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3d Respiratory Bronchioles, Alveolar Ducts, and Alveoli

3d Respiratory Bronchioles, Alveolar Ducts, and Alveoli

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Chapter Twenty-Five  Respiratory System








Branch of

pulmonary artery


Terminal bronchiole

LM 60x








Branch of

pulmonary vein


Respiratory bronchiole


bronchiole Alveoli





Alveolar duct


Alveolar pores



Elastic fibers

Connective tissue


SEM 40x


Figure 25.8

Bronchioles and Alveoli. Bronchioles and alveoli form the terminal ends of the respiratory passageway. (a) Terminal bronchioles branch into

respiratory bronchioles, which then branch into alveolar ducts and alveoli. A pulmonary vessel travels with the bronchioles, and the pulmonary

capillaries wrap around the alveoli for gas exchange. Elastic tissue is also wrapped around alveoli. (b) A photomicrograph shows the relationship of

respiratory bronchioles, alveolar ducts, and alveoli. (c) SEM of a terminal bronchiole, a respiratory bronchiole, alveolar duct, and alveoli reveals the

honeycomb appearance of alveoli.

(b) © McGraw-Hill Education/Al Telser, photographer; (c) © David Phillips/Science Source

Learning Strategy

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.


Chapter Twenty-Five  Respiratory System




Nucleus of capillary

endothelial cell


of alveolar

type I cell






type I cell

Diffusion of CO2


type II cell

Diffusion of O2




Alveolar epithelium



Fused basement membranes

of the alveolar epithelium and

the capillary endothelium

Alveolar pores



Capillary endothelium



Figure 25.9

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





Identify the structure and describe the function of the pleura.

Describe the gross anatomy of the lungs.

Identify and discuss the blood supply to and from the lungs.

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.



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 (plūr′ă), which is

formed from simple squamous epithelium called a mesothelium.

The mesothelium is attached to a thin layer of areolar connective tissue. The outer surface of each lung is tightly covered

by the visceral pleura, while the internal thoracic walls, the

lateral surfaces of the mediastinum, and the superior surface of

the diaphragm are lined by the parietal pleura (figure 25.10).

(These pleural layers may also be viewed in the transverse section

of the thoracic cavity shown in figure 22.2c.) The visceral and parietal pleural layers are continuous at the hilum of each lung. The

pleural cavity is a space located between the visceral and parietal

serous membrane layers. When the lungs are fully inflated, the

pleural cavity is considered a potential space because the visceral

and parietal pleurae are almost in contact with each other. An oily,

serous fluid is produced by the serous membranes and covers their

surface within the pleural cavity. Serous fluid acts as a lubricant,

ensuring the pleural surfaces slide by each other with minimal

friction during breathing. The serous fluid also creates tension

between the pleural membranes, so when the chest wall expands,

pulling the parietal pleura with it, then the serous fluid creates

tension and pulls the visceral pleura with it.

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 25.11). 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.

Chapter Twenty-Five  Respiratory System


Clinical View 25.6


Pneumothorax (nū-mō-thōr′aks; 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-ĕ-lek′tă-sis; ateles = incomplete, ektasis =

extension). The lung (or portion of lung) remains collapsed 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

Parietal pleura

Visceral pleura

Pleural cavity

Parietal pleura

Pleural cavity

Visceral pleura


Figure 25.10

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.

has been removed, an airtight 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.

Fluid also has the potential to 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.

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. Because 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 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, whereas 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 broncho­

pulmonary segments—10 in the right lung, and typically 8 to 10 in

the left lung (figure 25.12). (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 segmental 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

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, lymph vessels,

Both the pulmonary circulation and the bronchial circulation supply

the lungs. Recall from section 23.4 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


Chapter Twenty-Five  Respiratory System


Superior lobe

Horizontal fissure

Oblique fissure

Middle lobe

Cardiac notch

Inferior lobe



Right lung

Left lung

(a) Lateral views


Superior lobe

Oblique fissure

Pulmonary artery

Main bronchus


Pulmonary veins

Root of the lung

Horizontal fissure

Cardiac impression

Middle lobe

Cardiac notch

Inferior lobe

Oblique fissure

Oblique fissure



Right lung

Left lung

(b) Medial views

Figure 25.11

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.

