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THE RESPIRATORY SYSTEM I N T R O D U C T I O N Have you ever swallowed something and had it go down the wrong tube, making you cough or choke uncontrollably? This uncomfortable (and sometimes embarrassing) situation occurs because the respiratory and digestive systems both arise from the embryonic gut tube and share the nose, mouth, and throat as a common initial pathway. While the majority of the gut tube gives rise to the digestive system (Chapter 24), the tube that will become the respiratory system forms a highly branched network of respiratory airways that terminate in the lungs. The respiratory tubes have the basic design features shared by all tubular anatomy: an epithelial inner lining, a muscular and connective tissue middle layer, and an outer covering layer of connective tissue. Adaptations of this basic structural plan account for the principal functions associated with the respiratory system—gas transport and gas exchange. Cells continually use oxygen (O2) for the metabolic reactions that release energy from nutrient molecules to produce ATP. At the same time, these reactions release carbon dioxide (CO2). Because an excessive amount of the CO2 that is produced can be toxic to cells, excess CO2 must be eliminated quickly and efficiently by the cardiovascular and respiratory systems. The respiratory system is responsible for gas exchange—intake of O2 and elimination of CO2—and the cardiovascular system transports blood containing the gases between the lungs and body cells. The respiratory system also participates in regulating blood pH, contains receptors for the sense of smell, filters inhaled air, produces sounds, and rids the body of small amounts of water and heat in exhaled air. The branch of medicine that deals with the diagnosis and treatment of disease of the ears, nose, and throat (ENT) is called otorhinolaryngology



rhino-⫽nose; laryngo-⫽voice box, -logy⫽study of ). A pulmonologist (pul-mo¯-NOL-o¯-gist; pulmo-⫽lung) is a specialist in the diagnosis and treatment of diseases of the lungs. •

Did you ever wonder how smoking affects the respiratory system?


CONTENTS AT A GLANCE 23.1 Respiratory System Anatomy 728 • Nose 728 • Pharynx 732 • Larynx 732 • The Structures of Voice Production 735 • Trachea 736 • Bronchi 737 • Lungs 740 • Patency of the Respiratory System 746

23.3 Regulation of Breathing 749 • Role of the Respiratory Center 749 • Regulation of the Respiratory Center 751 23.4 Exercise and the Respiratory System 752 23.5 Development of the Respiratory System 753 23.6 Aging and the Respiratory System 754 Key Medical Terms Associated with the Respiratory System 754

23.2 Mechanics of Pulmonary Ventilation (Breathing) 748 • Inhalation 748 • Exhalation 748


• • • • • • •

Describe the anatomy and histology of the nose. Outline the structure and function of the pharynx. Identify the features and purpose of the larynx. List the structures of voice production. Describe the anatomy and histology of the trachea. Identify the functions of each bronchial structure. Explain how the anatomy of the lungs makes breathing possible.

The respiratory system consists of the nose, nasal cavity, pharynx (throat), larynx (voice box), trachea (windpipe), bronchi, and lungs (Figure 23.1). Structurally, the respiratory system consists of two parts: (1) the upper respiratory system includes the nose, nasal cavity, pharynx, and associated structures; (2) the lower respiratory system includes the larynx, trachea, bronchi, and lungs. Functionally, the respiratory system also consists of two parts. (1) The conducting zone consists of a series of interconnecting cavities and tubes both outside and within the lungs. These passageways include the nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles; their function is to filter, warm, and moisten air and conduct it into the lungs. (2) The respiratory zone consists of tubes and tissues within the lungs where gas exchange occurs. These tubes and tissues include the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli, and are the main sites of gas exchange between air and blood.

unpaired septal nasal cartilage forms the anterior portion of the nasal septum, a partition that divides the external and internal nose into right and left chambers (see Figure 7.9). The septal nasal cartilage is connected to the perpendicular plate of the ethmoid and vomer to form the remainder of the nasal septum. It is also connected to the nasal bones and the lateral nasal cartilages. The paired lateral nasal cartilages form the sides of the midportion of the external nose. They are connected to the nasal bones, maxillae, septal nasal cartilage, and major alar cartilages. The paired major alar cartilages (A¯-lar) form the sides of the inferior portion of the external nose. They are connected to the lateral nasal cartilages and septal nasal cartilage. The major alar cartilages form the medial and lateral borders of the nostrils. When the muscles of the nose contract and relax, the major alar cartilages dilate and constrict the nostrils. Finally, there are three or four small pieces of cartilage posterior to the major alar cartilages called the minor alar cartilages. Because it consists of pliable hyaline cartilage, the cartilaginous framework of the external nose is somewhat flexible. The surface anatomy of the nose is shown in Figure 27.5. CLIN ICA L CON N ECTION | Rhinoplasty


Rhinoplasty (RI¯-no¯-plas⬘-teˉ; rhin⫽nose; -plasty⫽to mold or to shape), commonly called a “nose job,” is a surgical procedure to alter the shape of the external nose. Although rhinoplasty is often done for cosmetic reasons, it is sometimes performed to repair a fractured nose or a deviated nasal septum. With anesthesia, instruments inserted through the nostrils are used to reshape the nasal cartilage and fracture and reposition the nasal bones to achieve the desired shape. An internal packing and splint keep the nose in the desired position while it heals. •

The head has two openings through which substances such as air and food can enter the body—the nose and the mouth. While air can enter through either of these passageways, it is the nose that forms the primary entryway for inhaled air. The nose consists of much more than what you see on someone⬘s face. In fact, the visible part of the nose makes up only about one-fourth of the entire nasal region. The nose is a special organ at the entrance to the respiratory system that is divided into a visible external portion and an internal portion inside the skull called the nasal cavity. The external nose, the skin and muscle-covered portion of the nose visible on the face, is an extension of bone and cartilage with an internal dividing wall and two entryways (the nostrils). The nasal bones project anteriorly to form the upper bony framework or “bridge” of the external nose on which a pair of glasses rest. The cartilaginous framework of the external nose is made up of several pieces of hyaline cartilage connected to each other and to the bones by tough fibrous connective tissue (Figure 23.2a). The

The openings into the external nose are the external nares (NAˉ-reˉz; singular is naris) or nostrils, which lead into cavities about the size of a finger tip called the nasal vestibules. (This is the area that is occupied by a finger that is placed in the nose.) The lower half of each nasal vestibule is lined with skin continuous with the skin of the face. This skin has numerous hairs, with sebaceous and sweat glands that secrete onto its surface. The upper lining of each nasal vestibule transitions into a mucous membrane that continues deeper into the nasal cavity. Deeper into the skull, beyond the region of the nasal vestibules, is the internal nose, also called the nasal cavity. It is a large space in the anterior aspect of the skull that lies inferior to the nasal bone and superior to the oral cavity and forms the majority of the nose. The bony and cartilaginous framework of the nose help to keep the vestibule and nasal cavity patent, that is, open or unobstructed. Anteriorly, the nasal cavity merges with the external nose, and posteriorly the internal nose communicates with the pharynx through





Figure 23.1 Structures of the respiratory system. The upper respiratory system includes the nose, pharynx, and associated structures; the lower respiratory system includes the larynx, trachea, bronchi, and lungs.

1. Provides for gas exchange— intake of O2 for delivery to body cells and removal of CO2 produced by body cells. 2. Helps regulate blood pH.

3. Contains receptors for the sense of smell, filters inspired air, produces sounds (phonation), and excretes small amounts of water and heat.


Pharynx Larynx Trachea

Right main bronchus


(a) Anterior view showing organs of respiration

LARYNX Right common carotid artery

Thyroid gland Anterior scalene muscle


Subclavian artery

Right subclavian artery

Phrenic nerve

Brachiocephalic artery

Left common carotid artery

Superior vena cava

Arch of aorta

Rib (cut) RIGHT LUNG


Heart in pericardial sac


Liver (b) Anterior view of lungs and heart after removal of the anterolateral thoracic wall and pleura

Which structures are part of the conducting portion of the respiratory system?





Figure 23.2 Respiratory structures in the head and neck. As air passes through the nose, it is warmed, filtered, and moistened, and olfaction occurs.



Lateral nasal cartilages Septal nasal cartilage Minor alar cartilages Major alar cartilages

Dense fibrous connective and adipose tissue

(a) Anterolateral view of nose showing cartilaginous and bony frameworks

Parasagittal plane

Superior Nasal meatuses

Middle Inferior

Frontal sinus Frontal bone Olfactory epithelium

Sphenoid bone Sphenoidal sinus

Pharyngeal tonsil

Superior Middle Inferior


Nasal vestibule

Opening of auditory tube

External naris

Internal naris



Oral cavity

Palatine tonsil Fauces

Nasal conchae


Palatine bone Soft palate


Lingual tonsil Epiglottis

Mandible Hyoid bone

LARYNGOPHARYNX Vestibular fold (false vocal cord) Vocal fold (true vocal cord)

Regions of the pharynx



Thyroid cartilage Cricoid cartilage


Thyroid gland (b) Parasagittal section of left side of head and neck showing location of respiratory structures

Nasopharynx Oropharynx Laryngopharynx



Sagittal plane

Frontal sinus Superior nasal concha


Superior nasal meatus Middle nasal concha Sphenoidal sinus

Middle nasal meatus Inferior nasal concha Inferior nasal meatus Hard palate

(c) Medial view of sagittal section

Frontal plane

View Brain

Optic nerve Ethmoidal cells

Periorbital fat Superior nasal concha Superior nasal meatus

Nasal septum: Perpendicular plate of ethmoid

Middle nasal concha Middle nasal meatus Maxillary sinus

Vomer Inferior nasal concha Inferior nasal meatus Hard palate


(d) Frontal section showing conchae

C LIN ICA L CON N ECTION | Tonsillectomy Tonsillectomy (ton-si-LEK-to¯-meˉ; -ektome⫽excision or to cut out) is surgical removal of the tonsils. The procedure is usually performed under general anesthesia on an outpatient basis. Tonsillectomies are performed in individuals who have frequent tonsillitis (ton⬘-si-Ll¯-tis), inflammation of the tonsils; those who have tonsils that develop an abcess or tumor; or when the tonsils obstruct breathing during sleep.

What is the path taken by air molecules into and through the nose?




two openings called the internal nares or choanae (koˉ-A-neˉ) (Figure 23.2b). Ducts from the paranasal sinuses (frontal, sphenoidal, maxillary, and ethmoidal paranasal sinuses) and the nasolacrimal ducts, which drain tears from the lacrimal glands, also open into the nasal cavity (see Figure 7.13). The lateral walls of the nasal cavity are formed by the ethmoid, maxillae, lacrimal, palatine, and inferior nasal conchae bones (see Figure 7.7); the ethmoid also forms the roof. The horizontal plates of the palatine bones and palatine processes of the maxillae, which together constitute the hard palate, form the floor of the nasal cavity. The nasal cavity is divided into two regions—the large inferior respiratory region and the small superior olfactory region. The nasal cavity, like the nasal vestibules, is divided by an intermediate nasal septum into right and left halves. A strong impact to the nasal region can break the delicate nasal septal bones or separate the cartilage portion of the nasal septum from the bony portion. During the healing process the bones and cartilage can become displaced to one side, resulting in a deviated septum. This displaced septum can lead to a narrowing of one side of the nasal cavity. This makes it more difficult to breathe through that side of the nose. Three shelves called conchae, formed by projections of the superior, middle, and inferior nasal conchae, extend out of each lateral wall of the nasal cavity. The conchae, almost reaching the bony nasal septum, subdivide each side of the nasal cavity into a series of groove-like passageways—the superior, middle, and inferior nasal meatuses (meˉ-Aˉ-tus-eˉz⫽openings or passages). A mucous membrane lines the nasal cavity and its shelves. The arrangement of conchae and meatuses increases surface area in the nasal cavity and prevents dehydration by acting as a baffle that traps water droplets during exhalation. The olfactory receptor cells, supporting cells, and basal cells lie in the olfactory region, the membrane lining the superior nasal conchae, and adjacent nasal septum. This region is called the olfactory epithelium (see Figure 21.1). It contains cilia but no goblet cells. Inferior to the olfactory epithelium, the mucous membrane contains capillaries and pseudostratified ciliated columnar epithelium with many goblet cells; this epithelium in the respiratory region is called the respiratory epithelium. As inhaled air whirls around the conchae and meatuses, it is warmed by blood circulating in the abundant capillaries. Mucus secreted by the goblet cells moistens the air and traps dust particles. Drainage from the nasolacrimal ducts and perhaps secretions from the paranasal sinuses also help moisten the air. The cilia move the mucus and trapped dust particles toward the pharynx, at which point they can be removed (i.e., swallowed or spit out) from the respiratory tract. In summary, the interior structures of the nose have three functions: (1) warming, moistening, and filtering incoming air; (2) detecting olfactory (smell) stimuli; and (3) modifying speech vibrations as they pass through the large, hollow resonating chambers. Resonance refers to prolonging, amplifying, or modifying a sound by vibration. CHECKPOINT

