparable to the strings of a harp. The short vocal folds in mature males help explain why men typically strings produce the high notes, while the long strings produce the have deeper voices than females. low notes. Thus, shorter vocal folds produce higher notes than longer ■ Pitch refers to the frequency of sound waves, and is vocal folds. determined by the amount of tension or tautness on the vocal folds as regulated by the intrinsic laryngeal muscles. Increasing the tension on the vocal folds causes the vocal folds to vibrate more when air passes by them and produces Keep in mind that recognizable speech also requires the a higher sound. Conversely, the less taut the vocal folds, the participation of the pharynx, nasal and oral cavities, paranasal less they vibrate and the lower the pitch of the sound. sinuses, lips, teeth, and tongue. If you have a stuffy nose, the ■ Loudness depends on the force of the air passing across the quality of your voice changes to a more nasal tone. Try this vocal folds. A lot of air forced through the rima glottidis experiment: Hold your nose and then speak. You will notice that produces a loud sound; a little air forced through the rima your voice sounds quite different when air doesn’t pass through glottidis produces a soft sound. When you whisper, only the nasal cavity. A sinus infection can also cause the sound of the most posterior portion of the rima glottidis is open, and the voice to change as fluid accumulation leads to decreased the vocal folds do not vibrate. Since the vocal folds are not space in the paranasal sinuses for sound resonance. In addition, vibrating, the whispered sounds are all of the same pitch. young children tend to have high, nasal-like voices because their

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Epiglottis CLINICAL VIEW Aspiration of Foreign Bodies, the Heimlich Maneuver, and

A foreign object (e.g., a piece of meat, candy, chewing gum, or grape) Vocal fold may be accidentally aspirated through the laryngeal opening into the Rima glottidis vestibule of the larynx where it becomes trapped. A foreign body trapped in the vestibule will cause the laryngeal muscles to spasm, which tenses the vocal cords. The rima glottidis closes and no air can Aryepiglottic fold enter the trachea. As a result, asphyxiation occurs—and the person will die in about 4 to 5 minutes from lack of oxygen if the obstruction is not dislodged and removed. The Heimlich maneuver should not be performed on anyone that is not . A person is choking if he or she cannot talk, cough, or breathe, and may turn gray or blue. The Heimlich maneuver is based on the concept that, because the lungs still contain air, sudden com- pression with quick thrusts of the just below the diaphragm causes the diaphragm to elevate and compress the lungs, expelling air through the trachea and into the larynx. This maneuver usually dislodges the foreign body from the larynx and propels the foreign Figure 25.6 object back up into the mouth. To perform the Heimlich maneuver Laryngoscopic View of the Larynx. A superior laryngoscopic view on a conscious adult, the rescuer stands behind the affected person, shows the vestibular folds, the vocal folds, and the rima glottidis who may be either sitting or standing. The rescuer makes a fist with opening into the trachea. one hand and places it, thumb toward the person choking, below the xiphoid process and above the umbilicus. The rescuer encircles the affected person’s waist, placing the other hand on top of the fist. In a series of six to ten sharp and distinct thrusts superiorly and inward, the rescuer attempts to develop enough pressure to create CLINICAL VIEW an artificial cough, which will force the foreign object out of the larynx. It may be necessary to repeat the procedure several times before the object is dislodged. If repeated attempts do not free the airway, an emergency cut in the lower larynx () may Laryngitis (lar-in-jı¯ tis)́ is of the larynx that may be necessary. You can also perform the maneuver on yourself if you extend to the surrounding structures. Viral or bacterial infection are alone. To apply the Heimlich maneuver to yourself, make a fist is the number one cause of laryngitis. Less frequently, laryngitis with one hand and place it in the middle of the body at a spot above follows overuse of the voice, such as yelling for several hours at the navel and below the , then grasp the fist with the other a football game. Symptoms include hoarse voice, , and hand and push sharply inward and upward. If this fails, the choking sometimes fever. In severe cases, the inflammation and swell- person should press the upper abdomen over the back of a chair, ing can extend to the epiglottis. In children, whose airways are edge of a table, porch railing or something similar, and thrust up and proportionately smaller, a swollen epiglottis may lead to sudden inward until the object is dislodged. airway obstruction and become a medical emergency. Smaller aspirated foreign bodies (e.g., sunflower seeds, pieces of bone, or tooth chips) sometimes do not get lodged in the larynx and Inflamed vocal folds travel to the bronchi or bronchioles. Due to the vertical orientation of the right , most foreign bodies that get past the larynx and trachea will end up lodged in the right bronchus and lower lobe of the right . In adults, choking or coughing is present in 95% of individuals presenting with . In cases where the foreign object goes into the bronchus, the individual usually can still breathe, but it remains an urgent medical condition and profes- sional help will be required. The medical practitioner will most likely use a procedure called bronchoscopy to locate and remove the object. Bronchoscopy is a procedure during which a practitioner uses a viewing tube (broncho- scope) to evaluate an individual’s trachea and bronchi for abnormali- ties including foreign bodies. The bronchoscope is inserted into the airways, usually through the nose or mouth, or occasionally through Vestibular fold Rima glottidis a tracheostomy. There are two types of bronchoscopes: rigid and A laryngoscopic view shows the inflamed, reddened vocal folds characteristic of laryngitis. flexible. In general, foreign body removal is best done with a rigid bronchoscope under general anesthesia.

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Larynx Thyroid cartilage

Cricoid Layer of mucus cartilage Anular ligament Esophagus Movement Tracheal of mucus cartilage toward of trachea pharynx Trachea Trachea

Pseudostratified ciliated columnar epithelium Lamina C-shaped cartilage propria LM 8x Particles (b) Cross section of debris Carina

Cilia

Left primary bronchus Right primary Pseudostratified bronchus ciliated columnar epithelium

(a) Anterior view (c) Microscopic view of tracheal lining Figure 25.7 Trachea. (a) The trachea connects to the larynx superiorly and the primary bronchi inferiorly. (b) A cross-sectional photomicrograph shows the relationship of the trachea (anteriorly) and the esophagus (posteriorly). The wall of the trachea is supported by C-shaped rings of cartilage. (c) The trachea is lined with a pseudostratified ciliated columnar epithelium that propels mucus and debris away from the lungs and toward the pharynx.

sinuses are not yet well-developed, so they lack large “chambers” loosen material (foreign objects or food materials) from the air where sounds can resonate. A child also has shorter, smaller passageway. vocal folds, which produce a higher voice. When a male goes The mucosa lining the trachea is a pseudostratified ciliated through puberty, his laryngeal cartilages and vocal folds grow columnar epithelium with numerous mucin-secreting goblet cells rapidly, producing the “cracking” voice that eventually leads to a and an underlying lamina propria that houses mucin-secreting deeper voice at maturity. glands (figure 25.7c). The movement of cilia propels mucus laden with dust and dirt particles toward the larynx and the pharynx, 25.3b Trachea where it is swallowed. A submucosal layer deep to the mucosa The trachea (trā ́k ē -ă; rough) is a flexible, slightly rigid tubular contains many submucous glands. organ often referred to as the “windpipe.” The trachea extends At the level of the , the trachea bifurcates into through the and lies immediately anterior to the two smaller tubes, called the right and left primary bronchi esophagus, inferior to the larynx, and superior to the primary (brong ́kı̄; bronchos = windpipe) (or main bronchi). Each primary bronchi of the lungs. The trachea averages approximately 2.5 cen- bronchus projects inferiorly and laterally toward a lung. The most timeters in diameter and 12 to 14 centimeters in length. inferior tracheal cartilage separates the primary bronchi at their The anterior and lateral walls of the trachea are supported origin and forms an internal ridge called the carina (kă-rı̄ n ́ ă; by 15 to 20 C-shaped tracheal cartilages (figure 25.7a). These keel). The right primary bronchus enters the lung more vertically cartilage “rings” reinforce and provide some rigidity to the than the left primary bronchus (figure 25.7a). The right bronchus is tracheal wall to ensure that the trachea remains open (patent) at also wider and shorter than the left. This can be clinically signifi- all times. The cartilage rings are connected by elastic connective cant because aspirated foreign objects are more likely to become tissue sheets called anular (an ́ū -lăr; anulus = ring) ligaments. lodged on the right side (see Clinical View). The open ends of each C-shaped piece are bound together by the trachealis muscle and an elastic, ligamentous membrane WHAT DO YOU THINK? (figure 25.7b). The trachealis distends during swallowing and ●2 In chronic smokers, the lining of the trachea and bronchi changes bulges into the lumen of the trachea to allow for expansion of from a pseudostratified ciliated columnar epithelium to a the esophagus to accommodate larger materials being swal- stratified squamous epithelium. Why do you think this change lowed. During coughing, it contracts to reduce trachea diameter, occurs? What are some consequences of this epithelium in thus facilitating the more rapid expulsion of air and helping to the trachea?

