Download 25. Respiratory System

O U T L I N E
25.1 General Organization and Functions of the Respiratory
System 748
25.1a Respiratory System Functions
25.2 Upper Respiratory Tract
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25.2a Nose and Nasal Cavity 750
25.2b Paranasal Sinuses 750
25.2c Pharynx 750
25.3 Lower Respiratory Tract
25.3a
25.3b
25.3c
25.3d
Larynx 753
Trachea 757
Bronchial Tree 758
Respiratory Bronchioles, Alveolar Ducts, and Alveoli
25.4 Lungs
25.4a
25.4b
25.4c
25.4d
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25
Respiratory
System
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Pleura and Pleural Cavities 762
Gross Anatomy of the Lungs 762
Blood Supply To and From the Lungs
Lymphatic Drainage 765
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25.5 Pulmonary Ventilation 766
25.6 Thoracic Wall Dimensional Changes During External
Respiration 767
25.7 Innervation of the Respiratory System 769
25.7a Ventilation Control by Respiratory Centers of the Brain
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25.8 Aging and the Respiratory System 771
25.9 Development of the Respiratory System 774
MODULE 11: RESPIR ATORY SYSTEM
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Respiratory System
he respiratory (res ṕ i-ra -̆ tōr ́ē; respiro = to breathe) system
provides the means for gas exchange required by living cells.
Oxygen must be supplied without interruption, and carbon dioxide, a
waste product generated by the cells, must be continuously expelled.
The respiratory and cardiovascular systems are inseparable partners.
While the respiratory system exchanges gases between the atmosphere and the blood, the cardiovascular system transports those
gases between the lungs and the body cells. This chapter examines
the cells, tissues, and organs involved in the complex and vital
process of respiration.
T
25.1 General Organization and
Functions of the Respiratory System
Learning Objectives:
1. Identify the components of the conducting and respiratory
portions of the respiratory system.
2. Describe and compare external and internal respiration.
3. Identify and describe the other functions of the respiratory
system.
Anatomically, the respiratory system consists of an upper
respiratory tract and a lower respiratory tract (figure 25.1).
Functionally, it can be divided into a conducting portion, which
transports air, and a respiratory portion, where gas exchange
with the blood occurs. The conducting portion includes the nose,
nasal cavity, and pharynx of the upper respiratory tract and the
larynx, trachea, and progressively smaller airways (from the primary bronchi to the terminal bronchioles) of the lower respiratory
tract. The respiratory portion is composed of small airways called
respiratory bronchioles and alveolar ducts as well as air sacs called
alveoli in the lower respiratory tract.
25.1a Respiratory System Functions
The primary function most of us associate with the respiratory
system is breathing, also termed pulmonary ventilation. Breathing
consists of two cyclic phases: inhalation (in-ha -̆ lā ś hun̆ ), also
called inspiration, and exhalation, also called expiration (eks-pirā ś hŭn). Inhalation draws gases into the lungs, and exhalation
forces gases out of the lungs.
Gas Exchange
The continuous movement of gases into and out of the lungs is
necessary for the process of gas exchange. There are two types
of gas exchange: external respiration and internal respiration.
External respiration involves the exchange of gases between the
atmosphere and the blood. Oxygen in the atmosphere is inhaled
into the lungs. It diffuses from the lungs into the blood within the
cardiovascular system at the same time carbon dioxide diffuses
from the blood into the lungs in order to be exhaled. Internal
respiration involves the exchange of gases between the blood and
the cells of the body. Blood transports oxygen from the lungs to
the body cells and transports carbon dioxide produced by the body
cells to the lungs.
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Sphenoidal sinus
Frontal sinus
Upper
respiratory
tract
Nasal cavity
Pharynx
Larynx
Trachea
Lower
respiratory
tract
Bronchi
Lungs
Pleura
Diaphragm
Figure 25.1
Gross Anatomy of the Respiratory System. The major components
of the respiratory system are organized into the upper and lower
respiratory tracts.
In addition to gas exchange, the respiratory system also functions in gas conditioning, sound production, olfaction, and defense.
Gas Conditioning
As inhaled gases pass through conducting airways, the gases are
“conditioned” prior to reaching the gas exchange surfaces of the
lungs. Specifically, the gases are warmed to body temperature,
humidified (moistened), and cleansed of particulate matter through
contact with the respiratory epithelium and its sticky mucous covering. Conditioning is facilitated by the twisted pathways through
the nasal cavity and paranasal sinuses, which cause the inhaled
air to become very turbulent during inhalation. This swirling
of inhaled gases means the air remains in the nasal cavity and
paranasal sinuses for a relatively longer time, providing greater
opportunity for conditioning.
Sound Production
As air is forced out of the lungs and moves through the larynx,
sound may be produced, such as speech or singing. Other anatomic structures aid sound production, including the nasal cavity,
paranasal sinuses, teeth, lips, and tongue.
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Olfaction
The superior region of the nasal cavity is covered with olfactory epithelium, which contains receptors for the sense of smell
(see chapter 19). These receptors are stimulated when airborne
molecules are inhaled and dissolved in the mucus covering this
olfactory epithelium.
Defense
Finally, both the structure of the respiratory system and some of
the cells within the respiratory epithelium protect the body against
infection by airborne molecules. The entrance to the respiratory
system (the nose) is inferiorly directed, is lined with coarse hairs,
and has twisted passageways to prevent large particles, microorganisms, and insects from entering. Additionally, numerous
goblet cells are dispersed throughout the pseudostratified ciliated
Respiratory System
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columnar epithelium lining much of the upper respiratory tract.
Mucous glands housed within the lamina propria deep to the epithelium contribute to the layer of mucus covering the epithelium
and keep it from drying out. Mucous glands also secrete lysozyme,
an enzyme that helps defend against inhaled bacteria. The layer
of sticky mucus traps inhaled dust, dirt particles, microorganisms,
and pollen. If we are exposed to airborne allergens, large quantities of small particulate material, irritating gases, or pathogens, the
rate of mucin production increases.
W H AT D I D Y O U L E A R N?
1
●
2
●
Explain the functions carried out by the respiratory system in
addition to gas exchange.
How does mucus help with respiratory system functions?
CLINICAL VIEW
Cystic Fibrosis
Cystic fibrosis (sis t́ ik f ı̄-brō ś is) is the most common serious genetic
disease in Caucasians, occurring with a frequency of 1 in 3200 births.
The condition is inherited as an autosomal recessive trait, and is rare
among people of Asian and African descent. The name cystic fibrosis
refers to the characteristic scarring and cyst formation within the pancreas, first recognized in the 1930s. Cystic fibrosis affects the organs
that secrete mucin, tears, sweat, digestive juices, and saliva. A defective gene produces an abnormal plasma membrane protein involved in
chloride ion transport, so individuals with cystic fibrosis cannot secrete
chloride. This lack of chloride secretion causes sodium and water to
move from the mucus back into the secretory cell itself, thus dehydrating the mucus covering the epithelial surface. The mucus becomes thick
and sticky, obstructing the airways of the lungs and the ducts of the
pancreas and salivary glands. In the lungs, the mucus becomes so
thick it results in airway obstruction. Pulmonary infections, secondary
to airway obstruction, are common and can be life-threatening. In the
case of the pancreas, the obstructed ducts lead to a backup of digestive
enzymes that eventually destroy the pancreas itself.
pulmonary infections are required chronically, because prevention
and early treatment of infection are vital to reducing long-term
complications. Absorption problems caused by pancreatic damage
are treated with orally administered digestive enzymes, vitamins,
and caloric supplements. Since the gene responsible for cystic
fibrosis has been identified, scientists have been investigating
ways to insert copies of the healthy gene into the epithelial cells
of the respiratory tracts of cystic fibrosis patients. In the most
promising method found thus far, the healthy gene is transmitted
via a modified adenovirus.
Mucus builds up and
blocks the bronchial
tree, leading to chronic
respiratory infections.
Interestingly, the normal chloride transport protein works in the
opposite direction in the sweat glands of the skin. Chloride and sodium
are not reabsorbed from the sweat, and so they become concentrated
on the skin in individuals with cystic fibrosis. Mothers of babies with
cystic fibrosis often find that the baby tastes “salty” when kissed.
Thus, clinically elevated chloride levels in sweat are one method of
diagnosing the disease.
Mucus buildup blocks
the pancreatic ducts
and prevents digestive
enzymes from entering
the small intestine.
The primary treatment for cystic fibrosis involves agents that
break up the thick mucus in the lungs. In addition, antibiotics for
Cystic fibrosis results in thickened mucus that obstructs both the
respiratory passageways and ducts of glands such as the pancreatic ducts.
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25.2 Upper Respiratory Tract
Learning Objectives:
1. Identify the structures and describe the organization and
functions of upper respiratory tract organs.
2. Identify and compare the regions of the pharynx.
The upper respiratory tract is composed of the nose and
nasal cavity, paranasal sinuses, pharynx, and associated structures. These structures are all part of the conducting portion of the
respiratory system. When an individual has an upper respiratory
tract infection, some or all of these structures are involved.
25.2a Nose and Nasal Cavity
The nose is the main conducting airway for inhaled air. The nose
is supported superiorly by paired nasal bones that form the bridge
of the nose. Anteroinferiorly from the bridge is the fleshy, cartilaginous dorsum nasi. The dorsum nasi is supported by one pair of lateral cartilages and two pairs of alar cartilages. Paired nostrils, or
nares (nā ŕ es; sing., nā ŕ is), open on the inferior surface of the nose.
The internal surface of the nose leads to the nasal cavity
(figure 25.2). The nasal cavity is continuous posteriorly with the
nasopharynx via paired openings called choanae (kō ́an-ē; sing.,
choana), or internal nares. The frontal bone, nasal bones, cribriform plate of the ethmoid, and sphenoid bone form the roof of the
nasal cavity. The palatine process of the maxillae and the horizontal plate of the palatine bones form the hard palate, which is the
nasal cavity floor. The anterior region of the nasal cavity, near the
nostrils, is called the vestibule.
The nasal cavity is lined with pseudostratified ciliated
columnar epithelium. Within this epithelium are numerous goblet
cells that produce mucin, and immediately deep to this epithelium
is an extensive vascular network. Near the vestibule are coarse
hairs called vibrissae (vı̄-bris ́ē; sing., vibrissa; vibro = to quiver)
that help trap larger particles before they pass through the nasal
cavity. The most superior part of the nasal cavity contains the
olfactory epithelium, which is composed of both a pseudostratified ciliated columnar epithelium and olfactory receptor cells.
The nasal septum divides the nasal cavity into left and right
portions. It is formed anteriorly by septal nasal cartilage. A thin,
bony sheet formed by the perpendicular plate of the ethmoid bone
(superiorly) and the vomer bone (inferiorly) forms the posterior
part of the nasal septum.
Along the lateral walls of the nasal cavity are three paired,
bony projections: the superior, middle, and inferior nasal conchae (kon ́kē; sing., concha; a shell). These conchae subdivide
the nasal cavity into separate air passages, each called a nasal
meatus (mē-ā t́ us̆ ). The superior, middle, and inferior meatuses are
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located immediately inferior to their corresponding nasal conchae.
As inhaled air passes over constricted, narrow grooves in each
meatus, the inhaled air becomes turbulent. Increased turbulence
ensures that the air remains in the nasal cavity for a longer time,
so that the air becomes warmed and humidified. Because the conchae help produce this turbulence, they are sometimes called the
“turbinate” bones.
Besides functioning in filtration, conditioning, and olfaction,
the nasal cavity is a resonating chamber that contributes to sound
production, discussed later in this chapter.
W H AT D O Y O U T H I N K ?
1
●
What does it mean if someone has a “deviated septum”? What
kinds of problems can arise with a deviated septum?
25.2b Paranasal Sinuses
Four bones of the skull contain paired air spaces called the paranasal (par-a -̆ nā ś a l̆ ; para = alongside) sinuses, which together
decrease skull bone weight. These spaces are named for the bones
in which they are housed; thus, from a superior to inferior direction,
they are the frontal, ethmoidal, sphenoidal, and maxillary sinuses
(figure 25.3; see also chapter 7). All sinuses communicate with
the nasal cavity by ducts and are lined with the same pseudostratified ciliated columnar epithelium as the nasal cavity.
Study Tip!
The nasal cavity and the paranasal sinuses are the primary structures that warm and humidify the air we inhale. To illustrate this,
breathe through your mouth instead of your nose on a cold day. Your
throat and trachea may feel “raw,” and you may cough because the air
entering the lungs from your mouth is not being properly conditioned.
Repeat this experiment by breathing through your nose. Once the air is
warmed and humidified in the nasal cavity, it is much easier to breathe.
25.2c Pharynx
The common space used by both the respiratory and digestive
systems is the pharynx (far ́ ingks), commonly called the throat
(figure 25.2). The pharynx is funnel-shaped, meaning that it is
slightly wider superiorly and narrower inferiorly. The pharynx
originates posterior to the nasal and oral cavities and extends
inferiorly to the level of the bifurcation of the larynx and
esophagus. For most of its length, the pharynx is the common
pathway for both inhaled and exhaled air (the respiratory system)
and ingested food (the digestive system).