1. What functions do the respiratory and cardiovascular systems have in common? 2. How do the the structures and functions of the upper and lower respiratory systems differ? 3. What is the difference between the structures and functions of the external nose and the internal nose?

the level of the cricoid cartilage, the most inferior cartilage of the larynx (voice box) (Figure 23.2). The pharynx lies just posterior to the nasal and oral cavities, superior to the larynx and esophagus, and just anterior to the cervical vertebrae. Its wall is composed of skeletal muscles and is lined with a mucous membrane. Relaxed skeletal muscles help keep the pharynx patent. Contraction of the skeletal muscles assists in deglutition (swallowing). The pharynx functions as a passageway for air and food, provides a resonating chamber for speech sounds, and houses the tonsils, which participate in immunological reactions against foreign invaders. The pharynx can be divided into three anatomical regions: (1) nasopharynx, (2) oropharynx, and (3) laryngopharynx. (See the lower orientation diagram in Figure 23.2b.) The superior portion of the pharynx, called the nasopharynx, lies posterior to the nasal cavity and extends to the plane of the soft palate. The soft palate, which forms the posterior portion of the roof of the mouth, is an arch-shaped muscular partition between the nasopharynx and oropharynx that is covered by mucous membrane. There are five openings in the wall of the nasopharynx: two internal nares, two openings that lead into the auditory (pharyngotympanic) tubes (commonly known as the eustachian tubes), and the single opening into the oropharynx. The posterior wall also contains the pharyngeal tonsil (fa-RIN-jeˉ-al) or adenoid. Through the internal nares, the nasopharynx receives air from the nasal cavity and receives packages of dust-laden mucus. The nasopharynx is lined with pseudostratified ciliated columnar epithelium, and the cilia move the mucus down toward the most inferior part of the pharynx. The nasopharynx also exchanges small amounts of air with the auditory tubes to equalize air pressure between the middle ear and the atmosphere. The intermediate portion of the pharynx, the oropharynx, lies posterior to the oral cavity and extends from the soft palate inferiorly to the level of the hyoid bone. In addition to communicating upward with the nasopharynx and downward with the laryngopharynx, it has an anterior opening, the fauces (FAWseˉz ⫽ throat), the opening from the mouth. This portion of the pharynx has both respiratory and digestive functions because it is a common passageway for air, food, and drink. Because the oropharynx is subject to abrasion by food particles, it is lined with nonkeratinized stratified squamous epithelium. Two pairs of tonsils, the palatine tonsils and lingual tonsils, are found in the oropharynx. The inferior portion of the pharynx, the laryngopharynx (la-rin⬘-goˉ -FAR-inks), or hypopharynx, begins at the level of the hyoid bone. At its inferior end, it opens into the esophagus (food tube) posteriorly and the larynx (voice box) anteriorly. Like the oropharynx, the laryngopharynx is both a respiratory and a digestive pathway and is lined by nonkeratinized stratified squamous epithelium. The arterial supply of the pharynx includes the ascending pharyngeal artery, the ascending palatine branch of the facial artery, the descending palatine and pharyngeal branches of the maxillary artery, and the muscular branches of the superior thyroid artery. The veins of the pharynx are similar in name to the arteries and drain into the pterygoid plexus and the internal jugular veins. Most of the muscles of the pharynx are innervated by nerve branches from the pharyngeal plexus supplied by the glossopharyngeal (IX) and vagus (X) nerves.



The pharynx (FAR-inks), or throat, is a funnel-shaped tube about 13 cm (5 in.) long that starts at the internal nares and extends to

The larynx (LAIR-inks), or voice box, is a short passageway that connects the laryngopharynx with the trachea. It lies in the



C L INIC AL C ON N E C T ION | Coryza, Seasonal Influenza, and H1N1 Influenza Hundreds of viruses can cause coryza (ko¯-Rl¯-za) or the common cold, but a group of viruses called rhinoviruses (Rl¯-no¯-vı¯ -rus-es) is responsible for about 40 percent of all colds in adults. Typical symptoms include sneezing, excessive nasal secretion, dry cough, and congestion. The uncomplicated common cold is not usually accompanied by a fever. Complications include sinusitis, asthma, bronchitis, ear infections, and laryngitis. Recent investigations suggest an association between emotional stress and the common cold. The higher the stress level, the greater the frequency and duration of colds. Seasonal influenza (flu) is also caused by a virus. Its symptoms include chills, fever (usually higher than 101⬚F ⫽ 39⬚C), headache, and muscular aches. Seasonal influenza can become life-threatening and may develop into pneumonia. It is important to recognize that influenza is a respiratory disease, not a gastrointestinal (GI) disease. Many people mistakenly report having seasonal flu when they are suffering from a GI illness. H1N1 influenza (flu), also known as swine flu, is a type of influenza caused by a virus called influenza H1N1. The term swine flu originated because early laboratory testing indicated that many of the genes in the new virus were similar to ones found in pigs (swine) in North America. However, subsequent testing revealed that the new virus is very different from the one that circulates in North American pigs. H1N1 flu is a respiratory disorder first detected in the United States in April 2009. In June 2009, the World Health Organization declared

H1N1 flu to be a global pandemic disease (a disease that affects large numbers of individuals in a short period of time and that occurs worldwide). The virus is spread in the same way that seasonal flu spreads: from person-to-person through coughing or sneezing or by touching infected objects and then touching one’s mouth or nose. Most individuals infected with the virus have mild disease and recover without medical treatment, but some people have severe disease, and some have even died. The symptoms of H1N1 flu include fever, cough, runny or stuffy nose, headache, body aches, chills, and fatigue. Some people also have vomiting and diarrhea. Most people who have been hospitalized for H1N1 flu have had one or more preexisting medical conditions such as diabetes, heart disease, asthma, kidney disease, or pregnancy. People infected with the virus can infect others from one day before symptoms occur to 5–7 days or more after they occur. Treatment of H1N1 flu involves taking antiviral drugs, such as Tamiflu and Relenza. A vaccine is also available. But the H1N1 flu vaccine is not a substitute for seasonal flu vaccines. In order to prevent infection, the Centers for Disease Control and Prevention (CDC) recommends washing your hands often with soap and water or with an alcohol-based hand cleaner; covering your mouth and nose with a tissue when coughing or sneezing and disposing of the tissue; avoiding touching your mouth, nose, or eyes; avoiding close contact (within six feet) with people who have flu-like symptoms; and staying home for seven days after symptoms begin or after being symptom-free for 24 hours, whichever is longer. •

midline of the neck anterior to the fourth through sixth cervical vertebrae (C4–C6). The wall of the larynx is composed of nine pieces of cartilage (Figure 23.3). Three occur singly (thyroid cartilage, epiglottis, and cricoid cartilage), and three occur in pairs (arytenoid, cuneiform, and corniculate cartilages). Of the paired cartilages, the arytenoid cartilages are the most important because they influence the posi-

tions and tensions of the vocal folds (true vocal cords). The extrinsic muscles of the larynx connect the cartilages to other structures in the throat; the intrinsic muscles connect the cartilages to each other (see Figures 11.9 and 11.10). The cavity of the larynx is the space that extends from the laryngeal entrance to the inferior border of the cricoid cartilage. The portion of the cavity of the larynx above the vestibular folds is called the laryngeal vestibule. The portion of the cavity of the larynx below the vocal folds is called the infraglottic cavity (infra⫽below).

Figure 23.3 Larynx. The larynx is composed of nine pieces of cartilage.

Epiglottis Hyoid bone Thyrohyoid membrane Epiglottis: Leaf Stem Larynx

Corniculate cartilage

Thyroid gland

Thyroid cartilage (Adam’s apple) Arytenoid cartilage Cricothyroid ligament Cricoid cartilage Cricotracheal ligament Thyroid gland Parathyroid glands (4) Tracheal cartilage

(a) Anterior view

(b) Posterior view

F I G U R E 23. 3




F I G U R E 23.3



Epiglottis Hyoid bone Thyrohyoid membrane Thyrohyoid membrane Cuneiform cartilage Sagittal plane

Fat body

Corniculate cartilage Vestibular fold

Arytenoid cartilage

Thyroid cartilage Laryngeal sinus

Vocal fold

Cricoid cartilage

Cricothyroid ligament

Cricotracheal ligament Tracheal cartilage

(c) Sagittal section Frontal plane


Epiglottic cartilage Thyrohyoid muscle Hyoid bone

Laryngeal vestibule

Vestibular fold

Thyrohyoid membrane Thyroepiglottic muscle

Vocal fold

Rima vestibuli Vocalis muscle Laryngeal ventricle

Inferior pharyngeal constrictor muscle

Thyroid cartilage

Sternothyroid muscle Rima glottidis Cricoid cartilage

Infraglottic cavity

Lateral cricoarytenoid muscle Cricothyroid muscle

Cricotracheal ligament First tracheal cartilage

Trachea Thyroid gland Parathyroid gland

How does the epiglottis prevent aspiration of foods and liquids?

The thyroid cartilage, the largest cartilage of the larynx, consists of two fused plates of hyaline cartilage that form the upper anterior and lateral walls of the larynx and give it a triangular shape. The anterior junction of the two plates forms the laryngeal prominence (Adam’s apple). It is usually larger in males than in females due to the influence of male sex hormones on its growth during puberty. Above the prominence is a V-shaped notch that can be palpated with your fingertip. The ligament that connects the thyroid cartilage to the hyoid bone just superior to it is called the thyrohyoid membrane.

(d) Frontal section

The epiglottis (epi-⫽over; glottis⫽tongue) is a large, leaf-shaped piece of elastic cartilage that is covered with epithelium (Figure 23.3b, c, h). The “stem” of the epiglottis is the tapered inferior portion that is attached to the anterior rim of the thyroid cartilage. The broad superior “leaf” portion of the epiglottis is unattached and is free to move up and down like a trap door. During swallowing, the pharynx and larynx rise. Elevation of the pharynx widens it to receive food or drink; elevation of the larynx causes the epiglottis to move down and form a lid over the opening into the larynx, closing it off. The narrowed passageway through the larynx is called the


glottis. The glottis consists of a pair of folds of mucous membrane, the vocal folds in the larynx, and the space between them called the rima glottidis (RI¯-ma GLOT-ti-dis; Figure 23.3h). The closing of the larynx during swallowing routes liquids and foods into the esophagus and keeps them out of the larynx and airways. When small particles of dust, smoke, food, or liquids pass into the larynx, a cough reflex occurs, usually expelling the material. The cricoid cartilage (KRI¯-koyd⫽ringlike) is a ring of hyaline cartilage that forms the inferior wall of the larynx. It is attached to the first ring of cartilage of the trachea by the cricotracheal ligament (kri¯⬘-ko¯-TRA¯-ke¯-al). The thyroid cartilage is connected to the cricoid cartilage by the cricothyroid ligament. The cricoid cartilage is the landmark for making an emergency airway called a tracheotomy. The paired arytenoid cartilages (ar⬘-i-TE¯-noyd⫽ladle-like) are triangular pieces of mostly hyaline cartilage located at the posterior, superior border of the cricoid cartilage. They form synovial joints with the cricoid cartilage and have a wide range of mobility. The paired corniculate cartilages (kor-NIK-u¯-la¯t⫽shaped like a small horn), horn-shaped pieces of elastic cartilage, are located at the apex of each arytenoid cartilage. The paired cunei¯ -ne¯-i-form⫽wedge-shaped) are club-shaped form cartilages (KU elastic cartilages anterior to the corniculate cartilages at the lateral aspect of the epiglottis. The lining of the larynx superior to the vocal folds is nonkeratinized stratified squamous epithelium. The lining of the larynx inferior to the vocal folds is pseudostratified ciliated columnar


epithelium consisting of ciliated columnar cells, goblet cells, and basal cells. The mucus secreted by these cells helps trap dust not removed in the upper passages. In contrast to the action of the cilia in the upper respiratory tract, which move mucus and trapped particles down toward the pharynx, the cilia in the lower respiratory tract move the mucus up toward the pharynx. Substances in cigarette smoke inhibit movement of cilia. If the cilia are paralyzed, only coughing can remove mucus–dust packages from the airways. This is why smokers cough so much and are more prone to respiratory infections.