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


Chapter Twenty-Five  Respiratory System




segments of

superior lobe










of middle lobe



of inferior lobe



segments of

superior lobe













segments of

inferior lobe





Right lung, lateral view

Left lung, lateral view

Figure 25.12

Bronchopulmonary Segments of the Lungs. The portion of each lung supplied by each segmental bronchus (represented by different colors) is a

bronchopulmonary segment. (The medial basal bronchopulmonary segment cannot be seen from this view.)

arteries and veins that supply the bronchi and bronchioles of the

lung. This part of the circulatory 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. About 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.

nodes within the lungs. Lymphatic ­vessels 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 section 24.2c). The right bronchomediastinal

trunk drains into the right lymphatic duct, whereas the left bronchomediastinal trunk drains into the thoracic duct.



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 25.13). 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

The lymph nodes of the lung become black and darkened

over time in both smokers and nonsmokers. Why do these

lymph nodes turn black?



What is the hilum of the lung, and how does it function?

Internal jugular vein

Right lymphatic duct

Thoracic duct

Subclavian vein



Brachiocephalic vein

Bronchomediastinal trunk

Tracheobronchial lymph nodes


lymph nodes


lymph nodes


lymph node


lymph nodes

Lymph vessels

Figure 25.13

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.


Chapter Twenty-Five  Respiratory System

Clinical View 25.7




Pneumonia (nū-mō′nē-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 accumulation

of fluid and leukocytes in the alveoli, 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 upon 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.

Alveolar duct


LM 30x

Normal lung tissues.

© McGraw-Hill Education/Al Telser, photographer


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.

Tissues within a lung affected by pneumonia.

© Collection CNRI/Phototake

© Carolina Biological Supply Company/Phototake

25.5  Pulmonary Ventilation

✓✓Learning Objective

12. Describe the process of pulmonary ventilation.

Breathing, also known as pulmonary ventilation, is the movement of

air into and out of the respiratory system. Breathing may be either quiet

or forced. Quiet breathing is the rhythmic breathing that occurs at rest;

forced breathing is vigorous breathing that accompanies exercise or hard

exertion. At rest, a normal adult breathes about 16 times per minute, and

about 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. Gas exchange is organized into four continuous and simultaneously occurring processes:

Pulmonary ventilation—movement of respiratory gases

Alveolar gas exchange (or external respiration)—exchange of

between the atmosphere and the alveoli of the lungs

respiratory gases between the alveoli and the blood

LM 75x

Gas transport—transport of respiratory gases within the

Systemic gas exchange (or internal respiration)—exchange of

blood between the lungs and systemic cells of the body

respiratory gases between the blood and systemic cells of the body

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, the

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.


1 0

What is pulmonary ventilation?

25.6  Mechanics of Breathing

✓✓Learning Objectives

13. Identify and describe the actions of the skeletal muscles of


14. Distinguish between quiet and forced breathing.

15. Define and describe how the thoracic cavity changes in size

and shape during respiration.

The mechanics of breathing involve coordinated movement of skeletal muscles, as well as dimensional changes in the thoracic cavity.

Here we first discuss which muscles are involved with both quiet

and forced breathing, and then we examine how the thoracic cavity

changes shape as we inhale and exhale.

25.6a  Skeletal Muscles of Breathing

The skeletal muscles of breathing are classified into the following

three categories: muscles of quiet breathing, muscles of forced inhalation, and muscles of forced exhalation (figure 25.14).


Chapter Twenty-Five  Respiratory System

movement of the diaphragm occur during forced exhalation because

the abdominal wall muscles also are contracting.

Lateral dimension changes occur either as the rib cage is elevated and the thoracic cavity widens, or as the rib cage depresses

and thoracic cavity narrows. This action can be mimicked by placing

your hands at the sides of your ribs and then abducting and adducting

the hands relative to the ribs.