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Paranasal
sinuses
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Frontal sinus
Sphenoidal sinus
Superior meatus
Middle meatus
Inferior meatus
Choanae
Superior nasal concha
Middle nasal concha
Nasal
cavity Inferior nasal concha
Vestibule
Nasal
cavity
Nostril
Hard palate
Pharyngeal tonsil
Opening of auditory tube
Soft palate
Uvula
Oral cavity
Nasopharynx
Oropharynx
Tongue
Pharynx
Laryngopharynx
Palatine tonsil
Lingual tonsil
Epiglottis
Larynx
Esophagus
Thyroid cartilage
Trachea
Cricoid cartilage
Pharynx:
Nasopharynx
Oropharynx
Laryngopharynx
Ethmoidal sinuses
Sphenoidal sinus
Superior nasal concha
Superior meatus
Middle nasal concha
(b) Regions of pharynx
Middle meatus
Inferior nasal concha
Vestibule
Hard palate
Inferior meatus
Nasopharynx
Soft palate
Uvula
Tongue
Oral cavity
Dentures
Oropharynx
Lingual tonsil
Laryngopharynx
Epiglottis
Thyroid cartilage
Esophagus
Cricoid cartilage
(a) Sagittal section
Figure 25.2
Anatomy of the Upper Respiratory Tract. The upper respiratory tract includes the nose, nasal cavity, paranasal sinuses, and pharynx.
(a) A diagrammatic sagittal section and cadaver photo of the head show the upper respiratory tract structures and their relationship to the
larynx, trachea, and esophagus. (b) The three specific regions of the pharynx (nasopharynx, oropharynx, and laryngopharynx) are highlighted
in a diagrammatic sagittal section.
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Frontal
Ethmoidal
Sphenoidal
Maxillary
drink enters the nasopharynx and the nasal cavity. For example,
if a person is swallowing and laughing at the same time, the soft
palate cannot form as good a seal for the nasopharynx. The force
from the laughing and the lack of a good seal may propel some of
the material into the nasal cavity. In other instances, severe vomiting can propel material so forcibly that the vomitus is expelled
through both the oral cavity and the nasal cavity.
In the lateral walls of the nasopharynx, paired auditory
tubes (eustachian tubes or pharyngotympanic tubes) connect the
nasopharynx to the middle ear. Recall that the auditory tubes
equalize air pressure between the middle ear and the atmosphere
by allowing excess air pressure to be released into the nasopharynx. The posterior nasopharynx wall also houses a single
pharyngeal tonsil (commonly called the adenoids [ad ́e -̆ noydz;
aden = gland, eidos = resemblance]).
Oropharynx
Figure 25.3
Paranasal Sinuses. The paranasal sinuses are air-filled cavities
named for the bones in which they are found: frontal, ethmoidal,
sphenoidal, and maxillary.
The pharynx is lined by a mucosa and contains skeletal muscles that are primarily used for swallowing. Its flexible lateral walls
are distensible in order to force swallowed food into the esophagus.
The pharynx is partitioned into three adjoining regions: the nasopharynx, oropharynx, and laryngopharynx (table 25.1). Figure
25.2b color-codes these regions of the pharynx for your reference.
Nasopharynx
The nasopharynx (nā ź ō-far ́ ingks) is the superiormost region of
the pharynx. The nasopharynx is located directly posterior to the
nasal cavity and superior to the soft palate, which separates it from
the posterior part of the oral cavity. It is lined with a pseudostratified ciliated columnar epithelium.
Normally, only air passes through the nasopharynx. Material
from the oral cavity and oropharynx is typically blocked from
entering the nasopharynx by the soft palate, which elevates when
we swallow. However, sometimes an accident occurs, and food or
Table 25.1
The middle pharyngeal region, the oropharynx (ōr ́ō-far ́ ingks), is
immediately posterior to the oral cavity. The oropharynx is bounded
by the edge of the soft palate superiorly and by the hyoid bone inferiorly. It is a common respiratory and digestive pathway through which
both air and swallowed food and drink pass. Nonkeratinized stratified squamous epithelium lines the oropharynx because this epithelium is strong enough to withstand the abrasion of swallowed food.
The fauces (faw ś ēz; throat) is the opening that represents
the threshold for entry into the oropharynx from the oral cavity.
Two pairs of muscular arches, the anterior palatoglossal arches
and the posterior palatopharyngeal arches, form the entrance into
the oropharynx from the oral cavity.
Lymphatic organs in the oropharynx provide the “first line
of defense” against ingested or inhaled foreign materials. The
palatine tonsils are on the lateral wall between the arches, and the
lingual tonsils are at the base of the tongue.
Laryngopharynx
The inferior, narrowed region of the pharynx is the laryngopharynx
(la-̆ ring ǵ ō-far ́ingks). It extends inferiorly from the hyoid bone and
is continuous with the larynx and esophagus. The laryngopharynx
terminates at the superior border of the esophagus, which is equivalent to the inferior border of the cricoid cartilage in the larynx. In
fact, the larynx forms the anterior wall of this part of the pharynx.
The laryngopharynx is lined with a nonkeratinized stratified squamous epithelium since it permits passage of both food and air.
Regions of the Pharynx
Function
Epithelial Lining
Characteristics
Nasopharynx
Conducts air
Pseudostratified ciliated columnar
epithelium
Posterior to nasal cavity
Pharyngeal tonsil on posterior wall
Auditory tubes open into nasopharynx to equalize air
pressure in the middle ear
Oropharynx
Conducts air; serves as passageway
for food and drink
Nonkeratinized stratified squamous
epithelium
Posterior to oral cavity
Paired palatine tonsils on lateral walls
Lingual tonsils on base of tongue (and thus in anterior
region of oropharynx)
Extends between soft palate and level of hyoid bone
Laryngopharynx
Conducts air; serves as passageway
for food and drink
Nonkeratinized stratified squamous
epithelium
Extends from level of hyoid bone to beginning of
esophagus (posterior to level of cricoid cartilage in
larynx)
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W H AT D I D Y O U L E A R N?
3
●
4
●
5
●
What changes occur in inhaled gases as they travel through the
respiratory system?
What is the function of the nasal conchae?
How is swallowed food prevented from entering the nasopharynx?
25.3 Lower Respiratory Tract
Learning Objectives:
1. Identify the structures and describe the organization and
functions of lower respiratory tract organs and regions.
2. Describe the characteristics of the respiratory membrane.
The lower respiratory tract is made up of conducting airways (larynx, trachea, bronchi, bronchioles, and their associated
structures) as well as the respiratory portion of the respiratory
system (respiratory bronchioles, alveolar ducts, and alveoli) (table
25.2; see figure 25.1). A lower respiratory tract infection affects
some or all of these structures.
25.3a Larynx
The larynx (lar ́ ingks), also called the voice box, is a short, somewhat cylindrical airway (figure 25.4). It is continuous superiorly
with the laryngopharynx, and inferiorly with the trachea; it is anterior to the esophagus. The superior aspect of the larynx is lined
Table 25.2
1
1
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with a nonkeratinized stratified squamous epithelium. Inferior to
the vocal cords, the larynx lining becomes a pseudostratified ciliated columnar epithelium. The larynx conducts air into the lower
respiratory tract, and produces sounds.
The larynx is supported by a framework of nine pieces of
cartilage (three individual pieces and three cartilage pairs) that
are held in place by ligaments and muscles. The largest cartilage
is the thyroid cartilage, which forms only the anterior and lateral walls of the larynx. It has no posterior component and it is
formed from hyaline cartilage. A dense connective tissue band
called the thyrohyoid membrane attaches the superior border
of the thyroid cartilage to the hyoid bone. The V-shaped anterior projection of the thyroid cartilage is called the laryngeal
(la -̆ rin ́ jē-a ̆l) prominence (commonly referred to as the “Adam’s
apple” in males). The overall growth of the thyroid cartilage is
stimulated by testosterone; thus, the Adam’s apple is usually
prominent and larger in males following puberty.
The ring-shaped cricoid (krı̄ ́ koyd; kridos = a ring) cartilage
forms the inferior base of the larynx and connects to the trachea
inferiorly. The cricoid cartilage is composed of hyaline cartilage. It
has a narrow anterior region, but its posterior region is wide to support the posterior larynx. A dense regular connective tissue band
called the cricothyroid ligament attaches the cricoid cartilage to
the inferior edge of the thyroid cartilage.
The large, spoon- or leaf-shaped epiglottis (ep-i-glot ́is; epi =
on, glottis = mouth of windpipe) is formed primarily of elastic cartilage. The epiglottis projects superiorly into the pharynx from its
Structures of the Lower Respiratory Tract
Structure
Anatomic Description
Wall Support
Epithelial Lining
Function
Larynx
Connects to pharynx and
trachea; composed of
cartilage, skeletal muscle,
and laryngeal ligaments; also
called the voice box
Nine pieces of cartilage;
supported by ligaments and
skeletal muscle
Nonkeratinized stratified
squamous epithelium
superior to vocal folds;
pseudostratified ciliated
columnar epithelium inferior
to vocal folds
Conducting: Air
Produces sound
Trachea
Flexible, but semirigid
tubular organ connecting
larynx to primary bronchi;
incomplete, C-shaped
cartilages keep trachea
patent (open)
C-shaped cartilage rings
Pseudostratified ciliated
columnar epithelium
Conducting: Air
Bronchi
Largest airways of the
bronchial tree; consist of
primary, secondary, tertiary,
and smaller bronchi
Incomplete rings and
irregular plates of cartilage;
some smooth muscle
Larger bronchi lined by
pseudostratified ciliated
columnar epithelium;
smaller bronchi lined by
simple columnar epithelium
Conducting: Air
Bronchioles
Smaller conducting airways
of bronchial tree; larger
bronchioles branch into
smaller bronchioles; terminal
bronchioles are the last part
of the conducting portion
No cartilage; proportionately
greater amounts of smooth
muscle in walls of terminal
bronchioles
Epithelium ranges from
simple columnar (for the
largest bronchioles) to
simple cuboidal (for smaller
bronchioles)
Conducting: Air
Smooth muscle in
walls allows for
bronchoconstriction and
bronchodilation
Respiratory bronchioles
Smallest conducting airways;
begin the respiratory portion
No cartilage; smooth muscle
is scarce in their walls
Simple cuboidal epithelium
Respiratory: Gas exchange
Alveolar ducts
Tiny airways that branch
off respiratory bronchioles;
multiple alveoli found along
walls of alveolar duct
No cartilage, no smooth
muscle
Simple squamous epithelium
Respiratory: Gas exchange
Alveoli
Tiny microscopic air sacs
No cartilage, no smooth
muscle
Simple squamous epithelium
Respiratory: Gas exchange
Structures are listed in the order that air passes through them during inhalation.
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Epiglottis
Aryepiglottic fold
Epiglottis
Hyoid
bone
Thyrohyoid
membrane
Hyoid bone
Thyrohyoid
membrane
Thyrohyoid
membrane
Adipose
connective tissue
Thyroid cartilage
Cuneiform
cartilage
Larynx
Thyroid
cartilage
Corniculate cartilage
Vestibular ligament
Vocal ligament
Arytenoid cartilage
Laryngeal
prominence
Cricothyroid
ligament
Cricothyroid ligament
Cricoid cartilage
Cricoid
cartilage
Trachea
Tracheal
cartilage
(a) Anterior
Tracheal
cartilage
(b) Posterior
(c) Midsagittal
Figure 25.4
Larynx. The larynx functions primarily to prevent food and fluid from entering the lower respiratory tract. Its secondary function is
sound production. Laryngeal anatomy and its relationship to the hyoid bone and trachea are compared in (a) anterior, (b) posterior, and
(c) midsagittal views.
attachment to the thyroid cartilage. When a person swallows, the
larynx moves anteriorly and superiorly, causing the epiglottis to close
over the laryngeal opening and prevent materials from entering the
larynx. After swallowing, the larynx returns to its normal position,
and the epiglottis elevates and returns to its original position.
The paired arytenoid (ar-i-tē n
́ oyd) cartilages have a pyramidal shape, and they rest on the superoposterior border of the cricoid cartilage. The paired corniculate (kōr-nik ū́ -lāt; corniculatus =
horned) cartilages attach to the superior surface of the arytenoid
cartilages. The paired cuneiform (kū n
́ ē-i-fōrm; cuneus = wedge)
cartilages do not directly attach to any other cartilages. Instead,
they are supported within a mucosa-covered connective tissue
sheet called the aryepiglottic fold. The aryepiglottic fold extends
between the lateral sides of each arytenoid cartilage and the epiglottis to support some of the laryngeal soft tissue structures.