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25.3c Bronchial Tree (or lobar bronchi). The left lung has two secondary bronchi since The bronchial tree is a highly branched system of air-con- it has two lobes; the right lung has three lobes and three sec- ducting passages that originate from the left and right primary ondary bronchi. Secondary bronchi are smaller in diameter than bronchi and progressively branch into narrower tubes as they primary bronchi. They further divide into tertiary bronchi. The diverge throughout the lungs before ending in terminal bron- right lung is supplied by 10 tertiary bronchi, and the left lung is chioles (figure 25.8). Incomplete rings of hyaline cartilage supplied by 8 to 10 tertiary bronchi. (The difference in the num- support the walls of the primary bronchi to ensure that they ber depends upon whether some of the left lung tertiary bronchi remain open. are combined or separate structures.) Each tertiary bronchus The primary bronchi enter the hilum of each lung together is called a segmental bronchus because it supplies a part of the with the pulmonary vessels, lymphatic vessels, and . lung called a , discussed later in Each primary bronchus then branches into secondary bronchi this chapter.

CLINICAL VIEW and Cricothyrotomy hemorrhage. Other potential complications include infection at the site, aspiration of foreign matter directly into the lungs, or tracheal The tracheotomy (tra¯-kē -ot o ́¯-mē ; tome = incision) is one of the oldest stenosis (a narrowing of the trachea at the incision site due to scar surgical procedures in medicine. A tracheotomy is performed when a tissue formation). patient requires extended ventilatory (breathing) assistance based on Cricothyrotomy is an alternative procedure often performed by EMTs one of three recognized indications: and paramedics to open an individual’s airway during certain emer- ■ The presence of an upper airway obstruction due to a foreign gency situations, such as when the throat or larynx are obstructed body, trauma, swelling, etc. by a foreign object or swelling, when an individual who is not able ■ Difficulty breathing due to advanced , emphysema, to breathe adequately on his or her own, or in cases of major facial or severe chest wall injury injury that prevent the insertion of an endotracheal tube through ■ Respiratory paralysis, as may result from head injury, polio, the mouth. A cricothyrotomy is generally performed by making a or tetanus infection vertical incision through the skin and fascia of the anterior just inferior to the thyroid cartilage (care must be taken when making Understanding the surface anatomy of the neck is critical to performing this incision not to cut the anterior jugular , which lie close a tracheotomy correctly. Typically, the physician makes a skin incision together on either side of the midline), then making another trans- about 1 to 1.5 centimeters superior to the . Care must be verse incision through the cricothyroid ligament, which lies deep to taken not to damage the anterior jugular veins, and sometimes the thyroid this incision (since the inferior end of the pharynx and the superior must be incised in the midline and divided to gain access to the trachea. end of the esophagus lie directly behind the cricoid ligament, care Retractors are used to separate the and expose the must be taken to avoid esophageal penetration). Once the incision is trachea. Then the surgeon makes an incision in the trachea between the made, a tube is placed into this opening, which allows air exchange third and fourth tracheal rings to allow the insertion of a tracheotomy to occur. The procedure should not be attempted on children under tube; this opening is called a tracheostomy. Once the tube has been taped 8 years old, if there is evidence of fractured laryngeal cartilages, or into place, the patient’s breathing bypasses the nasal cavity and larynx. if there is evidence of tracheal transection (transverse cut through The tracheotomy is an important and often life-saving procedure, the trachea). Potential complications are similar to tracheotomy and but it is not without risks. A misplaced incision in the anterior neck include bleeding, laryngotracheal injury, tension , and can lead to serious damage of the larynx and possibly even a fatal clogging of the inserted tube with blood or secretions.

Trachea

Thyroid Cricoid Tracheotomy cartilage cartilage tube

Thyroid gland Incision Scalpel Suprasternal Sutures notch

1 Incision is made superior to suprasternal 2 Retractors separate the tissue, and an 3 A tracheotomy tube is inserted, and the notch. Thyroid may have to be cut as well. incision is made through the third and fourth remaining incision is sutured closed. tracheal rings.

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Figure 25.8 Bronchial Tree. (a) The bronchial tree is composed of conducting passageways that originate at the primary bronchi and end at the terminal bronchioles (not visible in this view). Larynx (b) The major components of the bronchial tree are color-coded.

Primary bronchi Trachea Secondary bronchi Tertiary bronchi Smaller bronchi

(b)

Right primary Left primary bronchus bronchus Right superior secondary bronchus Left superior secondary Right middle bronchus secondary bronchus Left tertiary Right inferior bronchus secondary Left inferior bronchus secondary Right tertiary bronchus bronchus

Smaller bronchi Smaller bronchi

(a)

CLINICAL VIEW Chronic bronchitis results from long-term exposure to irritants such as chemical vapors, polluted air, or cigarette smoke. Medically, chronic bron- Bronchitis (brong-kı¯ tis)́ is inflammation of the bronchi caused by chitis is defined as the production of large amounts of mucus, associated viruses or bacteria, or by inhaling vaporized chemicals, particulate with a cough lasting 3 continuous months. If exposure to the irritant matter, or cigarette smoke from the air. Clinically, bronchitis is divided persists, permanent changes to the bronchi occur, including (1) thickened into two categories, acute and chronic. bronchial walls with subsequent narrowing of their lumens, (2) overgrowth (hyperplasia) of the mucin-secreting cells of the bronchi, and (3) accumula- develops rapidly either during or after an infection, tion of within the bronchial walls. These long-term changes such as a cold. Symptoms include cough, wheezing, pain upon inha- in the bronchi increase the likelihood of bacterial infections, and chronic lation, and fever. Most cases of acute bronchitis resolve completely bronchitis greatly increases the chance of developing pneumonia as well. within 10 to 14 days.

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There are approximately 9 to 12 different levels of bronchial approximately 300 to 400 million alveoli. Alveoli abut one another, branch division (not all shown in figure 25.8). Thus, the primary, so their sides become slightly flattened. Thus, an alveolus in secondary, and tertiary bronchi are the first, second, and third cross section actually looks more hexagonal or polygonal in shape generations of bronchi, respectively. All types of bronchi exhibit than circular (see figure 25.9a). The small openings in the walls some common characteristics: between adjacent alveoli, called alveolar pores, provide for collat- eral ventilation of alveoli. The spongy nature of the lung is due to ■ Incomplete rings of cartilage in the walls become less the packing of millions of alveoli together. numerous and smaller, eventually consisting only of Two cell types form the alveolar wall. The predominant cell scattered pieces of cartilage as the bronchi continue to is an alveolar type I cell, also called a squamous alveolar cell. divide and decrease in diameter. This simple squamous epithelial cell promotes rapid gas ■ The largest branches of bronchi are lined by a across the alveolar wall. The alveolar type II cell, called a septal pseudostratified ciliated columnar epithelium, whereas cell, is part of a smaller population of cells within the alveolar wall. smaller branches of bronchi are lined by a ciliated columnar Typically, it displays an almost cuboidal shape. Alveolar type II epithelium. cells secrete pulmonary (ser-fak t ́ ănt), a fluid composed ■ A complete ring of smooth muscle is found between the of lipids and proteins that coats the inner alveolar surface to reduce mucosa of the airways and the cartilaginous support in surface tension and prevent the collapse of the alveoli. Alveolar the wall. macrophages, or dust cells, may be either fixed or free. Fixed The bronchi branch into smaller and smaller that alveolar macrophages remain within the connective tissue of the eventually reach a diameter of less than 1 millimeter. These alveolar walls, while free alveolar macrophages are migratory cells smaller tubules, called bronchioles (brong ́kē -ō l), are no longer that continually crawl within the alveoli, engulfing any microor- lined with pseudostratified ciliated columnar epithelium, but with ganisms or particulate material that has reached the alveoli. The simple columnar or simple cuboidal epithelium. Unlike the small- alveolar macrophages are able to leave the lungs either by entering est bronchi, which have irregular plates of cartilage in their walls, alveolar lymphatics or by being coughed up in (matter the walls of bronchioles contain no cartilage, since their small from the respiratory tract, such as mucus mixed with saliva) and diameter alone prevents collapse. Instead, they have a thicker then expectorated from the mouth. layer of smooth muscle than do bronchi, a characteristic that helps The respiratory membrane is the thin wall between the regulate airway constriction or dilation. Contraction of the smooth alveolar lumen and the blood (see figure 25.10). It consists of the muscle narrows bronchioles (called bronchoconstriction), whereas plasma membranes of an alveolar type I cell, and an endothelial relaxation of the smooth muscle dilates bronchioles (called bron- cell of a , and their fused basement membranes. Oxygen chodilation). Constriction or dilation of the bronchioles regulates diffuses from the lumen of the alveolus across the respiratory the amount of air traveling through the bronchial tree. membrane into the pulmonary capillary, thereby allowing the The terminal bronchioles are the final segment of the con- erythrocytes in the blood to become oxygenated again. Conversely, ducting pathway. They conduct air into the respiratory portion of carbon dioxide diffuses from the blood in the capillary through the the respiratory system. respiratory membrane to enter the alveolus. Once in the alveoli, carbon dioxide is exhaled from the respiratory system into the WHAT DO YOU THINK? external environment.