Anterior-posterior dimension changes occur as the inferior portion of the sternum moves anteriorly and then posteriorly. This action

can be visualized by placing one hand on the front of your lower chest

and lifting it outwardly away from the chest and then back. In general, lateral and anterior-posterior dimensional changes both occur as

a result of the contraction and relaxation of all muscles of breathing

shown in figure 25.15, except for the diaphragm.


1 1

What types of dimensional changes occur to the thorax

when you inhale, and what muscles are responsible?

Muscles of quiet breathing are involved in normal rhythmic

breathing that occurs at rest. They are the diaphragm and the

external intercostals. These muscles alternately contract and

relax, resulting in movement of air into and out of the lungs

(see section 11.3).

■ Muscles of forced inhalation are used during a deep

inhalation, such as occurs during heavy exercise. They

include the sternocleidomastoid, scalenes, pectoralis minor,

serratus posterior superior, and erector spinae (see sections

11.1g, 11.2, 11.3, and 12.1a). All except the erector spinae are

located in a more superior position relative to the thoracic

cavity. They effectively move the rib cage to increase the

thoracic cavity volume more than occurs during quiet


■ Muscles of forced exhalation contract during a hard

exhalation when one blows up a balloon or coughs. These

muscles include the internal intercostals, abdominal

muscles, transversus thoracis, and serratus posterior inferior

(see sections 11.3 and 11.4). They usually either move the

rib cage inferiorly, medially, and posteriorly or compress the

abdominal contents to move the diaphragm superiorly into

the thoracic cavity. These movements cause a large decrease

in thoracic cavity volume.

Learning Strategy

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.

The muscles of forced inhalation and forced exhalation are collectively referred to as the accessory muscles of breathing.

Inhalation: Ribs (bucket

handles) elevated,

lateral dimension increased

25.6b  Volume Changes in the Thoracic Cavity

Cyclic activity of breathing muscles causes thoracic cavity volume

changes that occur in vertical, lateral, and anterior-posterior directions (figure 25.15).

Vertical changes of the thoracic cavity result from diaphragm

movement as it contracts and relaxes. It forms the rounded thoracic

cavity “floor” and is dome-shaped when relaxed. Contraction flattens

and moves its central portion inferiorly to press against the abdominal

viscera—consequently, the vertical dimensions of the thoracic cavity increase. When the diaphragm relaxes and returns to its original

position, the vertical dimensions decrease.

Inhalation: Ribs (bucket

Only small movements of the diaphragm are requiredhandles)

for elevated,

lateral dimension


breathing, and usually the changes in vertical dimension measure


few millimeters during quiet breathing. Greater changes in superior

Exhalation: Ribs (bucket

handles) depressed,

lateral dimension decreased

Exhalation: Ribs (b

handles) depres

lateral dimension de


Chapter Twenty-Five  Respiratory System

Muscles of forced inhalation:

Pull upward and outward



Serratus posterior superior

Pectoralis minor

Erector spinae

Muscles of forced exhalation:

Pull downward and inward

Transversus thoracis

Serratus posterior inferior

Internal intercostal

External oblique

Transversus abdominis

Anterior view

Muscles of quiet breathing

increase dimensions of

thoracic cavity

Posterior view


Figure 25.14

External intercostal

Skeletal Muscles of Breathing.

25.7  Innervation of the Respiratory System

✓✓Learning Objective

16. Identify the components of the autonomic nervous system that

regulate breathing.

The respiratory system is innervated by both somatic and autonomic

motor nerves. Centers in the cerebral cortex can affect respiratory

centers of pons and medulla oblongata and motor neurons that control respiratory muscles. Conscious control is mediated by somatic

motor innervation coming primarily from CN X (vagus) to move

pharyngeal and laryngeal muscles (see section 15.8). Additionally,

the phrenic nerve innervates the diaphragm.

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 (see

section 18.4). These preganglionic fibers enter the sympathetic trunk

and synapse with ganglionic neurons. The postganglionic sympathetic fibers (called 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 axons form

the pulmonary plexus, a weblike network of axons that surrounds

the main bronchi and enters the lungs at the hilum (see section 18.5a).