Two groups of laryngeal muscles are located within the
larynx. The intrinsic muscles attach to the arytenoid and corniculate cartilages. They cause the arytenoid cartilages to pivot, and
regulate tension on the vocal folds. The extrinsic muscles are the
infrahyoid muscles (see chapter 11) that attach the hyoid bone to
the thyroid cartilage. They normally stabilize the larynx and help
move it during swallowing.
Sound Production
Certain structures of the larynx function specifically in sound
production. Two pairs of ligaments extend from the posterior
surface of the thyroid cartilage to the arytenoid cartilages. The
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inferior ligaments, called vocal ligaments, are covered by a mucous
membrane. These ligaments together with their mucosa are called
the vocal folds (figure 25.5). Vocal folds are “true vocal cords”
because they produce sound when air passes between them. The
superior ligaments are called vestibular ligaments (see figure
25.4c). Together with the mucosa covering them, they are called the
vestibular folds (figure 25.6). These folds are “false vocal cords”
because they have no function in sound production, but protect the
vocal folds. The vestibular folds attach to the corniculate cartilages.
When intrinsic muscles of the larynx make the arytenoid
cartilages pivot, they can abduct or adduct the vocal folds. The
opening between the vocal folds is called the rima glottidis (rı̄ m
́ a ̆;
slit; glo-tı̄ -́ dis). This opening widens if the vocal folds are abducted
and becomes narrower if the vocal folds are adducted (see figure 25.5). The term glottis (glot ́is) refers to the rima glottidis plus
the vocal folds.
When air is forced through the rima glottidis, the vocal folds
begin to vibrate, and this vibration produces sound. The nonkeratinized stratified squamous epithelium lining the vocal folds
withstands this abrasive contact between the two vocal folds and
their vibrational activity during sound production. The length, tension, and position of the vocal folds determine the characteristics
of the sound, as follows:
■
The range of a voice (be it soprano or bass) is determined
by the length of the vocal folds. Longer vocal folds produce
lower sounds than shorter vocal folds. As we grow, our
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Abducted (open) vocal folds
Adducted (closed) vocal folds
Anterior
Thyroid cartilage
Cricoid cartilage
Vocal ligaments
Arytenoid cartilage
Corniculate cartilage
Posterior
(a) Cartilages and ligaments
Base of tongue
Epiglottis
Vestibular folds
Vocal folds
Aryepiglottic fold
Cuneiform cartilage
Corniculate cartilage
Rima glottidis
(b) Laryngoscopic view
Figure 25.5
Vocal Folds. The vocal folds (true vocal cords) are epithelium-covered elastic ligaments extending between the thyroid and arytenoid cartilages.
These folds surround the rima glottidis and are involved in sound production. Adducted (closed) and abducted (open) vocal folds are shown in
(a) a superior view of the cartilages and ligaments only and (b) a diagrammatic laryngoscopic view of the coverings around these cartilages and
ligaments.
■
■
vocal folds increase in length, which is why our voices
become deeper as we mature into adults. Also, both the
growth of the thyroid cartilage and the longer and thicker
vocal folds in mature males help explain why men typically
have deeper voices than females.
Pitch refers to the frequency of sound waves, and is
determined by the amount of tension or tautness on the
vocal folds as regulated by the intrinsic laryngeal muscles.
Increasing the tension on the vocal folds causes the vocal
folds to vibrate more when air passes by them and produces
a higher sound. Conversely, the less taut the vocal folds, the
less they vibrate and the lower the pitch of the sound.
Loudness depends on the force of the air passing across the
vocal folds. A lot of air forced through the rima glottidis
produces a loud sound; a little air forced through the rima
glottidis produces a soft sound. When you whisper, only
the most posterior portion of the rima glottidis is open, and
the vocal folds do not vibrate. Since the vocal folds are not
vibrating, the whispered sounds are all of the same pitch.
mck78097_ch25_747-778.indd 755
Study Tip!
The vocal folds are comparable to the strings of a harp. The short
strings produce the high notes, while the long strings produce the
low notes. Thus, shorter vocal folds produce higher notes than longer
vocal folds.
Keep in mind that recognizable speech also requires the
participation of the pharynx, nasal and oral cavities, paranasal
sinuses, lips, teeth, and tongue. If you have a stuffy nose, the
quality of your voice changes to a more nasal tone. Try this
experiment: Hold your nose and then speak. You will notice that
your voice sounds quite different when air doesn’t pass through
the nasal cavity. A sinus infection can also cause the sound of
the voice to change as fluid accumulation leads to decreased
space in the paranasal sinuses for sound resonance. In addition,
young children tend to have high, nasal-like voices because their
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CLINICAL VIEW
Epiglottis
Aspiration of Foreign Bodies, the
Heimlich Maneuver, and Bronchoscopy
Vestibular fold
A foreign object (e.g., a piece of meat, candy, chewing gum, or grape)
may be accidentally aspirated through the laryngeal opening into the
vestibule of the larynx where it becomes trapped. A foreign body
trapped in the vestibule will cause the laryngeal muscles to spasm,
which tenses the vocal cords. The rima glottidis closes and no air can
enter the trachea. As a result, asphyxiation occurs—and the person
will die in about 4 to 5 minutes from lack of oxygen if the obstruction
is not dislodged and removed.
Vocal fold
Rima glottidis
Aryepiglottic
fold
Figure 25.6
Laryngoscopic View of the Larynx. A superior laryngoscopic view
shows the vestibular folds, the vocal folds, and the rima glottidis
opening into the trachea.
CLINICAL VIEW
Laryngitis
Laryngitis (lar-in-jı̄ t́ is) is inflammation of the larynx that may
extend to the surrounding structures. Viral or bacterial infection
is the number one cause of laryngitis. Less frequently, laryngitis
follows overuse of the voice, such as yelling for several hours at
a football game. Symptoms include hoarse voice, sore throat, and
sometimes fever. In severe cases, the inflammation and swelling can extend to the epiglottis. In children, whose airways are
proportionately smaller, a swollen epiglottis may lead to sudden
airway obstruction and become a medical emergency.
Inflamed vocal folds
Vestibular fold
Rima glottidis
A laryngoscopic view shows the inflamed, reddened vocal folds
characteristic of laryngitis.
mck78097_ch25_747-778.indd 756
The Heimlich maneuver should not be performed on anyone that is
not choking. A person is choking if he or she cannot talk, cough, or
breathe, and may turn gray or blue. The Heimlich maneuver is based
on the concept that, because the lungs still contain air, sudden compression with quick thrusts of the abdomen just below the diaphragm
causes the diaphragm to elevate and compress the lungs, expelling
air through the trachea and into the larynx. This maneuver usually
dislodges the foreign body from the larynx and propels the foreign
object back up into the mouth. To perform the Heimlich maneuver
on a conscious adult, the rescuer stands behind the affected person,
who may be either sitting or standing. The rescuer makes a fist with
one hand and places it, thumb toward the person choking, below the
xiphoid process and above the umbilicus. The rescuer encircles the
affected person’s waist, placing the other hand on top of the fist.
In a series of six to ten sharp and distinct thrusts superiorly and
inward, the rescuer attempts to develop enough pressure to create
an artificial cough, which will force the foreign object out of the
larynx. It may be necessary to repeat the procedure several times
before the object is dislodged. If repeated attempts do not free the
airway, an emergency cut in the lower larynx (cricothyrotomy) may
be necessary. You can also perform the maneuver on yourself if you
are alone. To apply the Heimlich maneuver to yourself, make a fist
with one hand and place it in the middle of the body at a spot above
the navel and below the sternum, then grasp the fist with the other
hand and push sharply inward and upward. If this fails, the choking
person should press the upper abdomen over the back of a chair,
edge of a table, porch railing or something similar, and thrust up and
inward until the object is dislodged.
Smaller aspirated foreign bodies (e.g., sunflower seeds, pieces of
bone, or tooth chips) sometimes do not get lodged in the larynx and
travel to the bronchi or bronchioles. Due to the vertical orientation
of the right bronchus, most foreign bodies that get past the larynx
and trachea will end up lodged in the right bronchus and lower lobe
of the right lung. In adults, choking or coughing is present in 95% of
individuals presenting with foreign body aspiration. In cases where
the foreign object goes into the bronchus, the individual usually can
still breathe, but it remains an urgent medical condition and professional help will be required.
The medical practitioner will most likely use a procedure called
bronchoscopy to locate and remove the object. Bronchoscopy is a
procedure during which a practitioner uses a viewing tube (bronchoscope) to evaluate an individual’s trachea and bronchi for abnormalities including foreign bodies. The bronchoscope is inserted into the
airways, usually through the nose or mouth, or occasionally through
a tracheostomy. There are two types of bronchoscopes: rigid and
flexible. In general, foreign body removal is best done with a rigid
bronchoscope under general anesthesia.
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Respiratory System
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Thyroid
cartilage
Cricoid
cartilage
Layer of mucus
Trachealis muscle
Anular
ligament
Esophagus
Tracheal
cartilage
Lumen of trachea
Trachea
Movement
of mucus
toward
pharynx
Trachea
Pseudostratified
ciliated columnar
epithelium
C-shaped cartilage
LM 8x
Lamina
propria
Particles
of debris
(b) Cross section
Carina
Goblet cell
Cilia
Left primary
bronchus
Right
primary
bronchus
Pseudostratified
ciliated columnar
epithelium
(c) Microscopic view of tracheal lining
(a) Anterior view
Figure 25.7
Trachea. (a) The trachea connects to the larynx superiorly and the primary bronchi inferiorly. (b) A cross-sectional photomicrograph shows the
relationship of the trachea (anteriorly) and the esophagus (posteriorly). The wall of the trachea is supported by C-shaped rings of cartilage. (c) The
trachea is lined with a pseudostratified ciliated columnar epithelium that propels mucus and debris away from the lungs and toward the pharynx.
sinuses are not yet well-developed, so they lack large “chambers”
where sounds can resonate. A child also has shorter, smaller
vocal folds, which produce a higher voice. When a male goes
through puberty, his laryngeal cartilages and vocal folds grow
rapidly, producing the “cracking” voice that eventually leads to a
deeper voice at maturity.
25.3b Trachea
The trachea (trā ́ kē-a ̆; rough) is a flexible, slightly rigid tubular
organ often referred to as the “windpipe.” The trachea extends
through the mediastinum and lies immediately anterior to the
esophagus, inferior to the larynx, and superior to the primary
bronchi of the lungs. The trachea averages approximately 2.5 centimeters in diameter and 12 to 14 centimeters in length.
The anterior and lateral walls of the trachea are supported
by 15 to 20 C-shaped tracheal cartilages (figure 25.7a). These
cartilage “rings” reinforce and provide some rigidity to the
tracheal wall to ensure that the trachea remains open (patent) at
all times. The cartilage rings are connected by elastic connective
tissue sheets called anular (an ū́ -la r̆ ; anulus = ring) ligaments.
The open ends of each C-shaped piece are bound together by
the trachealis muscle and an elastic, ligamentous membrane
(figure 25.7b). The trachealis distends during swallowing and
bulges into the lumen of the trachea to allow for expansion of
the esophagus to accommodate larger materials being swallowed. During coughing, it contracts to reduce trachea diameter,
thus facilitating the more rapid expulsion of air and helping to
mck78097_ch25_747-778.indd 757
loosen material (foreign objects or food materials) from the air
passageway.
The mucosa lining the trachea is a pseudostratified ciliated
columnar epithelium with numerous mucin-secreting goblet cells
and an underlying lamina propria that houses mucin-secreting
glands (figure 25.7c). The movement of cilia propels mucus laden
with dust and dirt particles toward the larynx and the pharynx,
where it is swallowed. A submucosal layer deep to the mucosa
contains many submucous glands.
At the level of the sternal angle, the trachea bifurcates into
two smaller tubes, called the right and left primary bronchi
(brong ́ k ı̄; bronchos = windpipe) (or main bronchi). Each primary
bronchus projects inferiorly and laterally toward a lung. The most
inferior tracheal cartilage separates the primary bronchi at their
origin and forms an internal ridge called the carina (ka -̆ rı̄ n
́ a ̆;
keel). The right primary bronchus enters the lung more vertically
than the left primary bronchus (figure 25.7a). The right bronchus is
also wider and shorter than the left. This can be clinically significant because aspirated foreign objects are more likely to become
lodged on the right side (see Clinical View).
W H AT D O Y O U T H I N K ?
2
●
In chronic smokers, the lining of the trachea and bronchi changes
from a pseudostratified ciliated columnar epithelium to a
stratified squamous epithelium. Why do you think this change
occurs? What are some consequences of this epithelium in
the trachea?
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25.3c Bronchial Tree
The bronchial tree is a highly branched system of air-conducting passages that originate from the left and right primary
bronchi and progressively branch into narrower tubes as they
diverge throughout the lungs before ending in terminal bronchioles (figure 25.8). Incomplete rings of hyaline cartilage
support the walls of the primary bronchi to ensure that they
remain open.