●3 Can you think of any reasons why you would want your bronchioles to constrict? Why wouldn’t you want your bronchioles fully dilated all the time? Study Tip! To help understand the relationship between the respiratory bron- 25.3d Respiratory Bronchioles, Alveolar Ducts, chiole, alveolar duct, alveolar sac, and alveolus, try this analogy. Visualize and Alveoli a building at your school with multiple wings that radiate from a common As previously stated, the respiratory portion of the respiratory . A hallway connects each wing to the central atrium. Classrooms system consists of respiratory bronchioles, alveolar ducts, and pul- open into the hallways along their length. At the end of each hallway is monary alveoli (figure 25.9). Within this respiratory portion, the an expanded common space lined with more classrooms. (1) The atrium epithelium is much thinner than in the conducting portion, thus is like the respiratory . . . it distributes respiratory gases into facilitating gas diffusion between pulmonary and the every hallway. (2) The hallway leading to each wing is like the alveolar respiratory structures. duct . . . it conducts respiratory gases into each wing. (3) The end of Terminal bronchioles branch to form the respiratory bron- each alveolar duct (hallway) terminates in a large, expanded room (the chioles. Subsequent partitioning of the respiratory bronchioles alveolar sac) lined with multiple classrooms (alveoli). (4) Each classroom forms smaller respiratory bronchioles. Eventually, the smallest is like an alveolus . . . it is where gas exchange occurs. respiratory bronchioles subdivide into thin airways called alveolar ducts, which are lined with a simple squamous epithelium. The distal end of an alveolar duct terminates as a dilated alveolar sac. WHATW DID YOU LEARN? Both of these airways—respiratory bronchioles and alveolar ducts—contain small saccular outpocketings called alveoli (al- ●6 What function is served by the vocal folds? vē ́ō -lı̄; sing., al-vē ́ō -lŭs; alveus = hollow sac). An alveolus is about ●7 What are some anatomic differences between bronchi and 0.25 to 0.5 millimeter in diameter. Its thin wall is specialized to bronchioles? promote diffusion of gases between the alveolus and the blood ●8 How do terminal and respiratory bronchioles differ in structure in the pulmonary capillaries (figure 25.10). The lungs contain and function?

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Branch of Bronchiole pulmonary

Figure 25.9 Terminal bronchiole Bronchioles and Alveoli. Terminal bronchioles branch into respiratory bronchioles, which then branch into alveolar ducts and alveoli. (a) The pulmonary vessels travel with the bronchioles, and the pulmonary capillaries wrap around the alveoli for gas exchange. (b) A photomicrograph shows the relationship of respiratory Respiratory bronchiole bronchioles, alveolar ducts, and alveoli. (c) SEM of a terminal bronchiole, a respiratory bronchiole, alveolar ducts, and alveoli Arteriole Branch of Capillary reveals the honeycomb appearance of the alveoli. pulmonary beds Alveolar duct

Alveoli

Connective tissue

(a)

Terminal bronchiole Respiratory Alveolar bronchiole sac Alveolar ducts Alveoli Alveolar duct Alveoli Respiratory bronchiole

LM 60x SEM 180x

(b) (c)

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Alveolar Nucleus of capillary connective endothelial cell Nucleus tissue of alveolar Erythrocyte type I cell Erythrocyte Capillary

Pulmonary capillaries

Alveolar macrophages

Diffusion of CO2 Diffusion of O2 Alveolar pore Alveolar type II cells

Alveolus Alveolar epithelium Alveolar type I cell Fused basement membranes Respiratory of the alveolar epithelium and membrane the capillary endothelium Capillary endothelium (a) (b) Figure 25.10 Alveoli and the Respiratory Membrane. Gas exchange between the alveoli and the pulmonary capillaries occurs across a thin respiratory membrane. (a) A diagram shows the structural arrangement of several adjacent alveoli. (b) The respiratory membrane consists of an alveolar type I cell, an endothelial cell of a capillary, and their fused basement membranes. Oxygen diffuses from alveoli into the blood within the capillary, and carbon dioxide diffuses in the opposite direction. (Note: The pulmonary surfactant covering layer is not shown here.)

the mediastinum, and the superior surface of the diaphragm are 25.4 Lungs lined by the parietal pleura (figure 25.11). (These pleural layers may also be viewed in the transverse section of the Learning Objectives: shown in figure 22.2d.) The visceral and parietal pleural layers are 1. Identify the structure and describe the function of the pleura. continuous at the hilum of each lung. Between these serous mem- 2. Describe the gross anatomy of the lungs. brane layers is a . When the lungs are fully inflated, 3. Identify and discuss the blood supply to and from the the pleural cavity is a potential space because the visceral and lungs. parietal pleurae are almost in contact with each other. The pleural 4. Discuss the role of lymphatic structures in the function of membranes produce an oily, serous fluid that acts as a lubricant, the respiratory system. ensuring that opposing pleural membrane surfaces slide by each The lungs house the bronchial tree and the respiratory por- other with minimal friction during breathing. tion of the respiratory system. The lungs are located on the lateral 25.4b Gross Anatomy of the Lungs sides of the thoracic cavity and separated from each other by the The paired, spongy lungs are the primary organs of respiration. mediastinum. Each lung has a conical shape. Its wide, concave base rests inferi- WHAT DO YOU THINK? orly upon the muscular diaphragm, and its relatively blunt superior region, called the apex (or cupola), projects superiorly to a point ●4 As you will soon learn, the left lung is physically smaller than the that is slightly superior and posterior to the (figure 25.12). right lung. Why is the left lung smaller? Both lungs are bordered by the thoracic wall anteriorly, laterally, and posteriorly, and supported by the cage. Toward the midline, 25.4a Pleura and Pleural Cavities the lungs are separated from each other by the mediastinum. The outer lung surfaces and the adjacent internal thoracic wall The relatively broad, rounded surface in contact with the tho- are lined by a called pleura (ploor ́ă), which is racic wall is called the costal surface of the lung. The mediastinal formed from simple squamous epithelium called a mesothelium. surface of the lung is directed medially, facing the mediastinum The outer surface of each lung is tightly covered by the visceral and slightly concave in shape. This surface houses the vertical, pleura, while the internal thoracic walls, the lateral surfaces of indented hilum through which the bronchi, pulmonary vessels,

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CLINICAL VIEW Pneumothorax bandage is placed over the entry site to prevent air from reentering the pleural space. Pneumothorax (noo-mo¯-tho¯r aks; ́ pneuma = air) is a condition that A particularly dangerous condition is tension pneumothorax, in occurs when free air gets into the pleural cavity, the space between which a hole in the chest or lung allows air to enter and acts as a the parietal and visceral pleura. A pneumothorax may develop in one of one-way valve. As the patient struggles to breathe, air is pulled in two ways. Air may be introduced externally from a penetrating injury through the wound but cannot escape. Air pressure within the pleural to the chest, such as a knife wound or gunshot, or it may originate space becomes greater, causing of the lung and eventu- internally as when a broken rib lacerates the surface of the lung. ally displacing the and mediastinal structures. Both lungs then The presence of free air in the pleural space sometimes causes the become compressed, and respiratory distress and death occur unless affected lung or a portion of it to deflate, a condition termed ate- the tension pneumothorax is promptly treated. lectasis (at-e¯-lek tá ˘-sis; ateles = incomplete, ektasis = extension). The In addition to air, fluid can also accumulate in the pleural space. For example, collapsed portion of the lung remains down until the air has been blood may collect () due to a lacerated artery, a blood vessel removed from the pleural space. If a pneumothorax is small, the air that leaks as a result of , heart failure, or certain tumors. An accu- exits naturally within a few days. However, a large pneumothorax is a mulation of serous fluid within the pleural cavity is called hydrothorax, medical emergency requiring insertion of a tube into the pleural space and an accumulation of pus, as occurs with pneumonia, is called empyema. to suck out the free air. After the air has been removed, an airtight