The Structures of Voice Production The mucous membrane of the larynx forms two pairs of folds (Figure 23.3c): a superior pair called the vestibular folds (false vocal cords) and an inferior pair called simply the vocal folds (true vocal cords). The space between the vestibular folds is known as the rima vestibuli. The laryngeal ventricle (sinus) is a lateral expansion of the middle portion of the laryngeal cavity; it is bordered superiorly by the vestibular folds and inferiorly by the vocal folds. While the vestibular folds do not function in voice production, they do have other important functional roles. When the vestibular folds are brought together, they function in holding the breath against pressure in the thoracic cavity, such as might occur when a person strains to lift a heavy object. The vocal folds are the principal structures of voice production (Figure 23.4). Deep to the mucous membrane of the vocal folds, which is nonkeratinized stratified squamous epithelium, are bands

Figure 23.4 Movement of the vocal folds. The glottis consists of a pair of folds of mucous membrane, the vocal folds in the larynx, and the space between them (the rima glottidis). Tongue

Thyroid cartilage

Epiglottis Glottis: Vocal folds

Cricoid cartilage Vocal ligament

Rima glottidis Vestibular folds

Arytenoid cartilage

Superior view of cartilages and muscles

Posterior cricoarytenoid muscle

Cuneiform cartilage Corniculate cartilage View through a laryngoscope

(a) Movement of vocal folds apart (abduction)


Larynx Lateral cricoarytenoid muscle (b) Movement of vocal folds together (adduction)

What is the main function of the vocal folds?




of elastic ligaments stretched between the rigid cartilages of the larynx like the strings on a guitar. Intrinsic laryngeal muscles attach to both the rigid cartilages and the vocal folds. When the muscles contract they move the cartilages, which pulls the elastic ligaments tight; this stretches the vocal folds out into the airways, narrowing the rima glottidis. Contracting and relaxing the muscles varies the tension in the vocal folds, much like loosening or tightening a guitar string. Air passing through the larynx vibrates the folds and produces sound (phonation) by setting up sound waves in the column of air in the pharynx, nose, and mouth. The variation in the pitch of the sound is related to the tension in the vocal folds. The greater the pressure of air, the louder the sound produced by the vibrating vocal folds. When the intrinsic muscles of the larynx contract, they pull on the arytenoid cartilages, which causes the cartilages to pivot and slide. Contraction of the posterior cricoarytenoid muscles, for example, moves the vocal folds apart (abduction), opening the rima glottidis (Figure 23.4a). By contrast, contraction of the lateral cricoarytenoid muscles moves the vocal folds together (adduction), closing the rima glottidis (Figure 23.4b). Other intrinsic muscles can elongate (and place tension on) or shorten (and relax) the vocal folds. Pitch is controlled by the tension on the vocal folds. If they are pulled taut by the muscles, they vibrate more rapidly, and a higher vocal pitch results. Decreasing the muscular tension on the vocal folds produces lower-pitch sounds. Due to the influence of androgens (male sex hormones), the vocal folds are usually thicker and longer in males than in females, and therefore they vibrate more slowly. Thus, men’s voices generally have a lower range of pitch than women’s. Sound originates from the vibration of the vocal folds, but other structures are necessary for converting the sound into recognizable speech. The pharynx, mouth, nasal cavity, and paranasal sinuses all act as resonating chambers that give the voice its human and individual quality. We produce the vowel sounds by constricting and relaxing the muscles in the wall of the pharynx. Muscles of the face, tongue, and lips help us enunciate words.

Whispering is accomplished by closing all but the posterior portion of the rima glottidis. Because the vocal folds do not vibrate during whispering, there is no pitch to this form of speech. However, we can still produce intelligible speech while whispering by changing the shape of the oral cavity as we enunciate. As the size of the oral cavity changes, its resonance qualities change, which imparts a vowel-like pitch to the air as it rushes toward the lips. The arteries of the larynx are the superior and inferior laryngeal arteries. The superior and inferior laryngeal veins accompany the arteries. The superior laryngeal vein empties into the superior thyroid vein, and the inferior laryngeal vein empties into the inferior thyroid vein. The nerves of the larynx are both branches of the vagus (X) nerve. The superior laryngeal nerve enters the larynx from above, and the recurrent laryngeal nerve ascends through the base of the neck to enter the larynx from below.


Laryngitis and Cancer of the Larynx

Laryngitis is an inflammation of the larynx that is most often caused by a respiratory infection or irritants such as cigarette smoke. Inflammation of the vocal folds causes hoarseness or loss of voice by interfering with the contraction of the folds or by causing them to swell to the point where they cannot vibrate freely. Many long-term smokers acquire a permanent hoarseness from the damage done by chronic inflammation. Cancer of the larynx is found almost exclusively in individuals who smoke. The condition is characterized by hoarseness, pain on swallowing, or pain radiating to an ear. Treatment consists of radiation therapy and/or surgery. •

Trachea The trachea (TRA¯-ke¯-a⫽sturdy), or windpipe, is a tubular passageway for air that is about 12 cm (5 in.) long and 2.5 cm (1 in.) in diameter. It is located anterior to the esophagus (Figure 23.5) and

Figure 23.5 Location of the trachea in relation to the esophagus. The trachea is anterior to the esophagus and extends from the larynx to the superior border of the fifth thoracic vertebra. Esophagus Trachea ANTERIOR

Transverse plane

Tracheal cartilage

Right lateral lobe of thyroid gland Left lateral lobe of thyroid gland

Trachealis muscle in fibromuscular membrane Esophagus POSTERIOR

Superior view of transverse section of thyroid gland, trachea, and esophagus

What is the benefit of not having complete rings of tracheal cartilage between the trachea and the esophagus?


extends from the larynx to the superior border of the fifth thoracic vertebra (T5), where it divides into the right and left main bronchi (see Figure 23.6). The layers of the tracheal wall, from deep to superficial, are (1) the mucosa, (2) the submucosa, (3) the fibromusculocartilaginous layer, and (4) the adventitia. The mucosa of the trachea consists of an epithelial layer of pseudostratified ciliated columnar epithelium and an underlying layer of lamina propria that contains elastic and reticular fibers. The epithelium consists of ciliated columnar cells and goblet cells that reach the luminal surface, plus basal cells that do not (see Table 3.1E). The epithelium provides the same protection against dust as the membrane lining the nasal cavity and larynx. The submucosa consists of areolar connective tissue that contains seromucous glands and their ducts. In the fibromusculocartilaginous layer, the 16–20 incomplete horizontal rings of hyaline cartilage resemble the letter C. The rings are stacked one above another and are joined together by dense connective tissue. They may be felt through the skin inferior to the larynx. The open part of each C-shaped cartilage ring faces posteriorly toward the esophagus (Figure 23.5) and is spanned by a fibromuscular membrane. Within this membrane are transverse smooth muscle fibers, called the trachealis muscle (tra¯-ke¯-A-lis), and elastic connective tissue that allows the diameter of the trachea to change subtly during inhalation and exhalation, which is important in maintaining an efficient flow of air. The solid C-shaped cartilage rings provide a semi-rigid support to maintain patency so that the tracheal wall does not collapse inward (especially during inhalation) and obstruct the air passageway. The most superficial layer of the trachea, the adventitia, consists of areolar connective tissue that joins the trachea to surrounding tissues.


Tracheotomy and Intubation

Several conditions may block airflow by obstructing the trachea. The tracheal cartilage may be accidentally crushed, the mucous membrane may become inflamed and swell so much that it closes off the passageway, excess mucus secreted by inflamed membranes may clog the lower respiratory passages, a large object may be aspirated (breathed in), or a cancerous tumor may protrude into the airway. Two methods are used to reestablish airflow past a tracheal obstruction. If the obstruction is above the level of the larynx, a tracheotomy (tra-ke¯ -O-to¯-me) may be performed. In this procedure, also called a tracheostomy, a skin incision is followed by a short longitudinal incision into the trachea below the cricoid cartilage. A tracheal tube is then inserted to create an emergency air passageway. The second method is intubation (in⬘-too-BA¯-shun), in which a tube is inserted into the mouth or nose and passed inferiorly through the larynx and trachea. The firm wall of the tube pushes aside any flexible obstruction, and the lumen of the tube provides a passageway for air; any mucus clogging the trachea can be suctioned out through the tube. •

The arteries of the trachea are branches of the inferior thyroid, internal thoracic, and bronchial arteries. The veins of the trachea terminate in the inferior thyroid veins. The smooth muscle and glands of the trachea are innervated parasympathetically by branches of the vagus (X) nerves. Sympathetic innervation is through branches from the sympathetic trunk and its ganglia.


Bronchi At the superior border of the fifth thoracic vertebra, the trachea divides into a right main (primary) bronchus (BRONkus⫽windpipe), which goes into the right lung, and a left main (primary) bronchus, which goes into the left lung (Figure 23.6). The right main bronchus is more vertical, shorter, and wider than the left. As a result, an aspirated object is more likely to enter and lodge in the right main bronchus than the left. Like the trachea, the main bronchi (BRON-kı-) contain incomplete rings of cartilage and are lined by pseudostratified ciliated columnar epithelium. At the point where the trachea divides into right and left main bronchi is an internal ridge called the carina (ka-RI¯-na⫽keel of a boat). It is formed by a posterior and somewhat inferior projection of the last tracheal cartilage. The mucous membrane of the carina is one of the most sensitive areas of the entire larynx and trachea for triggering a cough reflex. Widening and distortion of the carina is a serious sign because it usually indicates a carcinoma of the lymph nodes around the region where the trachea divides. On entering the lungs, the main bronchi divide to form smaller bronchi—the lobar (secondary) bronchi, one for each lobe of the lung. (The right lung has three lobes; the left lung has two.) The lobar bronchi continue to branch, forming still smaller bronchi, called segmental (tertiary) bronchi (TER-she¯-e-re¯) that supply the specific bronchopulmonary segments within the lobes. The segmental bronchi then divide into bronchioles. Bronchioles in turn branch repeatedly, and the smallest ones branch into even smaller tubes called terminal bronchioles. These bronchioles contain Clara cells, columnar, nonciliated cells interspersed among the epithelial cells. Clara cells may protect against harmful effects of inhaled toxins and carcinogens, produce surfactant (discussed shortly), and function as stem cells (reserve cells), which can give rise to the various cells of the epithelium. The terminal bronchioles represent the end of the conducting zone of the respiratory system. Because this extensive branching from the trachea through the terminal bronchioles resembles an inverted tree, it is commonly referred to collectively as the bronchial tree. As the branching in the bronchial tree becomes more extensive, several structural changes may be noted. 1. The mucous membrane in the bronchial tree changes from pseudostratified ciliated columnar epithelium in the main bronchi, lobar bronchi, and segmental bronchi to ciliated simple columnar epithelium with some goblet cells in larger bronchioles, to mostly ciliated simple cuboidal epithelium with no goblet cells in smaller bronchioles, to mostly nonciliated simple cuboidal epithelium in terminal bronchioles. Recall that ciliated epithelium of the respiratory membrane removes inhaled particles in two ways. Mucus produced by goblet cells traps the particles and the cilia move the mucus and trapped particles toward the pharynx for removal. In regions where nonciliated simple cuboidal epithelium is present, inhaled particles are removed by macrophages. 2. Plates of cartilage gradually replace the incomplete rings of cartilage in main bronchi and finally disappear in the distal bronchioles. 3. As the amount of cartilage decreases, the amount of smooth muscle increases. Smooth muscle encircles the lumen in spiral bands and helps maintain patency. However, because there is no supporting cartilage, muscle spasms such as those that occur during an asthma attack can close off the airways, a potentially life-threatening situation.





Figure 23.6 Branching of airways from the trachea: the bronchial tree. The bronchial tree consists of macroscopic airways that begin at the trachea and continue through the terminal bronchioles. BRANCHING OF BRONCHIAL TREE Trachea


Main bronchi


Lobar bronchi Right lung

Left lung Segmental bronchi

Visceral pleura


Parietal pleura Terminal bronchioles

Pleural cavity Location of carina Right main bronchus

Left main bronchus Left lobar bronchus

Right lobar bronchus

Left segmental bronchus

Right segmental bronchus

Left bronchiole

Right bronchiole Right terminal bronchiole

Left terminal bronchiole Cardiac notch Diaphragm (a) Anterior view

Vestibular fold Vocal fold Cricoid cartilage of larynx Common carotid artery Left lateral lobe of thyroid gland Trachea

Right lung Aorta (cut) Right main bronchus Right lobar bronchi Left lung Right segmental bronchus Right bronchiole

Left pulmonary vein

Right pulmonary vein Esophagus Inferior vena cava Diaphragm (b) Anterior view of bronchial tree in lungs




Thyroid cartilage Arytenoid cartilage Cricoid cartilage


Left main bronchus

Left lobar bronchi

Left segmental bronchi Bronchioles (c) Anterior view of larynx and bronchial tree

(d) Posterior view of larynx and bronchial tree


Cilia Lumen of bronchiole

Cartilage Smooth muscle

Smooth muscle

Epithelium (ciliated simple columnar)

Epithelium (ciliated simple columnar)

Goblet cell




Details of epithelium

Transverse section of a bronchiole (e) Histology of a bronchiole

How many lobes and lobar bronchi are present in each lung?