The primary bronchi enter the hilum of each lung together
with the pulmonary vessels, lymphatic vessels, and nerves.
Each primary bronchus then branches into secondary bronchi
(or lobar bronchi). The left lung has two secondary bronchi since
it has two lobes; the right lung has three lobes and three secondary bronchi. Secondary bronchi are smaller in diameter than
primary bronchi. They further divide into tertiary bronchi. The
right lung is supplied by 10 tertiary bronchi, and the left lung is
supplied by 8 to 10 tertiary bronchi. (The difference in the number depends upon whether some of the left lung tertiary bronchi
are combined or separate structures.) Each tertiary bronchus
is called a segmental bronchus because it supplies a part of the
lung called a bronchopulmonary segment, discussed later in
this chapter.
CLINICAL VIEW
Tracheotomy and Cricothyrotomy
The tracheotomy (trā-kē-ot ṓ -mē; tome = incision) is one of the oldest
surgical procedures in medicine. A tracheotomy is performed when a
patient requires extended ventilatory (breathing) assistance based on
one of three recognized indications:
■
■
■
The presence of an upper airway obstruction due to a foreign
body, trauma, swelling, etc.
Difficulty breathing due to advanced pneumonia, emphysema,
or severe chest wall injury
Respiratory paralysis, as may result from head injury, polio,
or tetanus infection
Understanding the surface anatomy of the neck is critical to performing
a tracheotomy correctly. Typically, the physician makes a skin incision
about 1 to 1.5 centimeters superior to the suprasternal notch. Care must be
taken not to damage the anterior jugular veins, and sometimes the thyroid
must be incised in the midline and divided to gain access to the trachea.
Retractors are used to separate the subcutaneous tissue and expose the
trachea. Then the surgeon makes an incision in the trachea between the
third and fourth tracheal rings to allow the insertion of a tracheotomy
tube; this opening is called a tracheostomy. Once the tube has been taped
into place, the patient’s breathing bypasses the nasal cavity and larynx.
The tracheotomy is an important and often life-saving procedure,
but it is not without risks. A misplaced incision in the anterior neck
can lead to serious damage of the larynx and possibly even a fatal
hemorrhage. Other potential complications include infection at the
site, aspiration of foreign matter directly into the lungs, or tracheal
stenosis (a narrowing of the trachea at the incision site due to scar
tissue formation).
Cricothyrotomy is an alternative procedure often performed by EMTs
and paramedics to open an individual’s airway during certain emergency situations, such as when the throat or larynx are obstructed
by a foreign object or swelling, when an individual who is not able
to breathe adequately on his or her own, or in cases of major facial
injury that prevent the insertion of an endotracheal tube through
the mouth. A cricothyrotomy is generally performed by making a
vertical incision through the skin and fascia of the anterior neck just
inferior to the thyroid cartilage (care must be taken when making
this incision not to cut the anterior jugular veins, which lie close
together on either side of the midline), then making another transverse incision through the cricothyroid ligament, which lies deep to
this incision (since the inferior end of the pharynx and the superior
end of the esophagus lie directly behind the cricoid ligament, care
must be taken to avoid esophageal penetration). Once the incision is
made, a tube is placed into this opening, which allows air exchange
to occur. The procedure should not be attempted on children under
8 years old, if there is evidence of fractured laryngeal cartilages, or
if there is evidence of tracheal transection (transverse cut through
the trachea). Potential complications are similar to tracheotomy and
include bleeding, laryngotracheal injury, tension pneumothorax, and
clogging of the inserted tube with blood or secretions.
Trachea
Thyroid
cartilage
Thyroid gland
Tracheotomy
tube
Cricoid
cartilage
Incision
Scalpel
Sutures
Suprasternal
notch
1 Incision is made superior to suprasternal
notch. Thyroid may have to be cut as well.
mck78097_ch25_747-778.indd 758
2 Retractors separate the tissue, and an
incision is made through the third and fourth
tracheal rings.
3 A tracheotomy tube is inserted, and the
remaining incision is sutured closed.
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Figure 25.8
Bronchial Tree. (a) The
bronchial tree is composed
of conducting passageways
that originate at the primary
bronchi and end at the
terminal bronchioles (not
visible in this view).
(b) The major components
of the bronchial tree are
color-coded.
Larynx
Primary bronchi
Secondary bronchi
Tertiary bronchi
Smaller bronchi
Trachea
(b)
Right primary
bronchus
Right superior
secondary bronchus
Left primary
bronchus
Left superior
secondary
bronchus
Left tertiary
bronchus
Left inferior
secondary
bronchus
Right middle
secondary bronchus
Right inferior
secondary
bronchus
Right tertiary
bronchus
Smaller
bronchi
Smaller bronchi
(a)
CLINICAL VIEW
Bronchitis
Bronchitis (brong-kı̄ t́ is) is inflammation of the bronchi caused by
viruses or bacteria, or by inhaling vaporized chemicals, particulate
matter, or cigarette smoke from the air. Clinically, bronchitis is divided
into two categories, acute and chronic.
Acute bronchitis develops rapidly either during or after an infection,
such as a cold. Symptoms include cough, wheezing, pain upon inhalation, and fever. Most cases of acute bronchitis resolve completely
within 10 to 14 days.
mck78097_ch25_747-778.indd 759
Chronic bronchitis results from long-term exposure to irritants such as
chemical vapors, polluted air, or cigarette smoke. Medically, chronic bronchitis is defined as the production of large amounts of mucus, associated
with a cough lasting 3 continuous months. If exposure to the irritant
persists, permanent changes to the bronchi occur, including (1) thickened
bronchial walls with subsequent narrowing of their lumens, (2) overgrowth
(hyperplasia) of the mucin-secreting cells of the bronchi, and (3) accumulation of lymphocytes within the bronchial walls. These long-term changes
in the bronchi increase the likelihood of bacterial infections, and chronic
bronchitis greatly increases the chance of developing pneumonia as well.
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There are approximately 9 to 12 different levels of bronchial
branch division (not all shown in figure 25.8). Thus, the primary,
secondary, and tertiary bronchi are the first, second, and third
generations of bronchi, respectively. All types of bronchi exhibit
some common characteristics:
■
■
■
Incomplete rings of cartilage in the walls become less
numerous and smaller, eventually consisting only of
scattered pieces of cartilage as the bronchi continue to
divide and decrease in diameter.
The largest branches of bronchi are lined by a
pseudostratified ciliated columnar epithelium, whereas
smaller branches of bronchi are lined by a ciliated columnar
epithelium.
A complete ring of smooth muscle is found between the
mucosa of the airways and the cartilaginous support in
the wall.
The bronchi branch into smaller and smaller tubules that
eventually reach a diameter of less than 1 millimeter. These
smaller tubules, called bronchioles (brong ́ kē-ōl), are no longer
lined with pseudostratified ciliated columnar epithelium, but with
simple columnar or simple cuboidal epithelium. Unlike the smallest bronchi, which have irregular plates of cartilage in their walls,
the walls of bronchioles contain no cartilage, since their small
diameter alone prevents collapse. Instead, they have a thicker
layer of smooth muscle than do bronchi, a characteristic that helps
regulate airway constriction or dilation. Contraction of the smooth
muscle narrows bronchioles (called bronchoconstriction), whereas
relaxation of the smooth muscle dilates bronchioles (called bronchodilation). Constriction or dilation of the bronchioles regulates
the amount of air traveling through the bronchial tree.
The terminal bronchioles are the final segment of the conducting pathway. They conduct air into the respiratory portion of
the respiratory system.
W H AT D O Y O U T H I N K ?
3
●
Can you think of any reasons why you would want your
bronchioles to constrict? Why wouldn’t you want your bronchioles
fully dilated all the time?
25.3d Respiratory Bronchioles, Alveolar Ducts,
and Alveoli
As previously stated, the respiratory portion of the respiratory
system consists of respiratory bronchioles, alveolar ducts, and pulmonary alveoli (figure 25.9). Within this respiratory portion, the
epithelium is much thinner than in the conducting portion, thus
facilitating gas diffusion between pulmonary capillaries and the
respiratory structures.
Terminal bronchioles branch to form the respiratory bronchioles. Subsequent partitioning of the respiratory bronchioles
forms smaller respiratory bronchioles. Eventually, the smallest
respiratory bronchioles subdivide into thin airways called alveolar
ducts, which are lined with a simple squamous epithelium. The
distal end of an alveolar duct terminates as a dilated alveolar sac.
Both of these airways—respiratory bronchioles and alveolar
ducts—contain small saccular outpocketings called alveoli (alvē ́ō-lı̄; sing., al-vē ́ō-lu s̆ ; alveus = hollow sac). An alveolus is about
0.25 to 0.5 millimeter in diameter. Its thin wall is specialized to
promote diffusion of gases between the alveolus and the blood
in the pulmonary capillaries (figure 25.10). The lungs contain
mck78097_ch25_747-778.indd 760
approximately 300 to 400 million alveoli. Alveoli abut one another,
so their sides become slightly flattened. Thus, an alveolus in
cross section actually looks more hexagonal or polygonal in shape
than circular (see figure 25.9a). The small openings in the walls
between adjacent alveoli, called alveolar pores, provide for collateral ventilation of alveoli. The spongy nature of the lung is due to
the packing of millions of alveoli together.
Two cell types form the alveolar wall. The predominant cell
is an alveolar type I cell, also called a squamous alveolar cell.
This simple squamous epithelial cell promotes rapid gas diffusion
across the alveolar wall. The alveolar type II cell, called a septal
cell, is part of a smaller population of cells within the alveolar wall.
Typically, it displays an almost cuboidal shape. Alveolar type II
cells secrete pulmonary surfactant (ser-fak t́ a n̆ t), a fluid composed
of lipids and proteins that coats the inner alveolar surface to reduce
surface tension and prevent the collapse of the alveoli. Alveolar
macrophages, or dust cells, may be either fixed or free. Fixed
alveolar macrophages remain within the connective tissue of the
alveolar walls, while free alveolar macrophages are migratory cells
that continually crawl within the alveoli, engulfing any microorganisms or particulate material that has reached the alveoli. The
alveolar macrophages are able to leave the lungs either by entering
alveolar lymphatics or by being coughed up in sputum (matter
from the respiratory tract, such as mucus mixed with saliva) and
then expectorated from the mouth.
The respiratory membrane is the thin wall between the
alveolar lumen and the blood (see figure 25.10). It consists of the
plasma membranes of an alveolar type I cell, and an endothelial
cell of a capillary, and their fused basement membranes. Oxygen
diffuses from the lumen of the alveolus across the respiratory
membrane into the pulmonary capillary, thereby allowing the
erythrocytes in the blood to become oxygenated again. Conversely,
carbon dioxide diffuses from the blood in the capillary through the
respiratory membrane to enter the alveolus. Once in the alveoli,
carbon dioxide is exhaled from the respiratory system into the
external environment.
Study Tip!
To help understand the relationship between the respiratory bronchiole, alveolar duct, alveolar sac, and alveolus, try this analogy. Visualize
a building at your school with multiple wings that radiate from a common
atrium. A hallway connects each wing to the central atrium. Classrooms
open into the hallways along their length. At the end of each hallway is
an expanded common space lined with more classrooms. (1) The atrium
is like the respiratory bronchiole . . . it distributes respiratory gases into
every hallway. (2) The hallway leading to each wing is like the alveolar
duct . . . it conducts respiratory gases into each wing. (3) The end of
each alveolar duct (hallway) terminates in a large, expanded room (the
alveolar sac) lined with multiple classrooms (alveoli). (4) Each classroom
is like an alveolus . . . it is where gas exchange occurs.
W H AT D I D Y O U L E A R N?
6
●
7
●
8
●
What function is served by the vocal folds?
What are some anatomic differences between bronchi and
bronchioles?
How do terminal and respiratory bronchioles differ in structure
and function?
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761
Branch of
pulmonary artery
Bronchiole
Figure 25.9
Bronchioles and Alveoli. Terminal bronchioles branch into
respiratory bronchioles, which then branch into alveolar ducts and
alveoli. (a) The pulmonary vessels travel with the bronchioles, and
the pulmonary capillaries wrap around the alveoli for gas exchange.
(b) A photomicrograph shows the relationship of respiratory
bronchioles, alveolar ducts, and alveoli. (c) SEM of a terminal
bronchiole, a respiratory bronchiole, alveolar ducts, and alveoli
reveals the honeycomb appearance of the alveoli.