forms a groovelike impression on the medial surface of the left lung. Parietal pleura The right lung is subdivided into the superior (upper), mid- Visceral pleura dle, and inferior (lower) lobes by two fissures. The horizontal fis- Pleural cavity sure separates the superior from the middle lobe, while the oblique fissure separates the middle from the inferior lobe. The left lung has only two lobes, superior and inferior, which are subdivided by an oblique fissure. The lingula of the left lung is located on the superior lobe. The lingula is homologous to the middle lobe of the right lung. The left and right lungs may be partitioned into bronchopul- monary segments—10 in the right lung, and typically 8 to 10 in the Parietal pleura left lung (figure 25.13). (The discrepancy in bronchopulmonary segment number for the left lung comes from the merging or lump- Pleural cavity ing of some left bronchopulmonary segments into combined ones by some anatomists.) Each bronchopulmonary segment is supplied by Visceral pleura its own tertiary bronchus and a branch of the and vein. In addition, each segment is surrounded by connective tissue, thereby encapsulating one segment from another and ensuring that each bronchopulmonary segment is an autonomous unit. Thus, if a Diaphragm portion of a lung is diseased, a surgeon can remove the entire bron- chopulmonary segment that is affected, while the remaining healthy bronchopulmonary segments continue to function as before. Figure 25.11 Pleural Membranes. The serous membranes associated with 25.4c Blood Supply To and From the Lungs the lungs are called the pleura. The parietal pleura lines the inner Both the and the bronchial circulation sup- surface of the thoracic cavity, and the visceral pleura covers the outer ply the lungs. Recall from chapter 23 that the pulmonary circula- surface of the lungs. The thin space between these layers is called tion conducts blood to and from the gas exchange surfaces of the the pleural cavity. lungs to replenish its depleted oxygen levels and get rid of excess carbon dioxide (see figures 23.22 and 23.23). Deoxygenated blood is pumped from the right ventricle through the pulmonary trunk lymph vessels, and nerves pass. Collectively, all structures passing into pulmonary , which enter the lung. Thereafter, con- through the hilum are termed the . tinuous branching of these vessels leads to pulmonary capillaries The right and left lungs exhibit some obvious structural dif- that encircle all alveoli. The deoxygenated blood that enters these ferences. Since the heart projects into the left side of the thoracic capillaries becomes oxygenated before it returns to the left atrium cavity, the left lung is slightly smaller than the right lung. The left through a series of pulmonary venules and veins. lung has a medial surface indentation, called the cardiac impres- The bronchial circulation is a component of the systemic sion, that is formed by the heart. The left lung also has an anterior circulation. The bronchial circulation consists of tiny bronchial indented region called the cardiac notch. The descending thoracic arteries and veins that supply the bronchi and bronchioles of the

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Apex

Superior lobe

Horizontal fissure

Oblique fissure Oblique fissure

Middle lobe Cardiac notch Lingula Inferior lobe

Base

Right lung Left lung (a) Lateral views

Apex

Superior lobe

Oblique fissure Oblique fissure

Pulmonary arteries Hilum Hilum Primary bronchi Horizontal fissure

Cardiac impression Pulmonary Middle lobe veins Cardiac notch

Inferior lobe Oblique fissure Oblique fissure Base

Right lung Left lung (b) Medial views Figure 25.12 Gross Anatomy of the Lungs. The lungs are composed of lobes separated by distinct depressions called fissures. (a) Lateral views show the three lobes of the right lung and the two lobes of the left lung. (b) Medial views show the hilum of each lung, where the pulmonary vessels and bronchi enter and leave the lung.

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Apico- Apical posterior Anterior Bronchopulmonary Posterior segments of Superior Bronchopulmonary superior lobe Anterior lingular segments of superior lobe Inferior lingular Medial Bronchopulmonary Superior segments Lateral of middle lobe Superior Lateral basal Posterior Bronchopulmonary basal Bronchopulmonary Posterior segments of segments Lateral basal inferior lobe of inferior lobe basal Anterior Anterior basal basal

Right lung, lateral view Left lung, lateral view Figure 25.13 Bronchopulmonary Segments of the Lungs. The portion of each lung supplied by each tertiary bronchus (represented by different colors) is a bronchopulmonary segment. (The medial basal bronchopulmonary segment cannot be seen from this view.)

lung. This part of the circulation system is much smaller than the exit these lymph nodes and conduct lymph to bronchopulmonary pulmonary system, because most tiny respiratory structures (alve- lymph nodes located at the hilum of the lung. These vessels drain oli and alveolar ducts) exchange respiratory gases directly with the first into tracheobronchial lymph nodes and then into the left and inhaled air. Approximately three or four tiny bronchial arteries right bronchomediastinal trunks (discussed in chapter 24). The branch from the anterior wall of the descending thoracic aorta and right bronchomediastinal trunk drains into the right lymphatic divide to form capillary beds to supply structures in the bronchial duct, while the left bronchomediastinal trunk drains into the tho- tree. Increasingly larger bronchial veins collect venous blood and racic duct. drain into the azygos and hemiazygous systems of veins. WHAT DO YOU THINK? 25.4d Lymphatic Drainage ●5 The lymph nodes of the lung become black and darkened over time Lymph nodes and vessels are located within the connective tis- in both smokers and nonsmokers. Why do these lymph nodes turn sue of the lung as well as around the bronchi and pleura (figure black? 25.14). The lymph nodes collect carbon, dust particles, and pol- lutants that were not filtered out by the pseudostratified ciliated WHATW DID YOU LEARN? columnar epithelium. The lymph from the lung is conducted first to pulmonary lymph nodes within the lungs. Lymphatic vessels ●9 What is the hilum of the lung, and how does it function?

Internal jugular vein Right lymphatic duct Thoracic duct Subclavian vein Bronchomediastinal Bronchomediastinal trunk Figure 25.14 trunk Lung Lymphatic Drainage. Lymph vessels conduct Tracheobronchial lymph nodes lymph to the pulmonary, bronchopulmonary, and tracheobronchial lymph nodes. Bronchopulmonary Bronchopulmonary Lymph is then drained by the lymph nodes bronchomediastinal trunks into Pulmonary the right lymphatic duct or the Pulmonary lymph nodes thoracic duct. lymph nodes

Lymph vessels

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CLINICAL VIEW

Respiratory Pneumonia Alveolar duct Alveoli bronchiole Pneumonia (noo-mo¯ ne ́ ¯-a˘) is an infection of the alveoli of the lung. Common causative agents include viruses and bacteria, and sometimes fungi. The infection may involve an entire lung or just one lobe. Pneumonia results in tissue swelling and accumulated leukocytes in the affected area, thus greatly diminishing the capacity for gas exchange. Pneumonia is a contagious disease that is usually spread by respira- tory droplets. Symptoms include cough, fever, and rapid breathing. In addition, the bronchi produce and expel sputum (mucus and other matter), which may be rust- or green-tinged. Diagnosis of pneumonia depends on symptoms and characteristic changes seen on a chest x-ray. A is often helpful in identifying the specific organism. Treatment may include anti- biotics, respiratory support, and medications to relieve symptoms. LM 30x Patients with severe cases of pneumonia or those with coexisting lung diseases, such as chronic bronchitis or emphysema, may require Normal lung tissues. supplemental oxygen. Thickened Fluid and leukocytes alveolar walls in alveoli

Left lung

Chest x-ray of a patient with pneumonia in the left lung. A normal lung appears as a black space on an x-ray because its spongy LM 75x structure is not dense. In contrast, a pneumonia lung appears white or opaque on an x-ray due to accumulation of fluid and cells. Tissues within a lung affected by pneumonia.

■ Blood transports the carbon dioxide from the body cells to 25.5 Pulmonary Ventilation the lungs. ■ Carbon dioxide is added to the atmosphere during Learning Objective: exhalation. 1. Describe the process of pulmonary ventilation. The movement of gases into and out of the respiratory Breathing, also known as pulmonary ventilation, is the system follows Boyle’s law, which states, “The pressure of a movement of air into and out of the respiratory system. At rest, gas decreases if the volume of the container increases, and vice a normal adult breathes about 16 times per minute, and approxi- versa.” Thus, when the volume of the thoracic cavity increases mately 500 milliliters of air are exchanged with the atmosphere per even slightly during inhalation, the intrapulmonary pressure breath. The airflow exchange is caused by the muscular actions decreases slightly, and air flows into the lungs through the con- associated with inhalation and exhalation, as well as by differ- ducting airways. Therefore, air flows from a region of higher ences in atmospheric air pressure and lung (intrapulmonary) air pressure (the atmosphere) into a region of lower pressure within pressure. Gases are exchanged in the following cycle: the lungs (the intrapulmonary region). Similarly, when the vol- ume of the thoracic cavity decreases during exhalation, the intra- ■ Oxygen in the atmospheric air is drawn into the lungs by pulmonary pressure increases and forces air out of the lungs into inhalation. the atmosphere. ■ Oxygen is transported to the body cells from the lungs by blood circulating through the cardiovascular system. WHATW DID YOU LEARN? ■ Cells use the oxygen and generate carbon dioxide as a waste product. ●10 What is pulmonary ventilation?