CL I NI CAL C ONN E C T ION | Asthma and Chronic Bronchitis During an asthma (AZ-ma) attack, bronchiolar smooth muscle goes into spasm. Because there is no supporting cartilage, the spasms can reduce the lumen or even close off the air passageways. Movement of air through constricted bronchioles causes breathing to be more labored. The parasympathetic division of the ANS and mediators of allergic reactions such as histamine also cause narrowing of bronchioles (bronchoconstriction) due to contraction of bronchiolar smooth muscle. Because air moving through a restricted lumen causes a noise, the breathing of a true asthmatic can often be heard across the room. The principle is similar to that of a vacuum cleaner: It is so noisy because a large volume of air is moving through a small or restricted tube. Asthmatics typically react to low concentrations of stimuli that do not normally cause symptoms in people without asthma. Sometimes the trigger is an allergen such as pollen, dust mites, molds,

During exercise, activity in the sympathetic division of the autonomic nervous system (ANS) increases and causes the adrenal medullae to release the hormones epinephrine and norepinephrine; these hormones cause relaxation of smooth muscle in the bronchioles, which dilates the airways. The result is improved lung ventilation because air reaches the alveoli more quickly. The parasympathetic division of the ANS and mediators of allergic reactions such as histamine cause contraction of bronchiolar smooth muscle, resulting in constriction of distal bronchioles. The blood supply to the bronchi is via the left and right bronchial arteries. The veins that drain the bronchi are the right bronchial vein, which enters the azygos vein, and the left bronchial vein, which empties into the accessory hemiazygos vein or the left superior intercostal vein.

or a particular food. Other common triggers include emotional upset, aspirin, sulfating agents (used in wine and beer and to keep greens fresh in salad bars), exercise, and breathing cold air or cigarette smoke. Symptoms include difficult breathing, coughing, wheezing, chest tightness, tachycardia, fatigue, moist skin, and anxiety. Chronic bronchitis (brong-KI¯ -tis) is a disorder characterized by excessive secretion of bronchial mucus accompanied by a cough. Inhaled irritants lead to chronic inflammation with an increase in the size and number of mucous glands and goblet cells in the airway epithelium. The thickened and excessive mucus narrows the airway and impairs the action of cilia. Thus, inhaled pathogens become embedded in airway secretions and multiply rapidly. Besides a cough, symptoms of chronic bronchitis are shortness of breath, wheezing, cyanosis, and pulmonary hypertension. •


4. What are the roles of the three anatomical regions of the pharynx in respiration? 5. How does the larynx function in respiration and voice production? 6. Describe the location, structure, and function of the trachea. 7. What is the structure and function of the bronchial tree?

Lungs The lungs (⫽lightweights, because they float) (Figure 23.7) are paired cone-shaped organs in the thoracic cavity. The lungs are separated from each other by the heart and other structures of the

Figure 23.7 Relationship of the pleural membranes to the lungs. The parietal pleura lines the thoracic cavity; the visceral pleura covers the lungs. Transverse plane

Sternum Left lung VISCERAL PLEURA Ascending aorta Superior vena cava


Pulmonary arteries


Pulmonary vein

Right lung



Thoracic aorta Body of T4 Spinal cord

C L I NI C AL C ON N E C T ION | Pleurisy



Inflammation of the pleural membrane, called pleurisy or pleuritis, may in its early stages cause pain due to friction between the parietal and visceral layers of the pleura. If the inflammation persists, excess fluid accumulates in the pleural space, a condition known as pleural effusion. •

What type of membrane is the pleural membrane?

POSTERIOR Inferior view of a transverse section through the thoracic cavity showing the pleural cavity and pleural membranes


mediastinum, which separates the thoracic cavity into two anatomically distinct chambers. Because of this separation, if trauma causes one lung to collapse, the other may remain expanded. Each lung is surrounded by a protective, double-layered serous membrane called the pleural membrane (PLOOR-al; pleur-⍽side). This membrane is easily visualized with the following analogy: Imagine that the lung is your fist and you push your fist into a balloon. The two layers of the balloon wrap around your fist separated

Figure 23.8 Surface anatomy of the lungs. The oblique fissure divides the left lung into two lobes. The oblique and horizontal fissures divide the right lung into three lobes. First rib

Apex of lung Left lung

Base of lung Pleural cavity Pleura

(a) Anterior view of lungs and pleurae in thorax


by the space inside the balloon (see Figure 1.7e). This is similar to the design of the pleural membrane. The superficial layer of the pleural membrane lining the wall of the thoracic cavity is called the parietal pleura (the part of the balloon not touching your fist); the deep layer, the visceral pleura, adheres to the lungs (the part of the balloon in contact with your fist). The two layers are continuous with one another where the bronchi enter the lung (at your wrist, where the balloon folds off of your fist). Between the visceral and parietal pleurae is a small space, the pleural cavity (the inside of the balloon), which contains a small amount of lubricating fluid secreted by the two layers. This fluid reduces friction between the membranes, allowing them to slide easily over one another during breathing. Pleural fluid also causes the pleurae to adhere to one another just as a film of water causes two glass microscope slides to stick together, a phenomenon called surface tension. Separate pleural cavities surround the left and right lungs. The lungs extend from the diaphragm to just slightly superior to the clavicles and lie against the ribs anteriorly and posteriorly (Figure 23.8a). The broad inferior portion of the lung, the base, is concave and fits over the convex area of the diaphragm. The narrow superior portion of the lung is the apex. The surface of the lung lying against the ribs, the costal surface, matches the rounded curvature of the ribs. The mediastinal (medial) surface of each lung contains a region, the hilum, through which bronchi, pulmonary blood vessels, lymphatic vessels, and nerves enter and exit (Figure 23.8e, g). These structures are held together by the pleura and connective tissue and constitute the root of the lung. Medially, the left lung also contains a concavity, the cardiac notch, into which the apex of the heart projects. Due to the space occupied by the heart, the left lung is about 10 percent smaller than the right lung. The right lung is thicker and broader,

Apex Superior lobe View (b)


Oblique fissure Inferior lobe

Horizontal fissure

Oblique fissure

Cardiac notch Middle lobe

Inferior lobe


POSTERIOR Base (b) Lateral view of right lung

(c) Lateral view of left lung

Apex Superior lobe Oblique fissure

View (d)


View (e)

Hilum and its contents (root)

Horizontal fissure

Inferior lobe

Middle lobe

ANTERIOR (d) Medial view of right lung

Oblique fissure

Cardiac notch


ANTERIOR (e) Medial view of left lung

F I G U R E 23. 8




F I G U R E 23.8



SUPERIOR Apex Superior lobe Costal (rib) impressions Horizontal fissure

Oblique fissure

Oblique fissure

Cardiac notch Inferior lobe

Middle lobe Base


Inferior lobe POSTERIOR

ANTERIOR Right lung

Left lung (f) Lateral views


SUPERIOR Apex Superior lobe Oblique fissure Hilum and its contents (root) Horizontal fissure

Middle lobe

Inferior lobe

Cardiac notch

Oblique fissure ANTERIOR


Base Right lung


Left lung

(g) Medial views

CL I NI CAL C ONN E C T ION | Pneumothorax and Hemothorax In certain conditions, the pleural cavities may fill with air (pneumothorax; noo⬘-mo--THOR-aks; pneumo-⫽air or breath), blood (hemothorax), or pus. Air in the pleural cavities, most commonly introduced in a surgical opening of the chest or as a result of a stab or gunshot wound, may cause the lungs to collapse. This collapse of a part

of a lung, or rarely an entire lung, is called atelectasis (at⬘-e-LEK-ta-sis; ateles-⫽incomplete; -ectasis⫽expansion). The goal of treatment is the evacuation of air (or blood) from the pleural space, which allows the lung to reinflate. A small pneumothorax may resolve on its own, but it is often necessary to insert a chest tube to assist in evacuation. •

Why are the right and left lungs slightly different in size and shape?

but it is also somewhat shorter than the left lung because the diaphragm is higher on the right side to accommodate the liver, which lies inferior to it. The lungs almost fill the thorax (Figure 23.8a). The apex of the lungs lies superior to the medial third of the clavicles and this is the only area that can be palpated. The anterior, lateral, and posterior surfaces of the lungs lie against the ribs. The base of the lungs extends from the sixth costal cartilage anteriorly to the spinous process of the tenth thoracic vertebra posteriorly. The pleura extends about 5 cm (2 in.) below the base from the sixth costal cartilage anteriorly to the twelfth rib posteriorly. Thus, the lungs do not completely fill the pleural cavity in this area (see Figure 23.8a). Removal of excessive fluid in the pleural cavity can be accomplished without injuring lung tissue by inserting a needle anteriorly through the seventh intercostal space, a procedure termed thoracentesis (thor⬘-a-sen-TEˉ-sis; -centesis⫽puncture). The needle is passed along the superior border of the eighth rib to avoid damage to the intercostal nerves and blood vessels. Inferior to the seventh intercostal space there is danger of penetrating the diaphragm.

CLIN ICA L CON N ECTION | Malignant Mesothelioma ¯ -ma) is a rare cancer Malignant mesothelioma (me¯ -zo¯-the¯ -le¯ -O that affects the mesothelium of a serous membrane. The most common form, about 75 percent of all cases, affects the pleurae of the lungs (pleural mesothelioma). About 2000–3000 cases are diagnosed each year in the United States, accounting for about 3 percent of all cancers. The disease is almost entirely caused by asbestos, which has been widely used in insulation, textiles, cement, brake linings, gaskets, roof shingles, and floor products. The signs and symptoms of pleural mesothelioma, which may not appear until 20–50 years or more after asbestos exposure, include chest pain, shortness of breath, pleural effusion (fluid surrounding the lungs), fatigue, anemia, blood in the sputum (fluid) coughed up, wheezing, hoarseness, and unexplained weight loss. Diagnosis is based on medical history, physical examination, radiographs, CT scans, and biopsy. There is usually no cure, and the prognosis is poor. Chemotherapy, radiation therapy, immunotherapy, and multimodality therapy may be used to help decrease symptoms. •



gives rise to superior and inferior lobar bronchi. Within the lung, the lobar bronchi give rise to the segmental bronchi; there are 10 segmental bronchi in each lung. The segment of lung tissue that each segmental bronchus supplies is called a bronchopulmonary segment (brong⬘-koˉ-PUL-moˉ-nar⬘-eˉ) (Figure 23.9). Bronchial and pulmonary disorders (such as tumors or abscesses) that are localized in a particular bronchopulmonary segment may be surgically removed without seriously disrupting the surrounding lung tissue. Each bronchopulmonary segment of the lungs has many small compartments called lobules; each lobule is wrapped in elastic connective tissue and contains a lymphatic vessel, an arteriole, a

Lobes, Fissures, and Lobules Fissures divide each lung into lobes (Figure 23.8b–g). Both lungs have an oblique fissure, which extends inferiorly and anteriorly; the right lung also has a horizontal fissure. The oblique fissure in the left lung separates the superior lobe from the inferior lobe. In the right lung, the superior part of the oblique fissure separates the superior lobe from the inferior lobe; the inferior part of the oblique fissure separates the inferior lobe from the middle lobe, which is bordered superiorly by the horizontal fissure. Each lobe receives its own lobar bronchus. Thus, the right main bronchus gives rise to three lobar bronchi called the superior, middle, and inferior lobar bronchi, and the left main bronchus

Figure 23.9 Bronchopulmonary segments of the lungs. The bronchial branches are shown in the center of the figure. The bronchopulmonary segments within the lungs are numbered and named for convenience. There are 10 segmental bronchi in each lung; each is composed of smaller compartments called lobules. Superior lobe: 1 Apical 2 Posterior 3 Anterior




10 10

5 5

8 8

Lateral view of right lung Superior lobe: 1 Apical 2 Posterior 3 Anterior

Lobar bronchi

4 5

Segmental 6 bronchi

5 85 8

1 2

1 3









7 9

10 10


7 9




Segmental bronchi

7 Superior lobe: 1 Apical 2 Posterior Superior lobe: 3 Anterior 1 Apical 4 Superior 2 5 Posterior Inferior 3 Anterior 4 Superior Inferior lobe: 5 Inferior 6 Superior 7 Medial basal Inferior lobe: 8 Anterior basal 6 Superiorbasal 9 Lateral 7 Posterior Medial basal 10 basal 8 Anterior basal 9 Lateral basal 10 Posterior basal

Medial and basal views of right lung

Which bronchi supply a bronchopulmonary segment?

9 9

10 10

Superior lobe: 1 Apical 2 Posterior 3 Anterior 4 Superior 5 Inferior


Right 4

8 8


2 3



Lateral view of left lung 1

*Cannot be seen from this view.


5 5

Left main bronchus


Middle lobe: 4 Lateral* 5 Medial

Inferior lobe: 6 Superior 7 Medial basal 8 Anterior basal 9 Lateral basal 10 Posterior basal



Trachea Trachea Right main Right bronchus



*Cannot be seen from this view.