Terminal bronchiole
Respiratory bronchiole
Branch of
pulmonary vein
Arteriole
Capillary
beds
Alveolar duct
Alveoli
Connective
tissue
(a)
Terminal bronchiole
Respiratory
bronchiole
Alveolar
sac
Alveolar ducts
Alveolar duct
Alveoli
LM 60x
(b)
mck78097_ch25_747-778.indd 761
Alveoli
Respiratory bronchiole
SEM 180x
(c)
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Alveolar
Nucleus of capillary
connective
endothelial cell
Nucleus
tissue
of alveolar
Erythrocyte
type I cell
Erythrocyte
Capillary
Pulmonary
capillaries
Alveolar
macrophages
Diffusion of CO2
Diffusion of O2
Alveolar pore
Alveolar
type II cells
Alveolus
Alveolar epithelium
Respiratory
membrane
Fused basement membranes
of the alveolar epithelium and
the capillary endothelium
Alveolar
type I cell
Capillary endothelium
(a)
(b)
Figure 25.10
Alveoli and the Respiratory Membrane. Gas exchange between the alveoli and the pulmonary capillaries occurs across a thin respiratory
membrane. (a) A diagram shows the structural arrangement of several adjacent alveoli. (b) The respiratory membrane consists of an alveolar
type I cell, an endothelial cell of a capillary, and their fused basement membranes. Oxygen diffuses from alveoli into the blood within the
capillary, and carbon dioxide diffuses in the opposite direction. (Note: The pulmonary surfactant covering layer is not shown here.)
25.4 Lungs
Learning Objectives:
1. Identify the structure and describe the function of the pleura.
2. Describe the gross anatomy of the lungs.
3. Identify and discuss the blood supply to and from the
lungs.
4. Discuss the role of lymphatic structures in the function of
the respiratory system.
The lungs house the bronchial tree and the respiratory portion of the respiratory system. The lungs are located on the lateral
sides of the thoracic cavity and separated from each other by the
mediastinum.
W H AT D O Y O U T H I N K ?
4
●
As you will soon learn, the left lung is physically smaller than the
right lung. Why is the left lung smaller?
25.4a Pleura and Pleural Cavities
The outer lung surfaces and the adjacent internal thoracic wall
are lined by a serous membrane called pleura (ploor á )̆ , which is
formed from simple squamous epithelium called a mesothelium.
The outer surface of each lung is tightly covered by the visceral
pleura, while the internal thoracic walls, the lateral surfaces of
mck78097_ch25_747-778.indd 762
the mediastinum, and the superior surface of the diaphragm are
lined by the parietal pleura (figure 25.11). (These pleural layers
may also be viewed in the transverse section of the thoracic cavity
shown in figure 22.2d.) The visceral and parietal pleural layers are
continuous at the hilum of each lung. Between these serous membrane layers is a pleural cavity. When the lungs are fully inflated,
the pleural cavity is a potential space because the visceral and
parietal pleurae are almost in contact with each other. The pleural
membranes produce an oily, serous fluid that acts as a lubricant,
ensuring that opposing pleural membrane surfaces slide by each
other with minimal friction during breathing.
25.4b Gross Anatomy of the Lungs
The paired, spongy lungs are the primary organs of respiration.
Each lung has a conical shape. Its wide, concave base rests inferiorly upon the muscular diaphragm, and its relatively blunt superior
region, called the apex (or cupola), projects superiorly to a point
that is slightly superior and posterior to the clavicle (figure 25.12).
Both lungs are bordered by the thoracic wall anteriorly, laterally,
and posteriorly, and supported by the rib cage. Toward the midline,
the lungs are separated from each other by the mediastinum.
The relatively broad, rounded surface in contact with the thoracic wall is called the costal surface of the lung. The mediastinal
surface of the lung is directed medially, facing the mediastinum
and slightly concave in shape. This surface houses the vertical,
indented hilum through which the bronchi, pulmonary vessels,
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CLINICAL VIEW
Pneumothorax
Pneumothorax (noo-mō-thōr á ks; pneuma = air) is a condition that
occurs when free air gets into the pleural cavity, the space between
the parietal and visceral pleura. A pneumothorax may develop in one of
two ways. Air may be introduced externally from a penetrating injury
to the chest, such as a knife wound or gunshot, or it may originate
internally as when a broken rib lacerates the surface of the lung.
The presence of free air in the pleural space sometimes causes the
affected lung or a portion of it to deflate, a condition termed atelectasis (at-ē-lek t́ ă-sis; ateles = incomplete, ektasis = extension). The
collapsed portion of the lung remains down until the air has been
removed from the pleural space. If a pneumothorax is small, the air
exits naturally within a few days. However, a large pneumothorax is a
medical emergency requiring insertion of a tube into the pleural space
to suck out the free air. After the air has been removed, an airtight
Parietal pleura
Visceral pleura
Pleural cavity
Parietal pleura
Pleural cavity
Visceral pleura
Diaphragm
Figure 25.11
Pleural Membranes. The serous membranes associated with
the lungs are called the pleura. The parietal pleura lines the inner
surface of the thoracic cavity, and the visceral pleura covers the outer
surface of the lungs. The thin space between these layers is called
the pleural cavity.
lymph vessels, and nerves pass. Collectively, all structures passing
through the hilum are termed the root of the lung.
The right and left lungs exhibit some obvious structural differences. Since the heart projects into the left side of the thoracic
cavity, the left lung is slightly smaller than the right lung. The left
lung has a medial surface indentation, called the cardiac impression, that is formed by the heart. The left lung also has an anterior
indented region called the cardiac notch. The descending thoracic
mck78097_ch25_747-778.indd 763
bandage is placed over the entry site to prevent air from reentering
the pleural space.
A particularly dangerous condition is tension pneumothorax, in
which a hole in the chest or lung allows air to enter and acts as a
one-way valve. As the patient struggles to breathe, air is pulled in
through the wound but cannot escape. Air pressure within the pleural
space becomes greater, causing atelectasis of the lung and eventually displacing the heart and mediastinal structures. Both lungs then
become compressed, and respiratory distress and death occur unless
the tension pneumothorax is promptly treated.
In addition to air, fluid can also accumulate in the pleural space. For example,
blood may collect (hemothorax) due to a lacerated artery, a blood vessel
that leaks as a result of surgery, heart failure, or certain tumors. An accumulation of serous fluid within the pleural cavity is called hydrothorax,
and an accumulation of pus, as occurs with pneumonia, is called empyema.
aorta forms a groovelike impression on the medial surface of the
left lung.
The right lung is subdivided into the superior (upper), middle, and inferior (lower) lobes by two fissures. The horizontal fissure separates the superior from the middle lobe, while the oblique
fissure separates the middle from the inferior lobe. The left lung
has only two lobes, superior and inferior, which are subdivided by
an oblique fissure. The lingula of the left lung is located on the
superior lobe. The lingula is homologous to the middle lobe of the
right lung.
The left and right lungs may be partitioned into bronchopulmonary segments—10 in the right lung, and typically 8 to 10 in the
left lung (figure 25.13). (The discrepancy in bronchopulmonary
segment number for the left lung comes from the merging or lumping of some left bronchopulmonary segments into combined ones by
some anatomists.) Each bronchopulmonary segment is supplied by
its own tertiary bronchus and a branch of the pulmonary artery and
vein. In addition, each segment is surrounded by connective tissue,
thereby encapsulating one segment from another and ensuring that
each bronchopulmonary segment is an autonomous unit. Thus, if a
portion of a lung is diseased, a surgeon can remove the entire bronchopulmonary segment that is affected, while the remaining healthy
bronchopulmonary segments continue to function as before.
25.4c Blood Supply To and From the Lungs
Both the pulmonary circulation and the bronchial circulation supply the lungs. Recall from chapter 23 that the pulmonary circulation conducts blood to and from the gas exchange surfaces of the
lungs to replenish its depleted oxygen levels and get rid of excess
carbon dioxide (see figures 23.22 and 23.23). Deoxygenated blood
is pumped from the right ventricle through the pulmonary trunk
into pulmonary arteries, which enter the lung. Thereafter, continuous branching of these vessels leads to pulmonary capillaries
that encircle all alveoli. The deoxygenated blood that enters these
capillaries becomes oxygenated before it returns to the left atrium
through a series of pulmonary venules and veins.
The bronchial circulation is a component of the systemic
circulation. The bronchial circulation consists of tiny bronchial
arteries and veins that supply the bronchi and bronchioles of the
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Respiratory System
Apex
Superior lobe
Horizontal fissure
Oblique fissure
Oblique fissure
Middle
lobe
Cardiac
notch
Lingula
Inferior lobe
Base
Right lung
Left lung
(a) Lateral views
Apex
Superior lobe
Oblique fissure
Oblique fissure
Pulmonary arteries
Hilum
Hilum
Primary bronchi
Horizontal fissure
Cardiac impression
Pulmonary
veins
Middle lobe
Cardiac notch
Inferior lobe
Oblique fissure
Oblique fissure
Base
Right lung
Left lung
(b) Medial views
Figure 25.12
Gross Anatomy of the Lungs. The lungs are composed of lobes separated by distinct depressions called fissures. (a) Lateral views show the three
lobes of the right lung and the two lobes of the left lung. (b) Medial views show the hilum of each lung, where the pulmonary vessels and bronchi
enter and leave the lung.
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Chapter Twenty-Five
Respiratory System
Apicoposterior
Apical
Bronchopulmonary
segments of
superior lobe
765
Anterior
Posterior
Superior
lingular
Anterior
Inferior
lingular
Bronchopulmonary
segments of
superior lobe
Medial
Bronchopulmonary
segments
of middle lobe
Bronchopulmonary
segments
of inferior lobe
Superior
Lateral
Superior
Lateral
basal
Posterior
basal
Posterior
basal
Lateral
basal
Bronchopulmonary
segments of
inferior lobe
Anterior
basal
Anterior
basal
Right lung, lateral view
Left lung, lateral view
Figure 25.13
Bronchopulmonary Segments of the Lungs. The portion of each lung supplied by each tertiary bronchus (represented by different colors) is a
bronchopulmonary segment. (The medial basal bronchopulmonary segment cannot be seen from this view.)
lung. This part of the circulation system is much smaller than the
pulmonary system, because most tiny respiratory structures (alveoli and alveolar ducts) exchange respiratory gases directly with the
inhaled air. Approximately three or four tiny bronchial arteries
branch from the anterior wall of the descending thoracic aorta and
divide to form capillary beds to supply structures in the bronchial
tree. Increasingly larger bronchial veins collect venous blood and
drain into the azygos and hemiazygous systems of veins.
exit these lymph nodes and conduct lymph to bronchopulmonary
lymph nodes located at the hilum of the lung. These vessels drain
first into tracheobronchial lymph nodes and then into the left and
right bronchomediastinal trunks (discussed in chapter 24). The
right bronchomediastinal trunk drains into the right lymphatic
duct, while the left bronchomediastinal trunk drains into the thoracic duct.
W H AT D O Y O U T H I N K ?
25.4d Lymphatic Drainage
Lymph nodes and vessels are located within the connective tissue of the lung as well as around the bronchi and pleura (figure
25.14). The lymph nodes collect carbon, dust particles, and pollutants that were not filtered out by the pseudostratified ciliated
columnar epithelium. The lymph from the lung is conducted first
to pulmonary lymph nodes within the lungs. Lymphatic vessels
5
●
The lymph nodes of the lung become black and darkened over time
in both smokers and nonsmokers. Why do these lymph nodes turn
black?
W H AT D I D Y O U L E A R N?
9
●
What is the hilum of the lung, and how does it function?
Internal jugular vein
Right lymphatic duct
Thoracic duct
Subclavian vein
Bronchomediastinal
trunk
Brachiocephalic vein
Bronchomediastinal trunk
Tracheobronchial lymph nodes
Bronchopulmonary
lymph nodes
Pulmonary
lymph nodes
Bronchopulmonary
lymph node
Pulmonary
lymph nodes
Figure 25.14
Lung Lymphatic Drainage.
Lymph vessels conduct
lymph to the pulmonary,
bronchopulmonary, and
tracheobronchial lymph nodes.
Lymph is then drained by the
bronchomediastinal trunks into
the right lymphatic duct or the
thoracic duct.
Lymph vessels
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CLINICAL VIEW
Pneumonia
Respiratory
bronchiole
Alveoli
Alveolar duct
Pneumonia (noo-mō ń ē-ă) is an infection of the alveoli of the lung.
Common causative agents include viruses and bacteria, and sometimes
fungi. The infection may involve an entire lung or just one lobe.
Pneumonia results in tissue swelling and accumulated leukocytes in the
affected area, thus greatly diminishing the capacity for gas exchange.
Pneumonia is a contagious disease that is usually spread by respiratory droplets. Symptoms include cough, fever, and rapid breathing.
In addition, the bronchi produce and expel sputum (mucus and other
matter), which may be rust- or green-tinged.
Diagnosis of pneumonia depends on symptoms and characteristic
changes seen on a chest x-ray. A sputum culture is often helpful
in identifying the specific organism. Treatment may include antibiotics, respiratory support, and medications to relieve symptoms.
Patients with severe cases of pneumonia or those with coexisting
lung diseases, such as chronic bronchitis or emphysema, may require
supplemental oxygen.
LM 30x
Normal lung tissues.
Thickened
alveolar walls
Fluid and leukocytes
in alveoli
Left lung
Chest x-ray of a patient with pneumonia in the left lung. A normal
lung appears as a black space on an x-ray because its spongy
structure is not dense. In contrast, a pneumonia lung appears white
or opaque on an x-ray due to accumulation of fluid and cells.