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Finally, a slight anterior-posterior dimensional change 25.6 Thoracic Wall Dimensional occurs in the thoracic cavity due to movement of the inferior portion of the sternum. When you inhale, the inferior portion of Changes During External Respiration the sternum moves anteriorly, slightly increasing the anterior- Learning Objective: posterior dimensions of the . When you exhale, the inferior portion of the sternum moves posteriorly and returns to its origi- 1. Define and describe how the thoracic cavity changes in nal position. size and shape during respiration. As you inhale, the dimensions of your thoracic cavity gen- WHATW DID YOU LEARN? erally increase, forming a larger space for the expanding lungs. During exhalation, your thoracic cavity dimensions return to their ●11 What types of dimensional changes occur to the thorax when you original size. Thus, the thoracic cavity becomes larger during inhale, and what muscles are responsible? inhalation and smaller during exhalation, as diagrammed in fig- ure 25.15. Vertical dimensional changes occur with movements of the diaphragm, which forms the rounded “floor” of the thoracic cavity. The diaphragm contracts, causing its depression—that is, its dome-shaped central portion flattens and moves inferiorly to press against the abdominal viscera, resulting in inhalation. When you exhale, the diaphragm relaxes and returns to its origi- Study Tip! nal position. To visualize rib movement during external respiration, think of Lateral dimensional changes occur with rib movements. the thoracic cavity as a bucket and the as the bucket handles. Elevation of the ribs increases the lateral dimensions of the tho- When the bucket handles are lifted up, they move relatively farther racic cavity, while depression of the ribs decreases the lateral away from the edges of the bucket. Thus, the measurement from the dimensions of the thoracic cavity. bucket handles (ribs) to the bucket (thoracic cavity) increases, just as Figure 25.16 shows that several muscles of external respira- the lateral dimensions of the thoracic cavity increase. When the bucket tion move the ribs: handles are depressed, they move next to the edges of the bucket, and so the distance from the bucket handle (ribs) to the bucket (thoracic ■ The help increase thoracic cavity cavity dimension) decreases. dimensions by elevating the first and second ribs during forced inhalation. ■ The external intercostal muscles extend from a superior rib inferomedially to the adjacent inferior rib. The ribs elevate upon contraction of the external intercostals, thereby increasing the transverse dimensions of the thoracic cavity during inhalation. ■ The internal intercostal muscles lie at right angles to the external intercostals and deep to them. Contraction of the internal intercostals depresses the ribs, but this only occurs during forced exhalation. Normal exhalation requires no active muscular effort. ■ A small transversus thoracis (see also figure 11.13) extends Inhalation: Ribs (bucket handles) elevated, across the inner surface of the thoracic cage and attaches to lateral dimension increased ribs 2–6. It helps depress the ribs. Two posterior thoracic muscles also assist with external respiration. These muscles are located deep to the trapezius and latissimus dorsi, but superficial to the erector spinae muscles (see also figure 11.11). The serratus posterior superior elevates ribs 2–5 during inhalation, and the serratus posterior inferior depresses ribs 8–12 during exhalation. In addition, some accessory muscles assist with external respiration activities. The pectoralis minor and sternocleidomas- toid help with forced inhalation, while the abdominal muscles (external and internal obliques, transversus abdominis, and rectus Exhalation: Ribs (bucket handles) depressed, abdominis) assist in active exhalation. (Researchers are still debat- lateral dimension decreased ing the effects of some of the external respiration muscles.)

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Inhalation Exhalation

Thoracic Thoracic cavity cavity

Vertical changes

Figure 25.15 Thoracic Cavity Dimensional Changes Associated with Breathing. The boxlike contracts; vertical Diaphragm relaxes; vertical cavity changes size upon inhalation dimensions of thoracic cavity increase. dimensions of thoracic cavity narrow. and exhalation. During inhalation, the box increases in vertical, lateral, and anterior-posterior dimensions due to movement of the sternum, ribs, and diaphragm, respectively. Upon exhalation, these dimensions decrease, and the thoracic cavity becomes smaller. Lateral changes

Ribs are elevated and thoracic cavity widens. Ribs are depressed and thoracic cavity narrows.

Anterior-posterior changes

Inferior portion of sternum moves anteriorly. Inferior portion of sternum moves posteriorly.

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(a) Inhalation

Scalene muscles elevate 1st and 2nd ribs

External intercostal Inferior part of muscles elevate ribs sternum moves anteriorly

Diaphragm moves inferiorly during contraction

(b) Exhalation

Transversus thoracis depresses ribs Internal intercostal muscles depress ribs Inferior part of sternum moves posteriorly Diaphragm moves superiorly as it relaxes

Figure 25.16 Muscles Involved in External Respiration. (a) Inhalation requires contraction of the external intercostal muscles (to elevate the ribs) and the diaphragm (which moves inferiorly during contraction). Forced inhalation also requires contraction of the scalene muscles, which elevate the first and second ribs. (b) During exhalation, these muscles relax. Additionally, the transversus thoracis and internal intercostal muscles contract to depress the ribs during forced exhalation. Companion x-rays show the thoracic cavity during inhalation and exhalation.

the cardiac nerves) innervate both the heart and the lungs. The 25.7 Innervation of the Respiratory main function of the sympathetic innervation is to open up or dilate the bronchioles (bronchodilation). Parasympathetic innerva- System tion to the lungs is from the left and right vagus nerves (CN X). The Learning Objective: main function of the parasympathetic innervation is to decrease the airway diameter of the bronchioles (bronchoconstriction). 1. Identify the components of the autonomic Collectively, the sympathetic and parasympathetic fibers that regulate ventilation. form the , a weblike network of fibers that The larynx, trachea, bronchial tree, and lungs are inner- surrounds the primary bronchi and enters the lungs at the hilum. vated by the . The autonomic nerves Sensory information about the “stretch” in smooth muscle around that innervate the heart also send branches to these respiratory the bronchial tree is typically conducted by the to the structures (see figure 22.12 for a review of these nerves). The vagus brainstem and then relayed to centers involved with external res- nerve is the primary innervator of the larynx. Damage to one of piration as well as to other reflex centers, such as those involved the vagus nerve branches going to the larynx can cause a person in coughing and sneezing. to have a monotone or a permanently hoarse voice. Sympathetic innervation to the lungs originates from the T1–T5 (or occasionally T2–T5) segments of the . These WHAT DO YOU THINK? preganglionic fibers enter the sympathetic trunk and synapse with ●6 When an inhaler provides relief for bronchoconstriction, is ganglionic neurons. The postganglionic sympathetic fibers (called it mimicking sympathetic or parasympathetic stimulation?

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Stimulation Inhibition

Pneumotaxic center

Pons Apneustic center

Ventral respiratory group (VRG) Respiratory rhythmicity Dorsal respiratory group center (DRG) Medulla oblongata

Figure 25.17 Respiratory Control Centers in the Brainstem. The dorsal respiratory group (DRG) and ventral respiratory group (VRG) within the medulla oblongata Internal regulate normal ventilation rate. The intercostal pons houses the pneumotaxic and muscles apneustic centers, which influence the DRG and VRG. The pneumotaxic center is inhibitory to both respiration and the apneustic center. The apneustic center stimulates the DRG.

External intercostal muscles Diaphragm

25.7a Ventilation Control by Respiratory Centers impulses through both the phrenic and intercostal nerves to stimu- of the Brain late the diaphragm and external intercostal muscles. The apneustic (ap-noo stik) ́ center and the pneumotaxic The involuntary, rhythmic activities that deliver and remove respi- (noo-mō -tak sik) ́ center are nuclei housed within the pons. Both ratory gases are regulated in the brainstem. Regulatory respiratory areas influence the breathing rate by modifying the activity of the centers are located within the reticular formation through both the respiratory rhythmicity center. The apneustic center stimulates medulla oblongata and the pons. The regulatory centers are com- inspiration through the DRG; the pneumotaxic center inhibits both posed of specific nuclei, called the respiratory rhythmicity center, the activity of the DRG and that of the apneustic center. By inhibit- the apneustic center, and the pneumotaxic center (figure 25.17). ing the DRG, the VRG is able to function and initiate the process The respiratory rhythmicity center in the medulla oblongata of forced exhalation. For example, during vigorous exercise when establishes the rate and depth of breathing. Two distinct autonomic your respiratory rate must be increased, respiratory gases must be nuclei form this center. The dorsal respiratory group (DRG) is the exchanged more frequently than when at rest. Thus, the DRG, once inspiratory center that controls inhalation. It controls the motor stimulated, must be inhibited fairly quickly, with the simultaneous neurons that stimulate the muscles of inspiration. The ventral activation of the VRG, so that forced exhalation can occur and the respiratory group (VRG) is the expiratory center for forced exha- next inhalation can begin. lation. It functions only during forced exhalation. During normal quiet breathing, the VRG is inactive, and exhalation is a passive event that does not require nervous stimulation. When the VRG is WHATW DID YOU LEARN? activated, its neurons stimulate accessory respiratory muscles to cause maximal, rapid exhalation—for example, when you exercise ●12 What is the main function of sympathetic innervation to the lungs? and are breathing deeply and forcibly. The DRG is activated dur- ●13 Compare the activities of the DRG and the VRG in the brain’s ing both normal inhalation and forced inhalation. The DRG sends respiratory centers.