4 4 9 9


Inferior lobe: 6 Superior 7 Medial basal* 8 Anterior basal 9 Lateral basal 10 Posterior basal



Middle lobe: 4 Lateral 5 Medial

Inferior lobe: 6 Superior 7 Medial basal* 8 Anterior basal 9 Lateral basal 10 Posterior basal

*Cannot be seen from this view.

Left 78

6 10

9 7




1 2 1




3 4

10 10

7 97

5 4 5 8

8 9 Medial and basal views of left lung





venule, and a branch from a terminal bronchiole (Figure 23.10a). Terminal bronchioles subdivide into microscopic branches called respiratory bronchioles (Figure 23.10b). They also have alveoli (described shortly) budding from their walls. Because alveoli participate in gas exchange, respiratory bronchioles are the first structure in the respiratory zone of the respiratory system. As the respiratory bronchioles penetrate more deeply into the lungs, the epithelial lining changes from simple cuboidal to simple squamous. Respiratory bronchioles in turn subdivide into several (2–11) alveolar ducts (al-VEˉ-oˉ -lar), which consist of simple squamous epithelium. The respiratory passages from the trachea to the alveolar ducts contain about 25 orders of branching; that is, branching occurs about 25 times—from the trachea into main bronchi (first-order branching) into lobar bronchi (second-order branching) and so on down to the alveolar ducts.


Effects of Smoking on the Respiratory System

Smoking may cause a person to become easily “winded” during even moderate exercise because several factors decrease respiratory efficiency in smokers. Following are some of the effects of smoking on the respiratory system: (1) Nicotine constricts terminal bronchioles, which decreases airflow into and out of the lungs. (2) Carbon monoxide in smoke binds to hemoglobin and reduces its oxygen-carrying capability. (3) Irritants in smoke cause increased mucus secretion by the mucosa of the bronchial tree and swelling of the mucosal lining, both of which impede airflow into and out of the lungs. (4) Irritants in smoke also inhibit the movement of cilia and destroy cilia in the lining of the respiratory system. Thus, excess mucus and foreign debris are not easily removed, which further adds to the difficulty in breathing. The irritants can also convert the normal respiratory epithelium into stratified squamous epithelium, which lacks cilia and goblet cells. (5) With time, smoking leads to destruction of elastic fibers in the lungs and is the prime cause of emphysema. These changes cause collapse of small bronchioles and trapping of air in alveoli at the end of exhalation. The result is less efficient gas exchange. •

Alveoli Around the circumference of the alveolar ducts are numerous alveoli and alveolar sacs. An alveolus (al-VEˉ-oˉ-lus) is a cupshaped outpouching lined by simple squamous epithelium and

Figure 23.10 Microscopic anatomy of a lobule of the lungs. Alveolar sacs consist of two or more alveoli that share a common opening. MICROSCOPIC AIRWAYS Respiratory bronchioles Alveolar ducts Alveolar sacs Alveoli Terminal bronchiole Pulmonary venule

Elastic connective tissue

Pulmonary arteriole Lymphatic vessel

Terminal bronchiole


Blood vessel



ALVEOLAR DUCTS Pulmonary capillary


Visceral pleura ALVEOLI




(a) Diagram of a portion of a lobule of the lung

What types of cells make up the wall of an alveolus?

(b) Lung lobule

about 30x

Visceral pleura



Figure 23.11 Structural components of an alveolus. The respiratory membrane consists of a layer of type I and type II alveolar cells, an epithelial basement membrane, a capillary basement membrane, and the capillary endothelium. The exchange of respiratory gases occurs by diffusion across the respiratory membrane. Monocyte Reticular fiber Elastic fiber




Diffusion of O2

Red blood cell

Diffusion of CO2

Capillary endothelium

Alveolus Red blood cell in pulmonary capillary

Capillary basement membrane Epithelial basement membrane Type I alveolar cell Interstitial space

Alveolar fluid with surfactant (a) Section through an alveolus showing its cellular components

(b) Details of respiratory membrane





Red blood cells in blood vessel

LM 1000x

(c) Details of several alveoli

How thick is the respiratory membrane?

Emphysema (em⬘-fi-SE¯-ma⫽blown up or full of air) is a disorder characterized by destruction of the walls of the alveoli, which produces abnormally large air spaces that remain filled with air during exhalation. With less surface area for gas exchange, O2 diffusion across the respiratory membrane is reduced. Blood O2 level is somewhat lowered, and any mild exercise that raises the O2 requirements of the cells leaves the patient breathless. As increasing numbers of alveolar walls are damaged, lung elastic recoil decreases due to loss of elastic fibers, and an increasing amount of air becomes trapped in the lungs at the end of exhalation. Over several years, added respiratory exertion increases the size of the chest cage, resulting in a “barrel chest.” Emphysema is a common precursor to the development of lung cancer. •

supported by a thin elastic basement membrane; an alveolar sac consists of two or more alveoli that share a common opening (see Figure 23.10a, b). The walls of alveoli consist of two types of alveolar epithelial cells (Figure 23.11). Type I alveolar cells, the predominant cells, are simple squamous epithelial cells that form a nearly continuous lining of the alveolar wall. Type II alveolar cells, also called septal cells, are fewer in number and are found between type I alveolar cells. The thin type I alveolar cells are




the main sites of gas exchange. Type II alveolar cells, which are rounded or cuboidal epithelial cells with free surfaces containing microvilli, secrete alveolar fluid. This fluid keeps the surface between the cells and the air moist. Included in the alveolar fluid is surfactant (sur-FAK-tant), a complex mixture of phospholipids and lipoproteins. Surfactant lowers the surface tension of alveolar fluid, which reduces the tendency of alveoli to collapse and thus maintains their patency (described later). Associated with the alveolar wall are alveolar macrophages (dust cells), wandering phagocytes that remove fine dust particles and other debris in the alveolar spaces. Also present are fibroblasts that produce reticular and elastic fibers. Underlying the layer of type I alveolar cells is an elastic basement membrane. On the outer surface of the alveoli, the pulmonary arterioles and venules disperse into a network of blood capillaries (see Figure 23.10a) that consist of a single layer of endothelial cells and basement membrane. The exchange of O2 and CO2 between the air spaces in the lungs and the blood takes place by diffusion across the alveolar and capillary walls, which together form the respiratory membrane. Extending from the alveolar air space to blood plasma, the respiratory membrane consists of four layers (see Figure 23.11b): 1. A layer of type I and type II alveolar cells and associated alveolar macrophages that constitutes the alveolar wall 2. An epithelial basement membrane underlying the alveolar wall 3. A capillary basement membrane that is often fused to the epithelial basement membrane 4. The endothelial cells of the capillary wall Despite having several layers, the respiratory membrane is very thin—only 0.5 ␮m thick, about one-sixteenth the diameter of a red blood cell. This thinness allows rapid diffusion of gases. It has been estimated that the lungs contain 300 million alveoli, providing an immense surface area of 70 m2 (750 ft2)—about the size of a handball court—for the exchange of gases. A summary of the epithelial linings and special features of the organs of the respiratory system is presented in Table 23.1.

Blood Supply to the Lungs The lungs receive blood via two sets of arteries: pulmonary arteries and bronchial arteries. Deoxygenated blood passes through the pulmonary trunk, which divides into a left pulmonary artery that enters the left lung and a right pulmonary artery that enters the right lung. Return of the oxygenated blood to the heart occurs by way of the four pulmonary veins, which drain into the left atrium (see Figure 14.16). A unique feature of pulmonary

blood vessels is their constriction in response to localized hypoxia (low O2 level). In all other body tissues, hypoxia causes dilation of blood vessels, which serves to increase blood flow. In the lungs, however, blood vessels constrict in response to hypoxia, diverting deoxygenated blood from poorly ventilated areas to wellventilated regions of the lungs for more efficient gas exchange. Bronchial arteries, which branch from the aorta, deliver oxygenated blood to the lungs. This blood mainly passes to the muscular walls of the bronchi and bronchioles. Connections do exist between branches of the bronchial arteries and branches of the pulmonary arteries; most blood returns to the heart via pulmonary veins. Some blood, however, drains into bronchial veins, which are tributaries of the azygos system (see Exhibit 14.J), and returns to the heart via the superior vena cava. The nerve supply of the lungs is derived from the pulmonary plexus, located anterior and posterior to the roots of the lungs. The pulmonary plexus is formed by branches of the vagus (X) nerves and sympathetic trunks. Motor parasympathetic fibers arise from the dorsal nucleus of the vagus (X) nerve, and motor sympathetic fibers are postganglionic fibers of the second to fifth thoracic paravertebral ganglia of the sympathetic trunk.

Patency of the Respiratory System Throughout the discussion of the respiratory organs we have included examples of structures or secretions that help to maintain patency of the system so that air passageways are kept free of obstruction. These include the bony and cartilaginous frameworks of the nose, skeletal muscles of the pharynx, cartilages of the larynx, C-shaped rings of cartilage in the trachea and bronchi, smooth muscle in the bronchioles, and surfactant in the alveoli. Unfortunately, numerous factors can compromise patency. These include crushing injuries to bone and cartilage, a deviated nasal septum, nasal polyps, inflammation of mucous membranes, spasms of smooth muscle, and a deficiency of surfactant. CHECKPOINT

8. Where are the lungs located in relation to the clavicles, ribs, and heart? Where can the lungs be palpated? 9. Distinguish the parietal pleura from the visceral pleura. 10. Define each of the following parts of a lung: base, apex, costal surface, medial surface, hilum, root, cardiac notch, lobe, and lobule. 11. What is a bronchopulmonary segment? 12. Describe the histology and function of the respiratory membrane. 13. Give several examples of structures that maintain the patency of the respiratory system.

CL I NI CAL C ONN E C T ION | Lung Cancer In the United States, lung cancer is the leading cause of cancer death in both males and females, accounting for 160,000 deaths annually. At the time of diagnosis, lung cancer is usually well advanced, with distant metastases present in about 55 percent of patients, and regional lymph node involvement in an additional 25 percent. Most people with lung cancer die within a year of the initial diagnosis; the overall survival rate is only 10–15 percent. Cigarette smoke is the most common cause of lung cancer. Roughly 85 percent of lung cancer cases are related to smoking, and the disease is 10 to 30 times more common in smokers than nonsmokers.

Exposure to secondhand smoke is also associated with lung cancer and heart disease. In the United States, secondhand smoke causes an estimated 4000 deaths a year from lung cancer, and nearly 40,000 deaths a year from heart disease. Other causes of lung cancer are ionizing radiation and inhaled irritants, such as asbestos and radon gas. Emphysema is a common precursor to the development of lung cancer. The most common type of lung cancer, bronchogenic carcinoma (brong⬘-ko¯-JEN-ik), starts in the epithelium of the bronchial tubes. Bronchogenic tumors are named based on where they arise. For ex¯ -mas; adeno-⫽gland) ample, adenocarcinomas (ad-e¯ n-o¯-kar-si-NO



TABLE 23.1

Summary of the Respiratory System STRUCTURE






Nonkeratinized stratified squamous



Contains numerous hairs

Respiratory region

Pseudostratified ciliated columnar



Contains conchae and meatuses

Olfactory region

Olfactory epithelium (olfactory receptors)



Functions in olfaction


Pseudostratified ciliated columnar



Passageway for air; contains internal nares, openings for auditory tubes, and pharyngeal tonsil


Nonkeratinized stratified squamous



Passageway for both air and food and drink; contains opening from mouth (fauces)


Nonkeratinized stratified squamous



Passageway for both air and food and drink


Nonkeratinized stratified squamous above the vocal folds; pseudostratified ciliated columnar below the vocal folds

No above folds; Yes below folds

No above folds; Yes below folds

Passageway for air; contains vocal folds for voice production


Pseudostratified ciliated columnar



Passageway for air; contains C-shaped rings of cartilage to keep trachea open

Main bronchi

Pseudostratified ciliated columnar



Passageway for air; contain C-shaped rings of cartilage to maintain patency

Lobar bronchi

Pseudostratified ciliated columnar



Passageway for air; contain plates of cartilage to maintain patency

Segmental bronchi

Pseudostratified ciliated columnar



Passageway for air; contain plates of cartilage to maintain patency

Larger bronchioles

Ciliated simple columnar



Passageway for air; contain more smooth muscle than in the bronchi

Smaller bronchioles

Ciliated simple columnar



Passageway for air; contain more smooth muscle than in the larger bronchioles

Terminal bronchioles

Nonciliated simple columnar



Passageway for air; contain more smooth muscle than in the smaller bronchioles

Respiratory bronchioles

Simple cuboidal to simple squamous



Passageway for air; gas exchange

Alveolar ducts

Simple squamous



Passageway for air; gas exchange; produce surfactant


Simple squamous



Passageway for air; gas exchange; produce surfactant to maintain patency





Conducting structures

Respiratory structures

develop in peripheral areas of the lungs from bronchial glands and alveolar cells, squamous cell carcinomas develop from the squamous cells in the epithelium of larger bronchial tubes, and small (oat) cell carcinomas develop from epithelial cells in main bronchi near the hilum of the lungs. They get their name due to their flat cell shape with little cytoplasm, and they tend to involve the mediastinum early on. Depending on the type, bronchogenic tumors may be aggressive, locally invasive, and undergo widespread metastasis. The tumors begin as epithelial lesions that grow to form masses that obstruct the bronchial tubes or invade adjacent lung tissue. Bron-

chogenic carcinomas metastasize to lymph nodes, the brain, bones, liver, and other organs. Symptoms of lung cancer are related to the location of the tumor. These may include a chronic cough, spitting blood from the respiratory tract, wheezing, shortness of breath, chest pain, hoarseness, difficulty swallowing, weight loss, anorexia, fatigue, bone pain, confusion, problems with balance, headache, anemia, thrombocytopenia, and jaundice. Treatment consists of partial or complete surgical removal of a diseased lung (pulmonectomy), radiation therapy, and chemotherapy. •





• Distinguish among pulmonary ventilation, external respiration, and internal respiration. • Describe how inhalation and exhalation occur.