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




When an asthma inhaler provides relief for bronchoconstriction,

is it mimicking sympathetic or parasympathetic stimulation?

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 (figure 25.16). Regulatory

respiratory centers are located within the reticular formation through

both the medulla oblongata and the pons. The skeletal muscles of

breathing are coordinated by nuclei within the brainstem. These regulatory nuclei are housed specifically in the medullary respiratory

center within the medulla oblongata and the pontine respiratory

center (also called the pneumotaxic center) within the pons. These

two centers are collectively called the respiratory center.

Chapter Twenty-Five  Respiratory System









Diaphragm contracts; vertical

dimensions of thoracic cavity increase.

Diaphragm relaxes; vertical

dimensions of thoracic cavity narrow.



Ribs are elevated and thoracic cavity widens.

Ribs are depressed and thoracic cavity narrows.



Inferior portion of sternum moves anteriorly.

Inferior portion of sternum moves posteriorly.

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.



Chapter Twenty-Five  Respiratory System

Sensory input to

respiratory center

Motor output to

respiratory muscles

Output from cerebral cortex

Irritant receptors

Pontine respiratory center




of other





nerve (CN IX)



group (DRG)




group (VRG)

Carotid bodies

Vagus nerve (CN X)




Spinal cord




respiratory Proprioceptors


Cervical plexus

Phrenic nerve

Spinal cord


Intercostal nerves

Intercostal muscles

Accessory muscles

of respiration


Figure 25.16

Respiratory Centers. The pontine and medullary respiratory centers coordinate the contraction of the skeletal muscles required for breathing. The medullary

respiratory center consists of a ventral respiratory group (VRG) and a dorsal respiratory group (DRG). The DRG receives sensory input from multiple locations

and relays this information to the VRG. The VRG then transmits motor impulses through the spinal cord to the muscles of respiration.

There are two distinct groups of nuclei in the medullary respiratory center. The ventral respiratory group (VRG) is a column of

neurons located in the ventrolateral region of the medulla that contains

neurons of both inhalation and exhalation. Posterior to the VRG in the

dorsomedial region of the medulla is the dorsal respiratory group

(DRG), which relays its input to the VRG. The VRG initiates neural impulses for inhalation and exhalation; previously, physiologists

thought the DRG controlled this activity. Axons from upper motor

Chapter Twenty-Five  Respiratory System


Clinical View 25.8


Asthma (az′mă) 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




Normal bronchiole

Swollen submucosa


Narrowed airway

Extra mucous


where airborne industrial pollutants and tobacco smoke are

abundant. In most cases, the affected person develops a sensi­

tivity 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. In severe cases, oral doses of

steroids may control the allergic hyper-response and reduce the


Individuals suffering from

asthma may need to use

inhaled medications to dilate

their constricted bronchioles.

Bronchiole during an asthma attack

neurons extend from the VRG into the spinal cord. Axons from lower

motor neurons extend from the spinal cord and form both the phrenic

nerves that innervate the diaphragm (see section 16.4d) and intercostal

nerves that innervate the intercostal muscles (see section 16.4c).

The pontine respiratory center is located within the pons and

modifies the activity of the nuclei in the medulla. It appears to provide

for a smooth transition between inhalation and exhalation by sending

impulses to the VRG. Erratic breathing results if this area is damaged.

A breathing rhythm that involves 2 seconds of inhalation followed by 3 seconds of exhalation results in an average respiratory

rate of 12 times per minute. The average range for the rate of quiet

breathing is generally between 12 and 15 times per minute—a rate

referred to as eupnea (yūp-nē′ă).


1 2

1 3

What is the main function of sympathetic innervation to the


Compare the activities of the DRG and the VRG in the

brain’s respiratory centers.

© Coneyl Jay/Science Source

25.8  Aging and the Respiratory System

✓✓Learning Objective

17. Define and describe the age-related respiratory system


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 functionality. The resulting reduced capacity for gas

exchange can cause an older person to become “short of breath” upon


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.

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