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CLINICAL VIEW Asthma industrial pollutants and tobacco smoke are abundant. In most cases, the affected person develops a sensitivity to an airborne agent such Asthma (az ma ́ ˘) is a chronic condition characterized by episodes of as pollen, smoke, mold spores, dust , or particulate matter. bronchoconstriction and wheezing, coughing, , Upon reexposure to this triggering substance, a localized immune and excess pulmonary mucus. Its incidence is increasing among reaction occurs in the bronchi and bronchioles, resulting in bron- young people, particularly those living in urban areas where airborne choconstriction, swollen , and increased production of mucus. Episodes typically last an hour or two. Continual exposure to the triggering agent increases the severity and frequency of asthma attacks. Eventually, the walls of the bronchi and bronchioles may become permanently thickened, leading to chronic and unremitting airway narrowing and shortness of breath. If airway narrowing is extreme during a severe asthma attack, death could occur. Today, the primary treatment for asthma consists of administering inhaled steroids (cortisone-related compounds) to reduce the inflam- matory reaction, combined with to alleviate the bronchoconstriction. Avoidance of the triggering agent is also very Mucus Mucosa important. For some patients, allergy shots have proven helpful. In Submucosa cases of severe asthma, oral doses of steroids may control the allergic hyper-response and reduce the inflammation.

Normal airway

Swollen submucosa Mucosa Individuals suffering from Narrowed airway asthma may need to use Extra mucous inhaled medications to dilate secretion their constricted bronchioles.

Airway during an asthma attack

The resulting reduced capacity for gas exchange can cause an older 25.8 Aging and the Respiratory person to become “short of breath” upon exertion. Finally, as we get older, carbon, dust, and pollution material System gradually accumulate in our lymph nodes and lungs. If a person Learning Objective: also smokes regularly, the lungs become even darker and blacker throughout because of the deposition of carbon particles in the cells. 1. Define and describe the age-related respiratory system Two distinct diseases, emphysema and chronic bronchitis, together changes. encompass chronic obstructive pulmonary disease (COPD), which The respiratory system becomes less efficient with age due is often related to tobacco use. The condition is characterized by to several structural changes. First, aging results in a decrease lung structural abnormalities resulting from inflammation. The in elastic connective tissue in the lungs and the thoracic cavity resulting airflow obstruction makes it hard for the patient to exhale. wall. This loss of elasticity reduces the amount of gas that can be exchanged with each breath and results in a decrease in the WHATW DID YOU LEARN? ventilation rate. In addition, a condition such as emphysema may cause a loss of alveoli or a decrease in their size or functionality. ●14 What are some ways that aging can affect the respiratory system?

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CLINICAL VIEW: In Depth Smoking, Emphysema, cancer, asthma, and infections in the respiratory system. The most and common smoking-related diseases are emphysema and several types of lung cancer. Smoking is one of the most important modifiable factors contributing to disease and premature death in the United States. It significantly Emphysema (em-fi-ze¯ ma ́ ˘ ; en = in, physema = a blowing) is an irrevers- increases the risk and severity of atherosclerosis, and is directly related ible loss of pulmonary gas exchange areas due to inflammation of the to the development of cancers of the lung, esophagus, , and terminal bronchioles and alveoli, in conjunction with the widespread . Current studies also indicate an association between destruction of pulmonary elastic connective tissue. These combined secondhand smoke exposure and an increased risk of bronchitis, ear events lead to an increase in the diameter or dilation of individual infections, and asthma in children. Secondhand smoke is a mixture alveoli, resulting in a decrease in the total number of alveoli, and the of the gases and particulate materials released by the burning of subsequent loss of gas exchange surface area. A person with advanced tobacco in cigarettes, cigars, and pipes, as well as exhaled by smok- emphysema has a larger than normal chest circumference because ers. Unfortunately, secondhand smoke is inhaled by everyone within air is trapped within the abnormally expanded and nonfunctional the environment exposed to it. Potential health-care risks include alveoli. The patient is unable to exhale effectively, so that stagnant,

Dilated, nonfunctional air spaces

(a) Dilated, nonfunctional alveoli Nonsmoker’s lungs.

LM 15x

(b) Emphysema causes dilation of the alveoli and loss of elastic tissue, resulting in poorly functioning alveoli. (a) A gross section of an emphysemic lung shows the dilated alveoli. (b) Microscopically, the alveoli are abnormally large and nonfunctional.

Smoker’s lungs.

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oxygen-poor air builds up within the abnormally large (but numeri- Squamous cell carcinoma cally diminished) alveoli. Most cases of emphysema result from damage caused by smoking. Once the tissue in the lung has been destroyed, it cannot regenerate, and thus there is no cure for emphysema. The best therapy for an emphysema patient is to stop smoking and try to get optimal use from the remaining lung tissue by using a , seeking prompt treatment for pulmonary infections, and taking oxygen supplementation if necessary.

Gross section of a lung with squamous cell carcinoma (speckled white and black regions).

Adenocarcinoma is less common than the squamous cell type. An of the lung arises from the mucin-producing glands in the respiratory epithelium. It begins when DNA injury causes one of these cells to become malignant and begin to divide uncontrollably. Histologically, an adenocarcinoma displays some of the microscopic features of the gland from which it arose, thereby making it distin- guishable from the other forms of lung cancer. Small-cell carcinoma is a less common type of lung cancer that origi- nates in the primary bronchi and eventually invades the mediastinum. An individual with advanced emphysema must rely on a This type of cancer is especially known for its early metastasis to other portable oxygen tank, such as this backpack tank. organs. Small-cell carcinoma arises from the small neuroendocrine cells in the larger bronchi; their secretions help regulate muscle tone in the Lung cancer is a highly aggressive and frequently fatal malignancy that bronchi and vessels. As a consequence of their endocrine heritage, originates in the epithelium of the respiratory system. It claims over some of these tumors secrete hormones. For example, a small-cell 150,000 lives annually in the United States. Smoking causes about 85% cancer of the lung occasionally releases ACTH, producing symptoms of all lung cancers. Metastasis, the spread of cancerous cells to other of Cushing syndrome. tissues, occurs early in the course of the disease, making a surgical cure unlikely for most patients. Pulmonary symptoms include chronic Small-cell cough, coughing up blood, excess pulmonary mucus, and increased carcinoma likelihood of pulmonary infections. Some people are diagnosed based on symptoms that develop after the cancer has already metastasized to a distant site. For example, lung cancer commonly spreads to the brain, so in some cases lung cancer is not discovered until the patient seeks treatment for a seizure disorder related to cancer in the brain. Lung cancers are classified by their histologic appearance into three basic patterns: squamous cell carcinoma, adenocarcinoma, and small- cell carcinoma. Squamous cell carcinoma (kar-si-no¯ ma ́ ˘ ; karkinos = cancer, oma = tumor) is the most common form of lung cancer. At the microscopic level, the pseudostratified ciliated columnar epithelium lining the lungs changes to a sturdier stratified squamous epithelium to with- stand the chronic inflammation and injury caused by tobacco smoke. If the chronic injury continues, these transformed epithelial cells may accumulate enough genetic damage to become overtly malignant. The malignant cells divide uncontrollably, invade the surrounding tissue, Gross section of a lung with small-cell carcinoma (white regions) around a and then spread to distant sites. bronchus.