Respiration is the exchange of gases between the atmosphere, blood, and body cells. It takes place in three basic steps: 1. Pulmonary ventilation. The first process, pulmonary ventilation (pulmo⫽lung), or breathing, consists of inhalation (inflow) and exhalation (outflow) of air and is the exchange of air between the atmosphere and the air spaces of the lungs. 2. External (pulmonary) respiration. This is the exchange of gases between the air spaces of the lungs and blood in pulmonary capillaries across the respiratory membrane. The blood gains O2 and loses CO2. 3. Internal (tissue) respiration. This is the exchange of gases between systemic capillary blood and tissue cells. The blood loses O2 and gains CO2. The flow of air between the atmosphere and lungs occurs for the same reason that blood flows through the body: A pressure gradient (difference) exists. Air moves into the lungs when the pressure inside the lungs is less than the air pressure in the atmosphere. Air moves out of the lungs when the pressure inside the lungs is greater than the pressure in the atmosphere.

Inhalation Breathing in is called inhalation (inspiration). Just before each inhalation, the air pressure inside the lungs is equal to the pressure of the atmosphere, which at sea level is about 760 millimeters of mercury (mmHg), or 1 atmosphere (atm). For air to flow into the lungs, the pressure inside the alveoli must become lower than the atmospheric pressure. This condition is achieved by increasing the size of the lungs. For inhalation to occur, the lungs must expand. This increases lung volume and thus decreases the pressure in the lungs below atmospheric pressure. The first step in expanding the alveoli of the lungs during normal quiet breathing involves contraction of the principal muscle of inhalation—the diaphragm, with assistance from the external intercostals (Figure 23.12). The diaphragm, the most important muscle of inhalation, is a dome-shaped skeletal muscle that forms the floor of the thoracic cavity. It is innervated by fibers of the phrenic nerves, which emerge from both sides of the spinal cord at cervical levels 3, 4, and 5. Contraction of the diaphragm causes it to flatten, lowering its dome. This increases the vertical diameter of the thoracic cavity and accounts for the movement of about 75 percent of the air that enters the lungs during normal quiet inhalation. The distance the diaphragm moves during inspiration ranges from 1 cm (0.4 in.) during normal quiet breathing up to about 10 cm (4 in.) during strenuous exercise. Advanced pregnancy, excessive obesity, or confining abdominal clothing can prevent a complete descent of the diaphragm. At the same time the diaphragm is contracting, the external intercostals are at their most active stage (these muscles contract during all phases of breathing).

These skeletal muscles run obliquely downward and forward between adjacent ribs, and when these muscles contract, the ribs are pulled superiorly and the sternum is pushed anteriorly. This increases the anteroposterior and lateral diameters of the thoracic cavity. The primary role of all intercostal muscles is to keep the intercostal spaces from collapsing inward during the descent of the diaphragm, as this would reduce the thoracic volume and increase pressure. As the diaphragm and external intercostals contract and the overall size of the thoracic cavity increases, the walls of the lungs are pulled outward. The parietal and visceral pleurae normally adhere strongly to each other because of the below-atmospheric pressure between them and because of the surface tension created by their moist adjoining surfaces. As the thoracic cavity expands, the parietal pleura lining the cavity is pulled outward in all directions, and the visceral pleura and lungs are pulled along with it, increasing the volume of the lungs. When the volume of the lungs increases, alveolar pressure decreases from 760 to 758 mmHg. A pressure gradient is thus established between the atmosphere and the alveoli. Air moves from the atmosphere into the lungs due to a gas pressure difference, and inhalation takes place. Air continues to move into the lungs as long as the pressure difference exists. During deep, forceful inhalation, accessory muscles of inhalation also participate in increasing the size of the thoracic cavity (Figure 23.12a). The muscles are so named because they make little, if any, contribution during normal quiet inhalation, but during exercise or forced inhalation they may contract vigorously. The accessory muscles of inhalation include the sternocleidomastoid muscles, which elevate the sternum; the scalene muscles, which elevate the first two ribs; and the pectoralis minor muscles, which elevate the third through fifth ribs.

Exhalation Breathing out, called exhalation (expiration), is also achieved by a pressure gradient, but in this case the gradient is reversed: The pressure in the lungs is greater than the pressure of the atmosphere. Normal exhalation during quiet breathing depends on two factors: (1) the recoil of elastic fibers that were stretched during inhalation and (2) the inward pull of surface tension due to the film of alveolar fluid. Exhalation starts when the muscles of inhalation relax. As the diaphragm relaxes and the external intercostals become less active, the ribs move inferiorly; the diaphragm dome moves superiorly owing to its elasticity. These movements decrease the vertical, anteroposterior, and lateral diameters of the thoracic cavity. Also, surface tension exerts an inward pull between the parietal and visceral pleurae, and the elastic basement membranes of the alveoli and elastic fibers in bronchioles and alveolar ducts recoil. As a result, lung volume decreases and the alveolar pressure increases to 762 mmHg. Air then flows from the area of higher pressure in the alveoli to the area of lower pressure in the atmosphere. During labored breathing and when air movement out of the lungs is impeded, muscles of exhalation—abdominal and internal intercostals—contract. Contraction of the abdominal muscles moves the inferior ribs downward and compresses the abdominal viscera, thus forcing the diaphragm superiorly. Contraction of the internal intercostals, which extend inferiorly and posteriorly between adjacent ribs, also pulls the ribs downward.



Figure 23.12 Muscles of inhalation and exhalation and their actions. The pectoralis minor muscle, a muscle of deep inhalation (not shown here), is illustrated in Figure 11.13a. During deep, labored breathing, accessory muscles of inhalation (sternocleidomastoid, scalene, and pectoralis minor muscles) and accessory muscles of exhalation (internal intercostal and abdominal muscles) participate.




Sternocleidomastoid Scalenes

External intercostals

Diaphragm: Exhalation

Internal intercostals



External oblique

(b) Changes in size of thoracic cavity during inhalation and exhalation

Internal oblique Transversus abdominis Rectus abdominis (a) Muscles of inhalation (left); muscles of exhalation (right); arrows indicate the direction of muscle contraction

(c) During inhalation, the lower ribs (7–10) move upward and outward like the handle on a bucket

What is the main muscle that powers quiet breathing?

Breathing also provides humans with several methods for expressing emotions such as laughing, sighing, and sobbing. Moreover, the air involved in breathing can be used to expel foreign matter from the lower air passages through actions such as sneezing and coughing. Breathing movements can also be modified and controlled when you talk or sing. Some of the modified breathing movements that express emotion or clear the airways are listed in Table 23.2. All of these movements are reflexes, but some of them can be initiated voluntarily. CHECKPOINT

14. What are the basic differences among pulmonary ventilation, external respiration, and internal respiration? 15. Compare the events of quiet breathing and forceful breathing. 16. What causes hiccups?


• Explain the role of the respiratory center in breathing. • Describe the various factors that regulate the rate and depth of breathing.

Although breathing can be controlled voluntarily for short periods, the nervous system usually controls breathing automatically to meet the body’s demand without conscious effort.

Role of the Respiratory Center As you have already learned, the size of the thorax is altered by the action of the respiratory muscles, which contract and relax as a result of nerve impulses transmitted to them from centers in the




TABLE 23.2

Modified Breathing Movements MOVEMENT



A long-drawn and deep inhalation followed by a complete closure of the rima glottidis, which results in a strong exhalation that suddenly pushes the rima glottidis open and sends a blast of air through the upper respiratory passages. Stimulus for this reflex act may be a foreign body lodged in the larynx, trachea, or epiglottis.


Spasmodic contraction of muscles of exhalation that forcefully expels air through the nose and mouth. Stimulus may be an irritation of the nasal mucosa.


A long-drawn and deep inhalation immediately followed by a shorter but forceful exhalation.


A deep inhalation through the widely opened mouth producing an exaggerated depression of the mandible. It may be stimulated by drowsiness, or someone else’s yawning, but the precise cause is unknown.


A series of convulsive inhalations followed by a single prolonged exhalation. The rima glottidis closes earlier than normal after each inhalation so only a little air enters the lungs with each inhalation.


An inhalation followed by many short convulsive exhalations, during which the rima glottidis remains open and the vocal folds vibrate; accompanied by characteristic facial expressions and tears.


The same basic movements as crying, but the rhythm of the movements and the facial expressions usually differ from those of crying. Laughing and crying are sometimes indistinguishable.


Spasmodic contraction of the diaphragm followed by a spasmodic closure of the rima glottidis, which produces a sharp sound on inhalation. Stimulus is usually irritation of the sensory nerve endings of the gastrointestinal tract.

Valsalva (val-SAL-va) maneuver

Forced exhalation against a closed rima glottidis as may occur during periods of straining while defecating.

Pressurizing the middle ear

The nose and mouth are held closed and air from the lungs is forced through the auditory tube into the middle ear. Employed by those snorkeling or scuba diving during descent to equalize the pressure of the middle ear with that of the external environment.

brain. The area from which nerve impulses are sent to breathing muscles consists of clusters of neurons located bilaterally in the medulla oblongata and pons of the brain stem. This area, called the respiratory center, can be divided into two principal areas on the basis of location and function: (1) the medullary respiratory center in the medulla oblongata and (2) the pontine respiratory group in the pons (Figure 23.13a).

Medullary Respiratory Center The medullary respiratory center is made up of two collections of neurons called the dorsal respiratory group (DRG) formerly known as the inspiratory area, and the ventral respiratory group (VRG), formerly called the expiratory area (Figure 23.13a). During normal quiet breathing, neurons of the DRG generate impulses to the diaphragm via the phrenic nerves and the external intercostal muscles via the intercostal nerves (Figure 23.13a, b and Figure 23.14a). These impulses are released in bursts, which begin weakly, increase in strength for about two seconds, and then stop altogether. When the nerve impulses reach the diaphragm and external intercostal muscles, the diaphragm contracts, the external intercostal muscles contract during their most active phase, and inhalation occurs. When the DRG becomes inactive after two seconds, the diaphragm relaxes and the external intercostal muscles become less active and relax for about three seconds, allowing passive recoil of the lungs and thoracic wall. This results in exhalation. Then the cycle repeats itself. Even when all incoming nerve impulses to the DRG are cut or blocked, neurons in this area still rhythmically discharge impulses that cause inhalation. However, traumatic injury to both phrenic nerves causes paralysis of the diaphragm and cessation of breathing. The neurons of the VRG do not participate in normal quiet inhalation. However, when forceful breathing is required, such as during exercise, playing a wind instrument, or at high altitudes,

the VRG becomes activated as follows. During forceful inhalation (Figure 23.14b), nerve impulses from the DRG not only stimulate the diaphragm and external intercostal muscles to contract, they also activate neurons of the VRG involved in forceful inhalation to send nerve impulses to the accessory muscles of inhalation (sternocleidomastoid, scalene, and pectoralis minor muscles). Contraction of these muscles results in forceful inhalation. During forceful exhalation (Figure 23.14b), the DRG is inactive along with the neurons of the VRG that result in forceful exhalation, but neurons of the VRG involved in forceful exhalation send nerve impulses to the accessory muscles of exhalation (internal intercostal, external oblique, internal oblique, transversus abdominis, and rectus abdominis muscles). Contraction of these muscles results in forceful exhalation. Also located in the VRG is a cluster of neurons called the preBötzinger complex (BOT-zin-ger) that is believed to be important in the generation of the rhythm of breathing (Figure 23.13a). This rhythm generator, analogous to the cardiac conduction system of the heart, is composed of pacemaker cells that set the basic rhythm of breathing. The exact mechanism of the pacemaker cells is unknown and is the topic of much ongoing research. However, it is thought that the pacemaker cells provide input to the DRG, driving the rate at which DRG neurons fire nerve impulses.