LM 75x
Tissues within a lung affected by pneumonia.
■
25.5 Pulmonary Ventilation
Learning Objective:
1. Describe the process of pulmonary ventilation.
Breathing, also known as pulmonary ventilation, is the
movement of air into and out of the respiratory system. At rest,
a normal adult breathes about 16 times per minute, and approximately 500 milliliters of air are exchanged with the atmosphere per
breath. The airflow exchange is caused by the muscular actions
associated with inhalation and exhalation, as well as by differences in atmospheric air pressure and lung (intrapulmonary) air
pressure. Gases are exchanged in the following cycle:
■
■
■
Oxygen in the atmospheric air is drawn into the lungs by
inhalation.
Oxygen is transported to the body cells from the lungs by
blood circulating through the cardiovascular system.
Cells use the oxygen and generate carbon dioxide as a
waste product.
mck78097_ch25_747-778.indd 766
■
Blood transports the carbon dioxide from the body cells to
the lungs.
Carbon dioxide is added to the atmosphere during
exhalation.
The movement of gases into and out of the respiratory
system follows Boyle’s law, which states, “The pressure of a
gas decreases if the volume of the container increases, and vice
versa.” Thus, when the volume of the thoracic cavity increases
even slightly during inhalation, the intrapulmonary pressure
decreases slightly, and air flows into the lungs through the conducting airways. Therefore, air flows from a region of higher
pressure (the atmosphere) into a region of lower pressure within
the lungs (the intrapulmonary region). Similarly, when the volume of the thoracic cavity decreases during exhalation, the intrapulmonary pressure increases and forces air out of the lungs into
the atmosphere.
W H AT D I D Y O U L E A R N?
10
●
What is pulmonary ventilation?
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Chapter Twenty-Five
25.6 Thoracic Wall Dimensional
Changes During External Respiration
Learning Objective:
1. Define and describe how the thoracic cavity changes in
size and shape during respiration.
As you inhale, the dimensions of your thoracic cavity generally increase, forming a larger space for the expanding lungs.
During exhalation, your thoracic cavity dimensions return to their
original size. Thus, the thoracic cavity becomes larger during
inhalation and smaller during exhalation, as diagrammed in figure 25.15. Vertical dimensional changes occur with movements
of the diaphragm, which forms the rounded “floor” of the thoracic
cavity. The diaphragm contracts, causing its depression—that
is, its dome-shaped central portion flattens and moves inferiorly
to press against the abdominal viscera, resulting in inhalation.
When you exhale, the diaphragm relaxes and returns to its original position.
Lateral dimensional changes occur with rib movements.
Elevation of the ribs increases the lateral dimensions of the thoracic cavity, while depression of the ribs decreases the lateral
dimensions of the thoracic cavity.
Figure 25.16 shows that several muscles of external respiration move the ribs:
■
■
■
■
The scalene muscles help increase thoracic cavity
dimensions by elevating the first and second ribs during
forced inhalation.
The external intercostal muscles extend from a superior
rib inferomedially to the adjacent inferior rib. The ribs
elevate upon contraction of the external intercostals,
thereby increasing the transverse dimensions of the thoracic
cavity during inhalation.
The internal intercostal muscles lie at right angles to the
external intercostals and deep to them. Contraction of the
internal intercostals depresses the ribs, but this only occurs
during forced exhalation. Normal exhalation requires no
active muscular effort.
A small transversus thoracis (see also figure 11.13) extends
across the inner surface of the thoracic cage and attaches to
ribs 2–6. It helps depress the ribs.
Two posterior thoracic muscles also assist with external
respiration. These muscles are located deep to the trapezius and
latissimus dorsi, but superficial to the erector spinae muscles (see
also figure 11.11). The serratus posterior superior elevates ribs 2–5
during inhalation, and the serratus posterior inferior depresses
ribs 8–12 during exhalation.
In addition, some accessory muscles assist with external
respiration activities. The pectoralis minor and sternocleidomastoid help with forced inhalation, while the abdominal muscles
(external and internal obliques, transversus abdominis, and rectus
abdominis) assist in active exhalation. (Researchers are still debating the effects of some of the external respiration muscles.)
mck78097_ch25_747-778.indd 767
Respiratory System
767
Finally, a slight anterior-posterior dimensional change
occurs in the thoracic cavity due to movement of the inferior
portion of the sternum. When you inhale, the inferior portion of
the sternum moves anteriorly, slightly increasing the anteriorposterior dimensions of the thorax. When you exhale, the inferior
portion of the sternum moves posteriorly and returns to its original position.
W H AT D I D Y O U L E A R N?
11
●
What types of dimensional changes occur to the thorax when you
inhale, and what muscles are responsible?
Study Tip!
To visualize rib movement during external respiration, think of
the thoracic cavity as a bucket and the ribs as the bucket handles.
When the bucket handles are lifted up, they move relatively farther
away from the edges of the bucket. Thus, the measurement from the
bucket handles (ribs) to the bucket (thoracic cavity) increases, just as
the lateral dimensions of the thoracic cavity increase. When the bucket
handles are depressed, they move next to the edges of the bucket, and
so the distance from the bucket handle (ribs) to the bucket (thoracic
cavity dimension) decreases.
Inhalation: Ribs (bucket handles) elevated,
lateral dimension increased
Exhalation: Ribs (bucket handles) depressed,
lateral dimension decreased
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Chapter Twenty-Five
Respiratory System
Inhalation
Exhalation
Thoracic
cavity
Thoracic
cavity
Vertical
changes
Figure 25.15
Thoracic Cavity Dimensional
Changes Associated with
Breathing. The boxlike thoracic
cavity changes size upon inhalation
and exhalation. During inhalation,
the box increases in vertical, lateral,
and anterior-posterior dimensions
due to movement of the sternum,
ribs, and diaphragm, respectively.
Upon exhalation, these dimensions
decrease, and the thoracic cavity
becomes smaller.
Diaphragm contracts; vertical
dimensions of thoracic cavity increase.
Diaphragm relaxes; vertical
dimensions of thoracic cavity narrow.
Lateral
changes
Ribs are elevated and thoracic cavity widens.
Ribs are depressed and thoracic cavity narrows.
Anterior-posterior
changes
Inferior portion of sternum moves anteriorly.
mck78097_ch25_747-778.indd 768
Inferior portion of sternum moves posteriorly.
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Chapter Twenty-Five
Respiratory System
769
(a) Inhalation
Scalene muscles
elevate 1st and 2nd ribs
Inferior part of
sternum moves
anteriorly
External intercostal
muscles elevate ribs
Diaphragm moves
inferiorly during
contraction
(b) Exhalation
Transversus thoracis
depresses ribs
Internal intercostal
muscles depress ribs
Inferior part of
sternum moves
posteriorly
Diaphragm moves
superiorly as it relaxes
Figure 25.16
Muscles Involved in External Respiration. (a) Inhalation requires contraction of the external intercostal muscles (to elevate the ribs) and the
diaphragm (which moves inferiorly during contraction). Forced inhalation also requires contraction of the scalene muscles, which elevate the first
and second ribs. (b) During exhalation, these muscles relax. Additionally, the transversus thoracis and internal intercostal muscles contract to
depress the ribs during forced exhalation. Companion x-rays show the thoracic cavity during inhalation and exhalation.
25.7 Innervation of the Respiratory
System
Learning Objective:
1. Identify the components of the autonomic nervous system
that regulate ventilation.
The larynx, trachea, bronchial tree, and lungs are innervated by the autonomic nervous system. The autonomic nerves
that innervate the heart also send branches to these respiratory
structures (see figure 22.12 for a review of these nerves). The vagus
nerve is the primary innervator of the larynx. Damage to one of
the vagus nerve branches going to the larynx can cause a person
to have a monotone or a permanently hoarse voice.
Sympathetic innervation to the lungs originates from the
T1–T5 (or occasionally T2–T5) segments of the spinal cord. These
preganglionic fibers enter the sympathetic trunk and synapse with
ganglionic neurons. The postganglionic sympathetic fibers (called
mck78097_ch25_747-778.indd 769
the cardiac nerves) innervate both the heart and the lungs. The
main function of the sympathetic innervation is to open up or
dilate the bronchioles (bronchodilation). Parasympathetic innervation to the lungs is from the left and right vagus nerves (CN X). The
main function of the parasympathetic innervation is to decrease
the airway diameter of the bronchioles (bronchoconstriction).
Collectively, the sympathetic and parasympathetic fibers
form the pulmonary plexus, a weblike network of nerve fibers that
surrounds the primary bronchi and enters the lungs at the hilum.
Sensory information about the “stretch” in smooth muscle around
the bronchial tree is typically conducted by the vagus nerve to the
brainstem and then relayed to centers involved with external respiration as well as to other reflex centers, such as those involved
in coughing and sneezing.
W H AT D O Y O U T H I N K ?
6
●
When an asthma inhaler provides relief for bronchoconstriction, is
it mimicking sympathetic or parasympathetic stimulation?
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Respiratory System
Stimulation
Inhibition
Pneumotaxic center
Pons
Apneustic center
Respiratory
rhythmicity
center
Ventral respiratory group
(VRG)
Dorsal respiratory group
(DRG)
Medulla oblongata
Figure 25.17
Respiratory Control Centers in the
Brainstem. The dorsal respiratory group
(DRG) and ventral respiratory group
(VRG) within the medulla oblongata
regulate normal ventilation rate. The
pons houses the pneumotaxic and
apneustic centers, which influence the
DRG and VRG. The pneumotaxic center
is inhibitory to both respiration and the
apneustic center. The apneustic center
stimulates the DRG.
Internal
intercostal
muscles
External
intercostal muscles
25.7a Ventilation Control by Respiratory Centers
of the Brain
The involuntary, rhythmic activities that deliver and remove respiratory gases are regulated in the brainstem. Regulatory respiratory
centers are located within the reticular formation through both the
medulla oblongata and the pons. The regulatory centers are composed of specific nuclei, called the respiratory rhythmicity center,
the apneustic center, and the pneumotaxic center (figure 25.17).
The respiratory rhythmicity center in the medulla oblongata
establishes the rate and depth of breathing. Two distinct autonomic
nuclei form this center. The dorsal respiratory group (DRG) is the
inspiratory center that controls inhalation. It controls the motor
neurons that stimulate the muscles of inspiration. The ventral
respiratory group (VRG) is the expiratory center for forced exhalation. It functions only during forced exhalation. During normal
quiet breathing, the VRG is inactive, and exhalation is a passive
event that does not require nervous stimulation. When the VRG is
activated, its neurons stimulate accessory respiratory muscles to
cause maximal, rapid exhalation—for example, when you exercise
and are breathing deeply and forcibly. The DRG is activated during both normal inhalation and forced inhalation. The DRG sends
mck78097_ch25_747-778.indd 770
Diaphragm
impulses through both the phrenic and intercostal nerves to stimulate the diaphragm and external intercostal muscles.
The apneustic (ap-noo ś tik) center and the pneumotaxic
(noo-mō-tak ś ik) center are nuclei housed within the pons. Both
areas influence the breathing rate by modifying the activity of the
respiratory rhythmicity center. The apneustic center stimulates
inspiration through the DRG; the pneumotaxic center inhibits both
the activity of the DRG and that of the apneustic center. By inhibiting the DRG, the VRG is able to function and initiate the process
of forced exhalation. For example, during vigorous exercise when
your respiratory rate must be increased, respiratory gases must be
exchanged more frequently than when at rest. Thus, the DRG, once
stimulated, must be inhibited fairly quickly, with the simultaneous
activation of the VRG, so that forced exhalation can occur and the
next inhalation can begin.
W H AT D I D Y O U L E A R N?
12
●
13
●
What is the main function of sympathetic innervation to the lungs?
Compare the activities of the DRG and the VRG in the brain’s
respiratory centers.
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Chapter Twenty-Five
Respiratory System
771
CLINICAL VIEW
Asthma
Asthma (az m
́ ă) is a chronic condition characterized by episodes of
bronchoconstriction and wheezing, coughing, shortness of breath,
and excess pulmonary mucus. Its incidence is increasing among
young people, particularly those living in urban areas where airborne
Mucus
Mucosa
Submucosa
industrial pollutants and tobacco smoke are abundant. In most cases,
the affected person develops a sensitivity to an airborne agent such
as pollen, smoke, mold spores, dust mites, or particulate matter.
Upon reexposure to this triggering substance, a localized immune
reaction occurs in the bronchi and bronchioles, resulting in bronchoconstriction, swollen submucosa, and increased production of
mucus. Episodes typically last an hour or two. Continual exposure to
the triggering agent increases the severity and frequency of asthma
attacks. Eventually, the walls of the bronchi and bronchioles may
become permanently thickened, leading to chronic and unremitting
airway narrowing and shortness of breath. If airway narrowing is
extreme during a severe asthma attack, death could occur.