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25.9 Development of the Respiratory CLINICAL VIEW System Sudden Infant Death Syndrome (SIDS) Learning Objective: Sudden infant death syndrome (SIDS), also known as crib death, 1. Identify and describe how the respiratory system forms in is the sudden and unexplained death of an infant younger than 1 the embryo and fetus. year of age. Approximately 3000 SIDS deaths are reported annu- Early in the fourth week of development, a ventral outgrowth ally in the United States, and most occur in infants between 2 extends from the developing pharynx. This endodermal outgrowth and 4 months of age—about 60% of them males. To be counted is called the respiratory diverticulum (dı̄-ver-tik ́ū -lŭm; byroad), as a SIDS death, the child must die for no apparent reason other or , and it initially maintains communication with the than cessation of breathing. SIDS deaths are thoroughly evaluated pharynx (figure 25.18a). By late in the fourth week, a septum through investigation of the death scene, examination of family forms between the pharynx and the respiratory diverticulum, history, and of the child. partitioning them into two separate tubes. The respiratory diver- Although the definitive cause of SIDS is not known, current ticulum, formed from , undergoes intricate branching to research indicates that SIDS babies have trouble regulating and form the respiratory tree. Surrounding the respiratory diverticu- maintaining blood pressure, breathing, and body temperature. lum is , which later differentiates into the vasculature, Some form of stress, in combination with one or more of these muscle, and cartilage of each lung. three factors, appears to be involved in SIDS deaths. It is now The respiratory diverticulum grows inferiorly and forms known that babies sleeping on their stomachs are at greater risk the future trachea. At the end of the fourth week, the respiratory for SIDS than those sleeping on their backs. Thus, a national diverticulum branches into a left and right primary bronchial “back-to-sleep” (BTS) campaign has arisen and significantly bud. Each bud forms the rudiments of the left and right primary reduced the number of SIDS deaths in the United States. Although bronchi, respectively. Growing branches of the pulmonary arter- some pediatricians report that having babies sleep on their backs ies and veins travel with these developing bronchial buds. By the causes a slight delay in certain developmental milestones, such fifth week of development, the primary bronchial buds branch as sitting up and crawling, these delays typically are short-lived. into secondary bronchial buds (figure 25.18b). The secondary bronchial buds form the secondary bronchi of each lung. Thus, the

Right primary Left primary Pharynx Trachea bronchus bronchus

Right primary Left primary bronchus bronchus

Secondary Esophagus bronchi

Respiratory diverticulum Tertiary bronchi

Mesoderm Right primary Left primary Secondary bronchial bud bronchial bud bronchial buds

(a) Week 4: Respiratory diverticulum and (b) Week 5: Secondary bronchial (c) Week 6: Tertiary bronchi form primary bronchial buds form buds form

Figure 25.18 Development of the Respiratory System. The respiratory system forms as an outgrowth (called the respiratory diverticulum) from the developing pharynx beginning at week 4. (a) Primary bronchial buds appear later during week 4. (b) Secondary bronchial buds branch from the primary bronchi during week 5 and grow into the surrounding mesoderm. (c) By week 6, the tertiary bronchi of the left and right lungs have formed.

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left primary bronchial bud branches into two secondary bronchial which helps keep the alveoli patent (open) and facilitates inflation. buds (since the left lung has two secondary bronchi), and the right Without sufficient surfactant, the alveoli collapse upon exhalation, primary bronchial bud branches into three secondary bronchial and it becomes difficult to reinflate them. Prematurely born infants buds (for the three secondary bronchi of the right lung). sometimes experience respiratory distress due to inadequate pro- In the following weeks, the secondary bronchial buds duction of surfactant. undergo further branching. Week 6 is marked by the development Prior to birth, the respiratory system is nonfunctional because of tertiary bronchi and the rudiments of the bronchopulmonary gas exchange occurs between fetal blood and maternal blood at segments (figure 25.18c). From week 6 to week 16, the respiratory the . The lungs and pulmonary vessels of the fetus are col- tree forms smaller branches until finally the terminal bronchioles lapsed, and so most of the blood is shunted away from the lungs to form. Thus, the conducting portion of the respiratory system has the fetus’s systemic circulation. At birth, the first contraction of the developed by week 16. external intercostal muscles and diaphragm fills the lungs with air. Weeks 16–28 mark the branching and development of respi- (The pressure changes within the thoracic cavity drive the com- ratory bronchioles from the terminal bronchioles. From week 28 mencement of pulmonary circulation.) Blood is sent to the lungs, of development until birth, primitive alveoli (also called terminal where gas exchange occurs, and the newborn relies on its own lungs sacs) develop more profusely. These primitive alveoli have a thick (instead of the mother’s placenta) for respiratory gas exchange. epithelial lining that must thin into simple squamous epithelium in Even after we are born, our lungs continue to produce addi- order for the alveoli to become functional. It isn’t until after week tional primitive alveoli. Some research has indicated that alveoli 28 that this epithelium becomes sufficiently thinned for respira- continue to develop until we are about 8 years old, by which time tion. In addition, by about week 28 the alveolar type II cells start each lung has approximately 300 to 400 million alveoli. Table 25.3 to secrete pulmonary surfactant (described earlier in this chapter), summarizes the events in respiratory system development.

Table 25.3 Summary of Respiratory System Development Week of Development/ Respiratory System Structure Formed Age Early week 4 Respiratory diverticulum forms Late week 4 Primary bronchial buds form Week 5 Secondary bronchial buds form Week 6 Tertiary bronchial buds form Week 6–week 16 Successive branching of tertiary bronchial buds forms smaller bronchi and bronchioles; eventually, terminal bronchioles form; conducting portion of respiratory system is complete Week 16–week 28 Terminal bronchioles branch into respiratory bronchioles Week 28–birth Primitive alveoli form; pulmonary surfactant begins to be produced Birth–8 years Alveoli continue to develop; eventually, adult number of alveoli (300–400 million per lung) is attained

Clinical Terms

decompression sickness (the bends) A condition associated hyaline (hı̄ ́ă-lin; hyalos = glass) membrane disease Disease seen with the rapid decrease in pressures on the body during especially in premature neonates with reduced amounts of underwater ascent. The pressure changes particularly lung surfactant. affect the gases in tissues and those dissolved in the blood. (em ́bō -lizm) Obstruction or occlusion of a Because of the water pressure, body tissue absorbs nitrogen pulmonary vessel by an embolus (foreign material or blood gas faster as a diver descends than when ascending to the clot). surface. However, if a diver ascends too quickly, nitrogen gas tuberculosis (TB) A potentially serious bacterial infection that bubbles will form in body tissue rather than being exhaled. primarily affects the lungs. TB is caused by the bacterium The nitrogen bubbles cause severe pain and can be lethal. Mycobacterium tuberculosis. The bacteria usually infect the epistaxis (ep ́i-stak sis;́ epi = on, stazo = to fall in drops) Bleeding lungs, but they can also damage other parts of the body. from the nose; may be caused by allergies, hypertension, infection, or nasal trauma. Also called nosebleed or nasal hemorrhage.

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Chapter Summary 25.1 General ■ The respiratory system has a conducting portion to convey gas to and from the lungs and a respiratory portion for gas Organization exchange with the blood. and Functions of the Respiratory 25.1a Respiratory System Functions 748 System 748 ■ Respiratory system functions include gas exchange, gas conditioning, sound production, olfaction, and defense.

25.2 Upper ■ The conducting airways of the upper respiratory tract are the nose, nasal cavity, paranasal sinuses, pharynx, and their Respiratory associated structures. Tract 750 25.2a Nose and Nasal Cavity 750 ■ The nasal cavity is the primary site for conditioning inhaled air. It houses three pairs of nasal conchae that cause turbulence in inhaled air, which passes posteriorly into the nasopharynx through the choanae. 25.2b Paranasal Sinuses 750 ■ Paranasal sinuses are paired air spaces in the frontal, ethmoidal, and sphenoidal bones, and the maxillae. They decrease skull bone weight, help condition inhaled air, and contribute to sound resonance. 25.2c Pharynx 750 ■ The pharynx is composed of (1) the nasopharynx, with paired auditory openings on the lateral wall and a pharyngeal tonsil on the posterior wall; (2) the oropharynx, with paired palatine tonsils on the lateral walls and lingual tonsils at the base of the tongue; and (3) the laryngopharynx, which is continuous with the larynx and esophagus.

25.3 Lower ■ The conducting airways of the lower respiratory tract include the larynx, trachea, bronchi, bronchioles to the terminal Respiratory bronchioles, and their associated structures. Its respiratory portions include respiratory bronchioles, alveolar ducts, and Tract 753 alveoli. 25.3a Larynx 753 ■ The larynx conducts air into the trachea and lower respiratory tract, and produces sound. ■ The larynx is composed of cartilage and has paired vocal folds that produce sound when air passes between them. 25.3b Trachea 757 ■ The trachea is lined by pseudostratified ciliated columnar epithelium and has C-shaped tracheal cartilage rings that support the tracheal wall and prevent its collapse. 25.3c Bronchial Tree 758 ■ The bronchial tree conducts respiratory gases from the primary bronchi to the terminal bronchioles. ■ Bronchial tree passageways have cartilage and/or smooth muscle bands to support the walls. The passageway sequence is (1) primary bronchi, (2) secondary bronchi, (3) tertiary bronchi, (4) bronchioles, and (5) terminal bronchioles. 25.3d Respiratory Bronchioles, Alveolar Ducts, and Alveoli 760 ■ Respiratory bronchioles branch from terminal bronchioles and have alveoli outpocketings in their walls. ■ An alveolus is a small, thin sac with two types of cells in its wall. ■ Alveolar type I cells promote rapid gas diffusion; alveolar type II cells secrete pulmonary surfactant. ■ Alveolar macrophages remove inhaled particulate materials from alveolar surfaces. ■ The respiratory membrane consists of alveolar type I cells, an endothelial cell of a capillary, and their fused basement membranes.