Pontine Respiratory Group The pontine respiratory group (PRG) (PON-teˉn), formerly called the pneumotaxic center, is a collection of neurons in the pons (see Figure 23.13a). The neurons in the PRG are active during inhalation and exhalation. The PRG transmits nerve impulses to the DRG in the medulla. The PRG may play a role in both inhalation and exhalation by modifying the basic rhythm of breathing by the DRG, for example, when exercising, speaking, or sleeping.


Figure 23.13 Location of areas of the respiratory center. T respiratory center is composed of neurons in the The medullary respiratory center in the medulla oblongata m plus the pontine respiratory group in the pons.


Figure 23.14 Medullary respiratory center. Roles of the medullary respiratory center in controlling (a) the basic rhythm of breathing and (b) forceful breathing. During normal quiet breathing, the ventral respiratory group is inactive; during forceful breathing the dorsal g respiratory group activates the ventral respiratory group.


Dorsal respiratory group (DRG) 0LGEUDLQ




2 seconds

3 seconds




Diaphragm contracts and external intercostal muscles contract during their most active phase

Diaphragm relaxes and external intercostal muscles become less active and relax, followed by elastic recoil of lungs

Normal quiet inhalation

Normal quiet exhalation




(a) During normal quiet breathing

Activates Dorsal respiratory group (DRG)

Ventral respiratory group (VRG) (forceful inhalation neurons)

Diaphragm contracts and external intercostal muscles contract during their most active stage

Accessory muscles of inhalation (sternocleidomastoid, scalene, and pectoralis minor muscles) contract



Which area contains neurons that are active and then inactive in a repeating cycle?

Regulation of the Respiratory Center Although the basic rhythm of breathing is set and coordinated by the DRG of the medullary respiratory center, the rhythm can be modified in response to inputs from other brain regions, receptors in the peripheral nervous system, and other factors.

Forceful inhalation

Ventral respiratory group (VRG) (forceful exhalation neurons)

Cortical Influences on Breathing Because the cerebral cortex has connections with the respiratory center, you can voluntarily alter your pattern of breathing. You can even refuse to breathe at all for a short time. Voluntary control is protective because it enables us to prevent water or irritating gases from entering the lungs. However, the ability to not breathe is limited by the buildup of CO2 and H⍚ in the body. When CO2 and H⍚ concentrations increase to a certain level, the DRG neurons are strongly stimulated, nerve impulses are sent along the phrenic and intercostal nerves to inspiratory muscles, and breathing resumes, whether you want it to or not. It is impossible for people to kill themselves by voluntarily holding their breath. Even if you hold your breath for so long that you faint, breathing resumes when consciousness is lost. Nerve impulses from the hypothalamus and limbic system also stimulate the respiratory center, allowing emotional stimuli to alter breathing as, for example, when you laugh or cry.

Accessory muscles of exhalation (internal intercostal, external oblique, internal oblique, transversus abdominis, and rectus abdominis muscles) contract

Forceful exhalation

(b) During forceful breathing

Which nerves convey impulses from the respiratory center to the diaphragm?




Figure 23.15 Locations of peripheral chemoreceptors. C Chemoreceptors are sensory neurons that respond to cchanges in the levels of certain chemicals in the body.

These chemoreceptors are part of the peripheral nervous system and are sensitive to changes in O2, H⫹, and CO2 in the blood. Axons of sensory neurons from the aortic bodies are part of the vagus (X) nerves, and those from the carotid bodies are part of the right and left glossopharyngeal (IX) nerves and vagus (X) nerves (Figure 23.15). If there is even a slight increase in CO2, central and peripheral chemoreceptors are stimulated. The chemoreceptors send nerve impulses to the brain that cause the DRG to become highly active, and the rate of breathing increases. This allows the body to expel more CO2 until the CO2 is lowered to normal. If arterial CO2 is lower than normal, the chemoreceptors are not stimulated, and stimulatory impulses are not sent to the DRG. Consequently, the rate of breathing decreases until CO2 accumulates and the CO2 level rises to normal.

Medulla oblongata

Role of Lung Inflation in Stimulation of Breathing

Sensory axons in glossopharyngeal (IX) nerve Internal carotid artery CAROTID BODY External carotid artery

Carotid sinus

Common carotid artery

Sensory axons in vagus (X) nerve

Arch of aorta


In addition to all of the previously mentioned factors, receptors in the musculature of the bronchi and bronchioles throughout the lungs themselves can also modify breathing. Within these air passageways are stretch-sensitive receptors called baroreceptors (bar⬘-o¯-re-SEP-tors) or stretch receptors. When these receptors become stretched during overinflation of the lungs, nerve impulses are sent along the vagus (X) nerves to the dorsal respiratory group (DRG) of neurons in the medullary respiratory center. In response, the DRG is inhibited, the diaphragm relaxes and the external intercostals become less active, and further inhalation is stopped. As a result, exhalation begins. As air leaves the lungs during exhalation, the lungs deflate and the stretch receptors are no longer stimulated. Thus, the DRG is no longer inhibited, and a new inhalation begins. This reflex is referred to as the inflation (Hering-Breuer) reflex (HER-ing BROY-er). In infants, the reflex appears to function in normal breathing. In adults, however, the reflex is not activated until tidal volume (normally 500 mL) reaches more than 1500 mL. Therefore, the reflex in adults is a protective mechanism that prevents excessive inflation of the lungs (as during exercise), rather than a key component in the normal control of breathing. CHECKPOINT


Which chemicals stimulate peripheral chemoreceptors?

Chemoreceptor Regulation of Breathing Certain chemical stimuli determine how quickly and how deeply we breathe. The respiratory system functions to maintain proper levels of CO2 and O2 and is very responsive to changes in the levels of either in body fluids. Sensory neurons that are responsive to chemicals are termed chemoreceptors. Chemoreceptors in two locations of the respiratory system monitor levels of CO2, H⫹, and O2 and provide input to the respiratory center. Central chemoreceptors are located in the medulla oblongata in the central nervous system. They respond to changes in H⫹ or CO2 concentration, or both, in cerebrospinal fluid. Peripheral chemoreceptors are located in the aortic bodies, clusters of chemoreceptors located in the wall of the arch of the aorta, and in the carotid bodies, which are oval nodules in the wall of the left and right common carotid arteries where they divide into the internal and external carotid arteries.

17. How does the medullary respiratory center function in regulating breathing? 18. How is the pontine respiratory group related to the control of breathing? 19. Explain how each of the following modifies breathing: cerebral cortex, inflation reflex, CO2 levels, and O2 levels.


• Describe the effects of exercise on the respiratory system.

The respiratory and cardiovascular systems make adjustments in response to both intensity and duration of exercise. The effects of exercise on the heart are discussed in Chapter 13. Here we focus on how exercise affects the respiratory system. The heart pumps the same amount of blood to the lungs as to all the rest of the body. Thus, as cardiac output rises, the rate of blood flow through the lungs also increases. As blood flows more rapidly through the lungs, it picks up more O2. In addition, the rate at which O2 diffuses from alveolar air into the blood increases during maximal exercise because blood flows through a larger percentage of the pulmonary capillaries, providing a greater surface area for the diffusion of O2 into blood.

23.5 DEVELOPMENT OF THE RESPIRATORY SYSTEM C L INIC AL C ON N E C T ION | Pneumonia Pneumonia or pneumonitis (nu¯⬘-mo¯-NI¯-tis) is an acute infection or inflammation of the alveoli. It is the most common infectious cause of death in the United States, where an estimated 4 million cases occur annually. When certain microbes enter the lungs of susceptible individuals, they release harmful toxins, stimulating inflammation and immune responses that have damaging side effects. The toxins and immune response damage alveoli and bronchial mucous membranes; inflammation and edema cause the alveoli to fill with fluid, interfering with ventilation and gas exchange. The most common cause of pneumonia is the pneumococcal bacterium Streptococcus pneumoniae (see figure), but other SEM about 10,000x microbes may also cause pneumonia. Those who are most suscepti- Streptococcus pneumoniae, the most ble to pneumonia are the elderly, common cause of pneumonia infants, immunocompromised individuals, cigarette smokers, and individuals with an obstructive lung disease. Most cases of pneumonia are preceded by an upper respiratory infection that often is viral. Individuals then develop fever, chills, productive or dry cough, malaise, chest pain, and sometimes dyspnea (difficult breathing) and hemoptysis (spitting blood). Treatment may involve antibiotics, bronchodilators, oxygen therapy, increased fluid intake, and chest physiotherapy (percussion, vibration, and postural drainage). •

When muscles contract during exercise, they consume large amounts of O2 and produce large amounts of CO2. During vigorous exercise, O2 consumption and breathing both increase dramatically. At the onset of exercise, an abrupt increase in breathing is due to neural changes that send excitatory impulses to the DRG in the medulla oblongata. The more gradual increase in breathing during moderate exercise is due to chemical and physical changes in the bloodstream. During moderate exercise, the depth of breathing changes more than the breathing rate. When exercise is more strenuous, the frequency of breathing also increases. At the end of an exercise session, an abrupt decrease in breathing is followed by a more gradual decline to the resting level. The initial decrease is due mainly to changes in neural factors when movement stops or slows; the more gradual phase reflects the slower return of blood chemistry levels and temperature to the resting state.


At about four weeks of development, the respiratory system begins as an outgrowth of the foregut just inferior to the pharynx. This outgrowth is called the respiratory diverticulum (dı--verTIK-u¯-lum) or lung bud (Figure 23.16; see also Figure 22.8a). The endoderm lining the respiratory diverticulum gives rise to the epithelium and glands of the trachea, bronchi, and alveoli. Splanchnic mesoderm and neural crest tissue (see Figure 4.9d) surrounding the respiratory diverticulum gives rise to the connective tissue, cartilage, and smooth muscle of these structures. The epithelial lining of the larynx develops from the endoderm of the respiratory diverticulum; the cartilages and muscles originate from the fourth and sixth pharyngeal arches (see Figure 4.13).

Figure 23.16 Development of the bronchial tubes and lungs. T respiratory system develops from endoderm and The mesoderm. m Pharynx







Esophagus Fourth week

Left main bronchus Trachea

Left lobar bronchi

Right main bronchus

Right lobar bronchi Fifth week

Right superior lobe


Left segmental bronchi

Right segmental bronchi Sixth week

Trachea Left superior lobe

20. How does exercise affect the dorsal respiratory group? 21. Describe the changes in breathing caused by a brisk walk in the park (considered moderate exercise).


• Describe the development of the respiratory system.

The development of the mouth and pharynx are discussed in Chapter 24. Here we consider the development of the remainder of the respiratory system.

Right middle lobe

Right inferior lobe

Left inferior lobe

Developing pleura

Eighth week

When does the respiratory system begin to develop in an embryo?




As the respiratory diverticulum elongates, its distal end enlarges to form a globular tracheal bud, which gives rise to the trachea. Soon after, the tracheal bud divides into bronchial buds, which branch repeatedly and develop with the bronchi. By 24 weeks, 17 orders of branches have formed and respiratory bronchioles have developed. During weeks 6 to 16, all major elements of the lungs have formed, except for those involved in gaseous exchange (respiratory bronchioles, alveolar ducts, and alveoli). Since breathing is not possible at this stage, fetuses born during this time cannot survive. During weeks 16 to 26, lung tissue becomes highly vascular and respiratory bronchioles, alveolar ducts, and some primitive alveoli develop. Although it is possible for a fetus born near the end of this period to survive if given intensive care, death frequently occurs due to the immaturity of the respiratory and other systems. From 26 weeks to birth, many more primitive alveoli develop; they consist of type I alveolar cells (main sites of gaseous exchange) and type II surfactant-producing cells. Blood capillaries also establish close contact with the primitive alveoli. Recall that surfactant is necessary to lower surface tension of alveolar fluid and thus reduce the tendency of alveoli to collapse on exhalation. Although surfactant production begins by 20 weeks, it is present in only small quantities. Amounts sufficient to permit survival of a premature (preterm) infant are not produced until 26 to 28 weeks’ gestation. Infants born before 26–28 weeks are severely at risk of respiratory distress syndrome (RDS), in which the alveoli collapse during exhalation and must be reinflated during inhalation. The condition is treated by employing respirators that force air into the lungs and by administering surfactant. At about 30 weeks, mature alveoli develop. However, it is estimated that only about one-sixth of the full complement of alveoli develop before birth; the remainder develop after birth during the first eight years. As the lungs develop, they acquire their pleural sacs. The visceral pleura develops from splanchnic mesoderm and the parietal pleura develops from somatic mesoderm (see Figure 4.9d). During development, breathing movements of the fetus cause the aspiration of fluid into the lungs. The fluid is a mixture of

amniotic fluid, mucus from the bronchial glands, and surfactant. At birth, the lungs are about half-filled with fluid. When breathing begins at birth, most of the fluid is rapidly reabsorbed by blood and lymph capillaries; a small amount is expelled through the nose and mouth during delivery. CHECKPOINT

22. What structures develop from the respiratory diverticulum? 23. Which respiratory structures develop from endoderm? From mesoderm? 24. How many weeks old must a fetus be for it to survive as a preterm infant? Why?