Today, the primary treatment for asthma consists of administering
inhaled steroids (cortisone-related compounds) to reduce the inflammatory reaction, combined with bronchodilators to alleviate the
bronchoconstriction. Avoidance of the triggering agent is also very
important. For some patients, allergy shots have proven helpful. In
cases of severe asthma, oral doses of steroids may control the allergic
hyper-response and reduce the inflammation.
Normal airway
Swollen submucosa
Mucosa
Narrowed airway
Extra mucous
secretion
Individuals suffering from
asthma may need to use
inhaled medications to dilate
their constricted bronchioles.
Airway during an asthma attack
25.8 Aging and the Respiratory
System
Learning Objective:
1. Define and describe the age-related respiratory system
changes.
The respiratory system becomes less efficient with age due
to several structural changes. First, aging results in a decrease
in elastic connective tissue in the lungs and the thoracic cavity
wall. This loss of elasticity reduces the amount of gas that can
be exchanged with each breath and results in a decrease in the
ventilation rate. In addition, a condition such as emphysema may
cause a loss of alveoli or a decrease in their size or functionality.
mck78097_ch25_747-778.indd 771
The resulting reduced capacity for gas exchange can cause an older
person to become “short of breath” upon exertion.
Finally, as we get older, carbon, dust, and pollution material
gradually accumulate in our lymph nodes and lungs. If a person
also smokes regularly, the lungs become even darker and blacker
throughout because of the deposition of carbon particles in the cells.
Two distinct diseases, emphysema and chronic bronchitis, together
encompass chronic obstructive pulmonary disease (COPD), which
is often related to tobacco use. The condition is characterized by
lung structural abnormalities resulting from inflammation. The
resulting airflow obstruction makes it hard for the patient to exhale.
W H AT D I D Y O U L E A R N?
14
●
What are some ways that aging can affect the respiratory system?
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CLINICAL VIEW:
Respiratory System
In Depth
Smoking, Emphysema,
and Lung Cancer
Smoking is one of the most important modifiable factors contributing
to disease and premature death in the United States. It significantly
increases the risk and severity of atherosclerosis, and is directly related
to the development of cancers of the lung, esophagus, stomach, and
urinary bladder. Current studies also indicate an association between
secondhand smoke exposure and an increased risk of bronchitis, ear
infections, and asthma in children. Secondhand smoke is a mixture
of the gases and particulate materials released by the burning of
tobacco in cigarettes, cigars, and pipes, as well as exhaled by smokers. Unfortunately, secondhand smoke is inhaled by everyone within
the environment exposed to it. Potential health-care risks include
cancer, asthma, and infections in the respiratory system. The most
common smoking-related diseases are emphysema and several types
of lung cancer.
Emphysema (em-fi-zē m
́ ă; en = in, physema = a blowing) is an irreversible loss of pulmonary gas exchange areas due to inflammation of the
terminal bronchioles and alveoli, in conjunction with the widespread
destruction of pulmonary elastic connective tissue. These combined
events lead to an increase in the diameter or dilation of individual
alveoli, resulting in a decrease in the total number of alveoli, and the
subsequent loss of gas exchange surface area. A person with advanced
emphysema has a larger than normal chest circumference because
air is trapped within the abnormally expanded and nonfunctional
alveoli. The patient is unable to exhale effectively, so that stagnant,
Dilated, nonfunctional air spaces
(a)
Dilated, nonfunctional alveoli
Nonsmoker’s lungs.
LM 15x
(b)
Emphysema causes dilation of the alveoli and loss of elastic tissue, resulting
in poorly functioning alveoli. (a) A gross section of an emphysemic lung shows
the dilated alveoli. (b) Microscopically, the alveoli are abnormally large and
nonfunctional.
Smoker’s lungs.
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Chapter Twenty-Five
oxygen-poor air builds up within the abnormally large (but numerically diminished) alveoli. Most cases of emphysema result from damage
caused by smoking. Once the tissue in the lung has been destroyed, it
cannot regenerate, and thus there is no cure for emphysema. The best
therapy for an emphysema patient is to stop smoking and try to get
optimal use from the remaining lung tissue by using a bronchodilator,
seeking prompt treatment for pulmonary infections, and taking oxygen
supplementation if necessary.
Respiratory System
773
Squamous cell carcinoma
Gross section of a lung with squamous cell carcinoma (speckled white and
black regions).
Adenocarcinoma is less common than the squamous cell type. An
adenocarcinoma of the lung arises from the mucin-producing glands
in the respiratory epithelium. It begins when DNA injury causes one of
these cells to become malignant and begin to divide uncontrollably.
Histologically, an adenocarcinoma displays some of the microscopic
features of the gland from which it arose, thereby making it distinguishable from the other forms of lung cancer.
An individual with advanced emphysema must rely on a
portable oxygen tank, such as this backpack tank.
Lung cancer is a highly aggressive and frequently fatal malignancy that
originates in the epithelium of the respiratory system. It claims over
150,000 lives annually in the United States. Smoking causes about 85%
of all lung cancers. Metastasis, the spread of cancerous cells to other
tissues, occurs early in the course of the disease, making a surgical
cure unlikely for most patients. Pulmonary symptoms include chronic
cough, coughing up blood, excess pulmonary mucus, and increased
likelihood of pulmonary infections. Some people are diagnosed based on
symptoms that develop after the cancer has already metastasized to a
distant site. For example, lung cancer commonly spreads to the brain,
so in some cases lung cancer is not discovered until the patient seeks
treatment for a seizure disorder related to cancer in the brain.
Small-cell carcinoma is a less common type of lung cancer that originates in the primary bronchi and eventually invades the mediastinum.
This type of cancer is especially known for its early metastasis to other
organs. Small-cell carcinoma arises from the small neuroendocrine cells
in the larger bronchi; their secretions help regulate muscle tone in the
bronchi and vessels. As a consequence of their endocrine heritage,
some of these tumors secrete hormones. For example, a small-cell
cancer of the lung occasionally releases ACTH, producing symptoms
of Cushing syndrome.
Small-cell
carcinoma
Lung cancers are classified by their histologic appearance into three
basic patterns: squamous cell carcinoma, adenocarcinoma, and smallcell carcinoma.
Squamous cell carcinoma (kar-si-nō m
́ ă; karkinos = cancer, oma =
tumor) is the most common form of lung cancer. At the microscopic
level, the pseudostratified ciliated columnar epithelium lining the
lungs changes to a sturdier stratified squamous epithelium to withstand the chronic inflammation and injury caused by tobacco smoke.
If the chronic injury continues, these transformed epithelial cells may
accumulate enough genetic damage to become overtly malignant. The
malignant cells divide uncontrollably, invade the surrounding tissue,
and then spread to distant sites.
mck78097_ch25_747-778.indd 773
Gross section of a lung with small-cell carcinoma (white regions) around a
bronchus.
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774
Chapter Twenty-Five
Respiratory System
25.9 Development of the Respiratory
System
CLINICAL VIEW
Sudden Infant Death Syndrome (SIDS)
Learning Objective:
Sudden infant death syndrome (SIDS), also known as crib death,
is the sudden and unexplained death of an infant younger than 1
year of age. Approximately 3000 SIDS deaths are reported annually in the United States, and most occur in infants between 2
and 4 months of age—about 60% of them males. To be counted
as a SIDS death, the child must die for no apparent reason other
than cessation of breathing. SIDS deaths are thoroughly evaluated
through investigation of the death scene, examination of family
history, and autopsy of the child.
1. Identify and describe how the respiratory system forms in
the embryo and fetus.
Early in the fourth week of development, a ventral outgrowth
extends from the developing pharynx. This endodermal outgrowth
is called the respiratory diverticulum (dı̄-ver-tik ū́ -lu m
̆ ; byroad),
or lung bud, and it initially maintains communication with the
pharynx (figure 25.18a). By late in the fourth week, a septum
forms between the pharynx and the respiratory diverticulum,
partitioning them into two separate tubes. The respiratory diverticulum, formed from endoderm, undergoes intricate branching to
form the respiratory tree. Surrounding the respiratory diverticulum is mesoderm, which later differentiates into the vasculature,
muscle, and cartilage of each lung.
The respiratory diverticulum grows inferiorly and forms
the future trachea. At the end of the fourth week, the respiratory
diverticulum branches into a left and right primary bronchial
bud. Each bud forms the rudiments of the left and right primary
bronchi, respectively. Growing branches of the pulmonary arteries and veins travel with these developing bronchial buds. By the
fifth week of development, the primary bronchial buds branch
into secondary bronchial buds (figure 25.18b). The secondary
bronchial buds form the secondary bronchi of each lung. Thus, the
Pharynx
Although the definitive cause of SIDS is not known, current
research indicates that SIDS babies have trouble regulating and
maintaining blood pressure, breathing, and body temperature.
Some form of stress, in combination with one or more of these
three factors, appears to be involved in SIDS deaths. It is now
known that babies sleeping on their stomachs are at greater risk
for SIDS than those sleeping on their backs. Thus, a national
“back-to-sleep” (BTS) campaign has arisen and significantly
reduced the number of SIDS deaths in the United States. Although
some pediatricians report that having babies sleep on their backs
causes a slight delay in certain developmental milestones, such
as sitting up and crawling, these delays typically are short-lived.
Right primary
bronchus
Trachea
Right primary
bronchus
Left primary
bronchus
Left primary
bronchus
Secondary
bronchi
Esophagus
Respiratory
diverticulum
Tertiary
bronchi
Mesoderm
Right primary
bronchial bud
(a) Week 4: Respiratory diverticulum and
primary bronchial buds form
Left primary
bronchial bud
Secondary
bronchial buds
(b) Week 5: Secondary bronchial
buds form
(c) Week 6: Tertiary bronchi form
Figure 25.18
Development of the Respiratory System. The respiratory system forms as an outgrowth (called the respiratory diverticulum) from the developing
pharynx beginning at week 4. (a) Primary bronchial buds appear later during week 4. (b) Secondary bronchial buds branch from the primary
bronchi during week 5 and grow into the surrounding mesoderm. (c) By week 6, the tertiary bronchi of the left and right lungs have formed.
mck78097_ch25_747-778.indd 774
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Chapter Twenty-Five
left primary bronchial bud branches into two secondary bronchial
buds (since the left lung has two secondary bronchi), and the right
primary bronchial bud branches into three secondary bronchial
buds (for the three secondary bronchi of the right lung).
In the following weeks, the secondary bronchial buds
undergo further branching. Week 6 is marked by the development
of tertiary bronchi and the rudiments of the bronchopulmonary
segments (figure 25.18c). From week 6 to week 16, the respiratory
tree forms smaller branches until finally the terminal bronchioles
form. Thus, the conducting portion of the respiratory system has
developed by week 16.
Weeks 16–28 mark the branching and development of respiratory bronchioles from the terminal bronchioles. From week 28
of development until birth, primitive alveoli (also called terminal
sacs) develop more profusely. These primitive alveoli have a thick
epithelial lining that must thin into simple squamous epithelium in
order for the alveoli to become functional. It isn’t until after week
28 that this epithelium becomes sufficiently thinned for respiration. In addition, by about week 28 the alveolar type II cells start
to secrete pulmonary surfactant (described earlier in this chapter),
Respiratory System
775
which helps keep the alveoli patent (open) and facilitates inflation.
Without sufficient surfactant, the alveoli collapse upon exhalation,
and it becomes difficult to reinflate them. Prematurely born infants
sometimes experience respiratory distress due to inadequate production of surfactant.
Prior to birth, the respiratory system is nonfunctional because
gas exchange occurs between fetal blood and maternal blood at
the placenta. The lungs and pulmonary vessels of the fetus are collapsed, and so most of the blood is shunted away from the lungs to
the fetus’s systemic circulation. At birth, the first contraction of the
external intercostal muscles and diaphragm fills the lungs with air.
(The pressure changes within the thoracic cavity drive the commencement of pulmonary circulation.) Blood is sent to the lungs,
where gas exchange occurs, and the newborn relies on its own lungs
(instead of the mother’s placenta) for respiratory gas exchange.
Even after we are born, our lungs continue to produce additional primitive alveoli. Some research has indicated that alveoli
continue to develop until we are about 8 years old, by which time
each lung has approximately 300 to 400 million alveoli. Table 25.3
summarizes the events in respiratory system development.
Table 25.3
Summary of Respiratory System Development
Week of Development/
Age
Respiratory System Structure Formed
Early week 4
Respiratory diverticulum forms
Late week 4
Primary bronchial buds form
Week 5
Secondary bronchial buds form
Week 6
Tertiary bronchial buds form
Week 6–week 16
Successive branching of tertiary bronchial buds forms smaller bronchi and bronchioles; eventually, terminal bronchioles
form; conducting portion of respiratory system is complete
Week 16–week 28
Terminal bronchioles branch into respiratory bronchioles
Week 28–birth
Primitive alveoli form; pulmonary surfactant begins to be produced
Birth–8 years
Alveoli continue to develop; eventually, adult number of alveoli (300–400 million per lung) is attained
Clinical Terms
decompression sickness (the bends) A condition associated
with the rapid decrease in pressures on the body during
underwater ascent. The pressure changes particularly
affect the gases in tissues and those dissolved in the blood.