25.4 Lungs 762 ■ The lungs are lateral to the mediastinum in the thoracic cavity. 25.4a Pleura and Pleural Cavities 762 ■ The visceral pleura covers the lung outer surface, and the parietal pleura lines the internal thoracic walls; a pleural cavity is sandwiched between the pleural layers. The pleural membranes produce serous fluid. 25.4b Gross Anatomy of the Lungs 762 ■ Lung surfaces include the base (upon the diaphragm), the apex (superior surface), the costal surface (against the thoracic wall), and the mediastinal surface (facing the mediastinum). ■ The hilum is a medial opening through which bronchi, pulmonary vessels, lymph vessels, and nerves enter the lungs. 25.4c Blood Supply To and From the Lungs 763 ■ The pulmonary circulation transports blood to and from the gas exchange surfaces of the lungs, and the bronchial circulation supplies the bronchi and bronchioles. 25.4d Lymphatic Drainage 765 ■ The connective tissue in the lung houses lymph nodes and lymph vessels.

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25.5 Pulmonary ■ Breathing, called pulmonary ventilation, is the movement of air into and out of the respiratory tract. Change in air Ventilation 766 pressure between the atmosphere and the alveoli drives ventilation.

25.6 Thoracic ■ Inhalation causes the thoracic cavity space to increase vertically, laterally, and in an anterior-posterior direction; Wall Dimensional exhalation causes it to return to its original size. Changes During ■ During external respiration, the primary muscles that move the ribs are: for inhalation, (1) scalene, (2) external External intercostal, and (3) serratus posterior superior; for forced exhalation, (4) internal intercostal, (5) transversus thoracis, and Respiration 767 (6) serratus posterior inferior.

25.7 Innervation ■ Sympathetic stimulation causes bronchodilation; parasympathetic stimulation causes bronchoconstriction. of the Respiratory System 769 25.7a Ventilation Control by Respiratory Centers of the Brain 770 ■ The respiratory center in the medulla oblongata has a dorsal respiratory group (DRG) for inspiration and a ventral respiratory group (VRG) for forced expiration. ■ The apneustic and pneumotaxic centers within the pons influence the respiration rate by modifying the activity of the DRG.

25.8 Aging and ■ The respiratory system becomes less efficient with age due to loss of elasticity and loss or decreased size and the Respiratory functionality of alveoli. System 771

25.9 Development ■ Early in the fourth week of development, a respiratory diverticulum forms and leads to primary bronchial buds by late in of the Respiratory that same week. System 774 ■ By the fifth week of development, the primary bronchial buds branch into secondary bronchial buds. These bronchial buds undergo further branching until terminal bronchioles are formed by week 16. Respiratory bronchioles form from the terminal bronchioles during weeks 16–28. ■ Alveoli continue to form from week 28 of development until about 8 years of age, when the adult number of 300 to 400 million alveoli is attained.

Challenge Yourself

Matching Multiple Choice Match each numbered item with the most closely related lettered Select the best answer from the four choices provided. item. ______1. The visceral pleura covers the ______1. nasopharynx a. solid ring of hyaline cartilage a. outer surface of the lung. b. gas exchange surface of the alveoli. ______2. bronchiole b. branches directly from the c. inner wall of the thoracic cavity. ______3. trachea d. lining of the bronchi and bronchioles only. ______4. left lung c. has a cardiac notch and ______2. An area common to both the respiratory and cardiac impression digestive systems through which food, drink, and ______5. cricoid cartilage air pass is the d. phagocytic cell in alveoli ______6. primary bronchus a. nasopharynx. e. contains pharyngeal tonsil b. trachea. ______7. alveolar type II cell c. oropharynx. f. covers laryngeal opening d. glottis. ______8. arytenoid cartilage during swallowing ______3. Which statement is false about the trachea? ______9. alveolar macrophage g. causes air turbulence in nasal a. It is lined with a nonkeratinized stratified ______10. epiglottis cavity squamous epithelium. h. produces pulmonary b. It is continuous superiorly with the larynx. surfactant c. It bifurcates into left and right primary bronchi at the level of the sternal angle. i. lacks cartilage but has d. It contains C-shaped cartilage rings. significant amounts of smooth ______4. Which structure is the last, smallest portion of the muscle in wall conducting portion of the respiratory system? j. vocal folds attach to it a. nasopharynx b. terminal bronchiole c. respiratory bronchiole d. alveolus

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______5. Which is not a function of the paranasal sinuses? 2. What type of epithelium is found in the oropharynx, and a. warm inhaled air why is it well suited to this location? b. responsible for sound resonance 3. What must happen to the vocal folds in order to produce c. gas exchange a higher-pitched sound? A lower-pitched sound? What d. humidify inhaled air produces a louder sound? ______6. The ______cartilage of the larynx forms the 4. What are the components of the bronchial tree, from largest laryngeal prominence. to smallest? a. arytenoid 5. Why is cartilage unnecessary in the walls of the b. cuneiform bronchioles? c. thyroid d. cricoid 6. Why are alveolar type II cells important in maintaining the inflation of the lungs? ______7. The C-shaped cartilages in the trachea a. serve as a point of attachment for some muscles 7. How do the left and right lungs differ anatomically? of expiration. 8. How do the dimensions of the thoracic cavity change when b. support muscular attachments to the thyroid we inhale and exhale? What muscles assist with these cartilage and epiglottis. dimensional changes? c. prevent the trachea from collapsing. 9. Name the autonomic nervous system respiratory centers d. attach the trachea to the esophagus posteriorly. in the pons and the medulla oblongata, and describe their ______8. Which of the following is not a muscle of inspiration? functions. a. diaphragm 10. Contrast the functions and interactions of the DRG and the b. external intercostals VRG in the medulla oblongata. c. rectus abdominis d. scalene Developing Critical Reasoning ______9. The epithelium lining the alveolus is composed of a 1. Charlene has had a bad cold for the last few days. While a. simple squamous epithelium. preparing a presentation for her speech class, she records b. pseudostratified ciliated columnar epithelium. her talk so that she can critique it later. When she listens c. simple cuboidal epithelium. to the recording, her young daughter exclaims, “Mommy d. transitional epithelium. that doesn’t even sound like you. What happened to your ______10. The apneustic center is involved in voice?” How is Charlene’s cold related to the changes in a. inhibition of the pneumotaxic area. her voice? b. stimulation of DRG. 2. Your best friend George is an athletic 20-year-old who c. stimulation of the pneumotaxic area. smokes regularly. George tells you, “Smoking doesn’t affect d. inhibition of VRG. me—I can still run and do the sports I like. All that talk about smoking being dangerous doesn’t apply to me.” Do Content Review you agree with George? What would you tell him about 1. What is the function of the mucous lining of the epithelium the dangers of smoking and some of the conditions he may in the respiratory tract? expect to have later in life?

Answers to “What Do You Think?”

1. A “deviated septum” is off-center, so one side of the nasal 3. The constriction of the bronchioles allows for a more cavity is larger than the other. This alters the normal flow forceful expulsion of air from the lungs, which may help of air through the nose, and if the narrower side becomes dislodge accumulated mucus or inhaled foreign particulate blocked, or sinus problems may result. materials. 2. The epithelium changes because a stratified squamous 4. The left lung is smaller because the heart projects into the epithelium is more sturdy and protective against smoke left side of the thoracic cavity. than a pseudostratified ciliated columnar epithelium. 5. Lymph nodes darken and turn black as they accumulate the Unfortunately, since stratified squamous epithelium lacks dust, particles, and pollution we inhale over a lifetime. cilia and goblet cells, less mucus is produced, and no cilia 6. An asthma inhaler mimics sympathetic stimulation because are present to propel particles away from the bronchi toward it causes bronchodilation. the pharynx. Thus, the main way to eliminate these particles is by coughing, leading to the chronic “smoker’s cough.”

www.mhhe.com/mckinley3 Enhance your study with practice tests and activities to assess your understanding. Your instructor may also recommend the interactive eBook, individualized learning tools, and more.

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