• Describe the effects of aging on the respiratory system.

With advancing age, the airways and tissues of the respiratory tract, including the alveoli, become less elastic and more rigid; the chest wall becomes more rigid as well. The result is a decrease in lung capacity. In fact, vital capacity (the maximum amount of air that can be exhaled after maximal inhalation) can decrease as much as 35 percent by age 70. A decrease in blood level of O2, decreased activity of alveolar macrophages, and diminished ciliary action of the epithelium lining the respiratory tract also occur. Because of all these age-related factors, older people are more susceptible to pneumonia, bronchitis, emphysema, and other pulmonary disorders. Age-related changes in the structure and functions of the lung can also contribute to an older person’s reduced ability to perform vigorous exercises, such as running. CHECKPOINT

25. What accounts for the decrease in lung capacity with aging?

KEY MEDIC AL TERMS ASSOCIATED WITH THE RESPIRATORY SYSTEM Abdominal thrust (Heimlich) maneuver (HI¯M-lik ma-NOO-ver) First-aid procedure designed to clear the airways of obstructing objects. It is performed by applying a quick upward thrust between the navel and costal margin that causes sudden elevation of the diaphragm and forceful, rapid expulsion of air from the lungs; this action forces air out of the trachea to eject the obstructing object. The abdominal thrust maneuver is also used to expel water from the lungs of neardrowning victims before resuscitation is begun. Apnea (AP-ne¯-a; a⫽without; pnoia⫽air or breath) Absence of breathing movements. Carbon monoxide (CO) poisoning Elevated level of carbon monoxide in the body, which can cause the lips and oral mucosa to appear bright, cherry-red (the color of hemoglobin with carbon monoxide bound to it). Without prompt treatment, carbon monoxide poisoning is fatal. It is possible to rescue a victim of CO poisoning by administering pure oxygen, which speeds up the separation of carbon monoxide from hemoglobin. Chronic obstructive pulmonary disease (COPD) A type of respiratory disorder characterized by chronic and recurrent obstruction

of airflow, which increases airway resistance. COPD affects about 30 million Americans and is the fourth leading cause of death behind heart disease, cancer, and cerebrovascular disease. The principal types of COPD are emphysema and chronic bronchitis. Hypoxia (hı--POK-se¯-a; hypo-⫽below or under) A deficiency of O2 at the tissue level. Pulmonary edema An abnormal accumulation of fluid in the interstitial spaces and alveoli of the lungs. The edema may arise from increased permeability of the pulmonary capillaries (pulmonary origin) or increased pressure in the pulmonary capillaries (cardiac origin); the latter cause may coincide with congestive heart failure. The most common symptom is dyspnea. Others include wheezing, tachypnea (rapid breathing rate), restlessness, a feeling of suffocation, cyanosis, pallor (paleness), diaphoresis (excessive perspiration), and pulmonary hypertension. Pulmonary embolism (EM-bo¯-lizm) The blockage of a pulmonary artery or its branches by a blood clot that travels to the lungs, usually from a vein in a leg or the pelvis. Respirator (RES-pi-ra¯⬘-tor) An apparatus fitted to a mask over the nose and mouth, or hooked directly to an endotracheal or tracheotomy

CHAPTER REVIEW AND RESOURCE SUMMARY tube, that is used to assist or support breathing or to provide nebulized medication to the air passages. Sudden infant death syndrome (SIDS) Death of infants between the ages of 1 week and 12 months thought to be due to hypoxia while


sleeping in a prone position (on the stomach) and the rebreathing of exhaled air trapped in a depression of the mattress. It is now recommended that normal newborns be placed on their backs for sleeping: “back to sleep.”



23.1 Respiratory System Anatomy

Anatomy Overview - Overview of Respiratory Organs Anatomy Overview - Nasal Cavity and Pharynx Anatomy Overview - Respiratory Tissues Anatomy Overview - Respiratory Epithelium Figure 23.3 - Larynx Figure 23.7 - Relationship of the Pleural Membrane to the Lung Figure 23.11 - Structure of an Alveolus Exercise - Build an Airway Exercise - Concentrate on Respiratory Structures Concepts and Connections Functional Anatomy of the Respiratory System

1. The respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, and lungs. The respiratory system acts with the cardiovascular system to supply oxygen (O2) to and remove carbon dioxide (CO2) from the blood. 2. The external nose is made of cartilage and skin and is lined with a mucous membrane. Openings to the exterior are the external nares. The internal nose communicates with the paranasal sinuses and nasopharynx through the internal nares. The nasal cavity is divided by a nasal septum. The anterior portion of the cavity is called the nasal vestibule. The nose warms, moistens, and filters air and functions in olfaction and speech. 3. The pharynx (throat) is a muscular tube lined by a mucous membrane. The anatomical regions of the pharynx are the nasopharynx, oropharynx, and laryngopharynx. The nasopharynx functions in respiration. The oropharynx and laryngopharynx function both in respiration and digestion. 4. The larynx (voice box) is a passageway that connects the pharynx with the trachea. It contains the thyroid cartilage (Adam’s apple); the epiglottis, which prevents food from entering the larynx; the cricoid cartilage, which connects the larynx and trachea; and the paired arytenoid, corniculate, and cuneiform cartilages. The vocal folds of the larynx produce sound as they vibrate; taut folds produce high pitches, and relaxed ones produce low pitches. 5. The trachea (windpipe) extends from the larynx to the main bronchi. It is composed of C-shaped rings of cartilage and smooth muscle and is lined with pseudostratified ciliated columnar epithelium. 6. The bronchial tree consists of the trachea, main bronchi, lobar bronchi, segmental bronchi, bronchioles, and terminal bronchioles. Walls of bronchi contain rings of cartilage; walls of bronchioles contain increasingly smaller plates of cartilage and increasing amounts of smooth muscle. 7. Lungs are paired organs in the thoracic cavity enclosed by the pleural membrane. The parietal pleura is the superficial layer that lines the thoracic cavity; the visceral pleura is the deep layer that covers the lungs. The right lung has three lobes separated by two fissures; the left lung has two lobes separated by one fissure, along with a depression, the cardiac notch. 8. Lobar bronchi give rise to branches called segmental bronchi, which supply segments of lung tissue called bronchopulmonary segments. Each bronchopulmonary segment consists of lobules, which contain lymphatics, arterioles, venules, terminal bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. 9. Alveolar walls consist of type I alveolar cells, type II alveolar cells, and associated alveolar macrophages. 10. Gas exchange occurs across the respiratory membranes.

23.2 Mechanics of Pulmonary Ventilation (Breathing) 1. Pulmonary ventilation, or breathing, consists of inhalation (inspiration) and exhalation (expiration). 2. Inhalation occurs when alveolar pressure falls below atmospheric pressure. Contraction of the diaphragm and external intercostals increases the size of the thorax, decreasing the intrapleural pressure so that the lungs expand. Expansion of the lungs decreases alveolar pressure so that air moves down a pressure gradient from the atmosphere into the lungs. 3. During forceful inhalation, accessory muscles of inhalation (sternocleidomastoids, scalenes, and pectoralis minors) are also used. 4. Exhalation occurs when alveolar pressure is higher than atmospheric pressure. Relaxation of the diaphragm and external intercostals results in elastic recoil of the chest wall and lungs, which increases intrapleural pressure; lung volume decreases and alveolar pressure increases, so air moves from the lungs to the atmosphere. 5. Forceful exhalation involves contraction of the internal intercostal and abdominal muscles.

23.3 Regulation of Breathing 1. The respiratory center consists of a medullary respiratory center in the medulla oblongata and the pontine respiratory group in the pons. 2. The medullary respiratory center is made up of a dorsal respiratory group (DRG), which controls normal quiet breathing, and a ventral respiratory group (VRG), which is used during forceful breathing and controls the rhythm of breathing. 3. The pontine respiratory group may modify the rhythm of breathing during exercise, speaking, and sleep. 4. Breathing may be modified by a number of factors, including cortical influences; the inflation reflex; and chemical stimuli, such as O2, CO2, and H⫹ levels.

Animation - Pulmonary Ventilation Animation - Gas Exchange Introduction Animation - Gas Exchange - Internal and External Respiration Animation - Gas Transport Exercise - Carbon Dioxide Transport Try-out Interactive Exercise - Gas Exchange Match-up Exercise - Concentrate on Respiration Concepts and Connections - Carbon Dioxide Transport Concepts and Connections - Ventilation Anatomy Overview - Structures That Control Respiration Animation - Regulation of Ventilation






23.4 Exercise and the Respiratory System 1. The rate and depth of breathing change in response to both the intensity and duration of exercise. 2. An increase in pulmonary blood flow and O2-diffusing capacity occurs during exercise. 3. The abrupt increase in breathing at the start of exercise is due to neural changes that send excitatory impulses to the DRG in the medulla oblongata. The more gradual increase in breathing during moderate exercise is due to chemical and physical changes in the bloodstream.

23.5 Development of the Respiratory System 1. The respiratory system begins as an outgrowth of endoderm called the respiratory diverticulum. 2. Smooth muscle, cartilage, and connective tissue of the bronchial tubes and pleural sacs develop from mesoderm.

23.6 Aging and the Respiratory System 1. Aging results in decreased vital capacity, decreased blood level of O2, and diminished alveolar macrophage activity. 2. Older people are more susceptible to pneumonia, emphysema, bronchitis, and other pulmonary disorders.

CRITICAL THINKING QUESTIONS 1. Your friend Hedge wants to pierce his nose to go along with the 6 earrings in his ear. He thinks a ring through the center would be awesome but wonders if there’s a difference between piercing the center versus the side of the nose. Is there? 2. Suzanne is traveling high into the Andes Mountains of South America to visit some ancient Incan ruins. Though she is in excellent physical condition, she finds herself breathing rapidly. What is happening to Suzanne and why? 3. Upon placement of an endotracheal tube (ET) into an anesthetized patient, the anesthesiology resident realized that air sounds were coming from the epigastric region rather than from the lungs. What went wrong?

4. Gretchen, who is nine years old, is upset because after finally persuading her daddy to play soccer with her, he had to sit down and rest after only 15 minutes. She indignantly berated him, saying, “You know, Daddy, if you’d stop smoking, you could play longer.” Outline the specific breathing difficulties that make it so hard for Gretchen’s father to catch his breath. 5. Latasha is losing her patience with her little sister, LaTonya. “I’m going to hold my breath ‘til I turn blue and die and then you’re gonna get it!” screams LaTonya. Latasha is not too worried. Why not?

ANSWERS TO FIGURE QUESTIONS A 23.1 The conducting portion of the respiratory system includes the nose, nasal cavity, pharynx, larynx, trachea, bronchi, and bronchioles (except the respiratory bronchioles). 23.2 The path of air is external nares n nasal vestibule n nasal cavity n internal nares. 23.3 During swallowing, the epiglottis closes over the rima glottidis, the entrance to the trachea, to prevent aspiration of food and liquids into the lungs. 23.4 The main function of the vocal folds is voice production. 23.5 Because the tissues between the esophagus and trachea are soft, the esophagus can bulge and press against the trachea during swallowing. 23.6 The left lung has two lobes and two lobar bronchi; the right lung has three of each. 23.7 The pleural membrane is a serous membrane. 23.8 Because two-thirds of the heart lies to the left of the midline, the left lung contains a cardiac notch to accommodate the

position of the heart. The right lung is shorter than the left because the diaphragm is higher on the right side to accommodate the liver. 23.9 Segmental bronchi supply a bronchopulmonary segment. 23.10 The wall of an alveolus is made up of type I alveolar cells, type II alveolar cells, and associated alveolar macrophages. 23.11 The respiratory membrane averages 0.5 mm in thickness. 23.12 The diaphragm is responsible for about 75 percent of each inhalation during quiet breathing. 23.13 The DRG contains neurons that have cycles of activity/inactivity. 23.14 The phrenic nerves innervate the diaphragm. 23.15 Peripheral chemoreceptors are responsive to changes in the partial pressures of oxygen and carbon dioxide, and concentrations of H⫹ in the blood. 23.16 The respiratory system begins to develop about 4 weeks after fertilization.

Chapter 23: The Respiratory System  

Chapter 23: The Respiratory System

Chapter 23: The Respiratory System  

Chapter 23: The Respiratory System