Because of the water pressure, body tissue absorbs nitrogen
gas faster as a diver descends than when ascending to the
surface. However, if a diver ascends too quickly, nitrogen gas
bubbles will form in body tissue rather than being exhaled.
The nitrogen bubbles cause severe pain and can be lethal.
epistaxis (ep ́i-stak ́sis; epi = on, stazo = to fall in drops) Bleeding
from the nose; may be caused by allergies, hypertension,
infection, or nasal trauma. Also called nosebleed or nasal
hemorrhage.
mck78097_ch25_747-778.indd 775
hyaline (hı̄ ́ă-lin; hyalos = glass) membrane disease Disease seen
especially in premature neonates with reduced amounts of
lung surfactant.
pulmonary embolism (em ́bō-lizm) Obstruction or occlusion of a
pulmonary vessel by an embolus (foreign material or blood
clot).
tuberculosis (TB) A potentially serious bacterial infection that
primarily affects the lungs. TB is caused by the bacterium
Mycobacterium tuberculosis. The bacteria usually infect the
lungs, but they can also damage other parts of the body.
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776
Chapter Twenty-Five
Respiratory System
Chapter Summary
25.1 General
Organization
and Functions of
the Respiratory
System 748
25.2 Upper
Respiratory
Tract 750
■
The respiratory system has a conducting portion to convey gas to and from the lungs and a respiratory portion for gas
exchange with the blood.
25.1a Respiratory System Functions
Respiratory system functions include gas exchange, gas conditioning, sound production, olfaction, and defense.
■
The conducting airways of the upper respiratory tract are the nose, nasal cavity, paranasal sinuses, pharynx, and their
associated structures.
25.2a Nose and Nasal Cavity
■
■
750
The nasal cavity is the primary site for conditioning inhaled air. It houses three pairs of nasal conchae that cause
turbulence in inhaled air, which passes posteriorly into the nasopharynx through the choanae.
25.2b Paranasal Sinuses
750
Paranasal sinuses are paired air spaces in the frontal, ethmoidal, and sphenoidal bones, and the maxillae. They decrease
skull bone weight, help condition inhaled air, and contribute to sound resonance.
25.2c Pharynx
25.3 Lower
Respiratory
Tract 753
748
■
750
■
The pharynx is composed of (1) the nasopharynx, with paired auditory openings on the lateral wall and a pharyngeal
tonsil on the posterior wall; (2) the oropharynx, with paired palatine tonsils on the lateral walls and lingual tonsils at the
base of the tongue; and (3) the laryngopharynx, which is continuous with the larynx and esophagus.
■
The conducting airways of the lower respiratory tract include the larynx, trachea, bronchi, bronchioles to the terminal
bronchioles, and their associated structures. Its respiratory portions include respiratory bronchioles, alveolar ducts, and
alveoli.
25.3a Larynx
753
■
The larynx conducts air into the trachea and lower respiratory tract, and produces sound.
■
The larynx is composed of cartilage and has paired vocal folds that produce sound when air passes between them.
25.3b Trachea
■
757
The trachea is lined by pseudostratified ciliated columnar epithelium and has C-shaped tracheal cartilage rings that
support the tracheal wall and prevent its collapse.
25.3c Bronchial Tree
758
■
The bronchial tree conducts respiratory gases from the primary bronchi to the terminal bronchioles.
■
Bronchial tree passageways have cartilage and/or smooth muscle bands to support the walls. The passageway sequence
is (1) primary bronchi, (2) secondary bronchi, (3) tertiary bronchi, (4) bronchioles, and (5) terminal bronchioles.
25.3d Respiratory Bronchioles, Alveolar Ducts, and Alveoli
25.4 Lungs 762
Respiratory bronchioles branch from terminal bronchioles and have alveoli outpocketings in their walls.
■
An alveolus is a small, thin sac with two types of cells in its wall.
■
Alveolar type I cells promote rapid gas diffusion; alveolar type II cells secrete pulmonary surfactant.
■
Alveolar macrophages remove inhaled particulate materials from alveolar surfaces.
■
The respiratory membrane consists of alveolar type I cells, an endothelial cell of a capillary, and their fused basement
membranes.
■
The lungs are lateral to the mediastinum in the thoracic cavity.
25.4a Pleura and Pleural Cavities
■
762
The visceral pleura covers the lung outer surface, and the parietal pleura lines the internal thoracic walls; a pleural cavity
is sandwiched between the pleural layers. The pleural membranes produce serous fluid.
25.4b Gross Anatomy of the Lungs
762
■
Lung surfaces include the base (upon the diaphragm), the apex (superior surface), the costal surface (against the thoracic
wall), and the mediastinal surface (facing the mediastinum).
■
The hilum is a medial opening through which bronchi, pulmonary vessels, lymph vessels, and nerves enter the lungs.
25.4c Blood Supply To and From the Lungs
■
■
763
The pulmonary circulation transports blood to and from the gas exchange surfaces of the lungs, and the bronchial
circulation supplies the bronchi and bronchioles.
25.4d Lymphatic Drainage
mck78097_ch25_747-778.indd 776
760
■
765
The connective tissue in the lung houses lymph nodes and lymph vessels.
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Chapter Twenty-Five
Respiratory System
777
25.5 Pulmonary
Ventilation 766
■
Breathing, called pulmonary ventilation, is the movement of air into and out of the respiratory tract. Change in air
pressure between the atmosphere and the alveoli drives ventilation.
25.6 Thoracic
Wall Dimensional
Changes During
External
Respiration 767
■
Inhalation causes the thoracic cavity space to increase vertically, laterally, and in an anterior-posterior direction;
exhalation causes it to return to its original size.
■
During external respiration, the primary muscles that move the ribs are: for inhalation, (1) scalene, (2) external
intercostal, and (3) serratus posterior superior; for forced exhalation, (4) internal intercostal, (5) transversus thoracis, and
(6) serratus posterior inferior.
25.7 Innervation
of the Respiratory
System 769
■
Sympathetic stimulation causes bronchodilation; parasympathetic stimulation causes bronchoconstriction.
25.7a Ventilation Control by Respiratory Centers of the Brain
770
■
The respiratory center in the medulla oblongata has a dorsal respiratory group (DRG) for inspiration and a ventral
respiratory group (VRG) for forced expiration.
■
The apneustic and pneumotaxic centers within the pons influence the respiration rate by modifying the activity of the
DRG.
25.8 Aging and
the Respiratory
System 771
■
The respiratory system becomes less efficient with age due to loss of elasticity and loss or decreased size and
functionality of alveoli.
25.9 Development
of the Respiratory
System 774
■
Early in the fourth week of development, a respiratory diverticulum forms and leads to primary bronchial buds by late in
that same week.
■
By the fifth week of development, the primary bronchial buds branch into secondary bronchial buds. These bronchial
buds undergo further branching until terminal bronchioles are formed by week 16. Respiratory bronchioles form from the
terminal bronchioles during weeks 16–28.
■
Alveoli continue to form from week 28 of development until about 8 years of age, when the adult number of 300 to 400
million alveoli is attained.
Challenge Yourself
Matching
Multiple Choice
Match each numbered item with the most closely related lettered
item.
Select the best answer from the four choices provided.
______ 1. nasopharynx
a. solid ring of hyaline cartilage
______ 2. bronchiole
b. branches directly from the
trachea
______ 3. nasal meatus
______ 4. left lung
______ 5. cricoid cartilage
______ 6. primary bronchus
______
7. alveolar type II cell
______ 8. arytenoid cartilage
c. has a cardiac notch and
cardiac impression
d. phagocytic cell in alveoli
e. contains pharyngeal tonsil
f. covers laryngeal opening
during swallowing
______ 9. alveolar macrophage g. causes air turbulence in nasal
cavity
______ 10. epiglottis
h. produces pulmonary
surfactant
i. lacks cartilage but has
significant amounts of smooth
muscle in wall
j. vocal folds attach to it
mck78097_ch25_747-778.indd 777
______ 1. The visceral pleura covers the
a. outer surface of the lung.
b. gas exchange surface of the alveoli.
c. inner wall of the thoracic cavity.
d. lining of the bronchi and bronchioles only.
______ 2. An area common to both the respiratory and
digestive systems through which food, drink, and
air pass is the
a. nasopharynx.
b. trachea.
c. oropharynx.
d. glottis.
______ 3. Which statement is false about the trachea?
a. It is lined with a nonkeratinized stratified
squamous epithelium.
b. It is continuous superiorly with the larynx.
c. It bifurcates into left and right primary bronchi at
the level of the sternal angle.
d. It contains C-shaped cartilage rings.
______ 4. Which structure is the last, smallest portion of the
conducting portion of the respiratory system?
a. nasopharynx
b. terminal bronchiole
c. respiratory bronchiole
d. alveolus
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Chapter Twenty-Five
Respiratory System
______ 5. Which is not a function of the paranasal sinuses?
a. warm inhaled air
b. responsible for sound resonance
c. gas exchange
d. humidify inhaled air
2. What type of epithelium is found in the oropharynx, and
why is it well suited to this location?
______ 6. The ______ cartilage of the larynx forms the
laryngeal prominence.
a. arytenoid
b. cuneiform
c. thyroid
d. cricoid
4. What are the components of the bronchial tree, from largest
to smallest?
______ 7. The C-shaped cartilages in the trachea
a. serve as a point of attachment for some muscles
of expiration.
b. support muscular attachments to the thyroid
cartilage and epiglottis.
c. prevent the trachea from collapsing.
d. attach the trachea to the esophagus posteriorly.
______ 8. Which of the following is not a muscle of inspiration?
a. diaphragm
b. external intercostals
c. rectus abdominis
d. scalene
______ 9. The epithelium lining the alveolus is composed of a
a. simple squamous epithelium.
b. pseudostratified ciliated columnar epithelium.
c. simple cuboidal epithelium.
d. transitional epithelium.
______ 10. The apneustic center is involved in
a. inhibition of the pneumotaxic area.
b. stimulation of DRG.
c. stimulation of the pneumotaxic area.
d. inhibition of VRG.
Content Review
1. What is the function of the mucous lining of the epithelium
in the respiratory tract?
3. What must happen to the vocal folds in order to produce
a higher-pitched sound? A lower-pitched sound? What
produces a louder sound?
5. Why is cartilage unnecessary in the walls of the
bronchioles?
6. Why are alveolar type II cells important in maintaining the
inflation of the lungs?
7. How do the left and right lungs differ anatomically?
8. How do the dimensions of the thoracic cavity change when
we inhale and exhale? What muscles assist with these
dimensional changes?
9. Name the autonomic nervous system respiratory centers
in the pons and the medulla oblongata, and describe their
functions.
10. Contrast the functions and interactions of the DRG and the
VRG in the medulla oblongata.
Developing Critical Reasoning
1. Charlene has had a bad cold for the last few days. While
preparing a presentation for her speech class, she records
her talk so that she can critique it later. When she listens
to the recording, her young daughter exclaims, “Mommy
that doesn’t even sound like you. What happened to your
voice?” How is Charlene’s cold related to the changes in
her voice?
2. Your best friend George is an athletic 20-year-old who
smokes regularly. George tells you, “Smoking doesn’t affect
me—I can still run and do the sports I like. All that talk
about smoking being dangerous doesn’t apply to me.” Do
you agree with George? What would you tell him about
the dangers of smoking and some of the conditions he may
expect to have later in life?
Answers to “What Do You Think?”
1. A “deviated septum” is off-center, so one side of the nasal
cavity is larger than the other. This alters the normal flow
of air through the nose, and if the narrower side becomes
blocked, nasal congestion or sinus problems may result.
3. The constriction of the bronchioles allows for a more
forceful expulsion of air from the lungs, which may help
dislodge accumulated mucus or inhaled foreign particulate
materials.
2. The epithelium changes because a stratified squamous
epithelium is more sturdy and protective against smoke
than a pseudostratified ciliated columnar epithelium.
Unfortunately, since stratified squamous epithelium lacks
cilia and goblet cells, less mucus is produced, and no cilia
are present to propel particles away from the bronchi toward
the pharynx. Thus, the main way to eliminate these particles
is by coughing, leading to the chronic “smoker’s cough.”
4. The left lung is smaller because the heart projects into the
left side of the thoracic cavity.
5. Lymph nodes darken and turn black as they accumulate the
dust, particles, and pollution we inhale over a lifetime.
6. An asthma inhaler mimics sympathetic stimulation because
it causes bronchodilation.
www.mhhe.com/mckinley3 Enhance your study with practice tests and
activities to assess your understanding. Your instructor may also recommend
the interactive eBook, individualized learning tools, and more.
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