Download Respiratory System

An Overview of the Respiratory System
The respiratory system includes many different structures working together for a common
purpose. These structures include the nose, the nasal cavity and sinuses, pharynx, larynx,
trachea, and smaller conducting passageways leading to the gas exchange surfaces of the lungs.
The respiratory tract consists of the airways that carry air to and from the surfaces of these
surfaces and can be divided into two parts. The conducting zone extends from the entrance of
the nose to the nasal cavity to the smallest bronchioles of the lungs. It consists of the nose,
pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles. Their function is to
filter, warm, and humidify the air and conduct it into the lungs. The respiratory zone of the
tract includes the delicate bronchioles and air sacs, also called alveoli, where gas exchange
takes place between the air and blood.
The respiratory system also consists of the respiratory tract and its associated tissues, organs,
and supporting structures. These are also divided into two parts: (1) The upper respiratory
system includes the nose, pharynx, and associated structures. These passageways also filter,
warm, and humidify the air, protecting the more fragile conduction and exchange surfaces of
the lower respiratory system from debris and pathogens. (2) The lower respiratory system
includes the larynx, trachea, bronchi, and lungs.
The filtering, warming, and moistening of inhaled air begins at the entrance of the nose to the
upper respiratory system and continues throughout the rest of the conducting zone. Most
foreign particles and pathogens have been removed by the time the air reaches lung alveoli.
The Upper Respiratory System
The Nose and Nasal Cavity
The head has two openings through which substances such as air and food can enter the bodythe nose and the mouth. Although air can enter through either of these passageways, the nose
is the primary passageway for air entering the respiratory system. The nose consists of more
than what you see on someone’s face. In fact, the nose can be divided into two parts, the
external and internal portions. The external nose, the skin and muscle-covered portion of the
nose visible on the face, is an extension of bone and cartilage with two entryways and an
internal dividing wall. Three large cartilages that project from the skull to form the framework
of the external nose are the septal nasal cartilage, the paired major alar cartilages, and the
minor alar cartilages, which define the cartilage skeleton of the nose. The major and minor alar
cartilages form the inner part of the lateral sides of the nose. The septal cartilage divides the
external nose into right and left chambers. The external nose is flexible to some extent because
it consists of hyaline cartilage. Air normally enters the respiratory system through the external
nares, also called, nostrils, which open into the nasal cavity. The nasal vestibule is the portion
of the nasal cavity enclosed by the flexible tissues of the nose. The skin of the nasal vestibule is
composed of many coarse hairs that extend across the external nares and sebaceous and sweat
glands that secrete onto its surface. Large airborne particles such as sand, sawdust, and even
insects are trapped in these hairs and are prevented from entering the nasal cavity. The upper
lining of each nasal vestibule turns into a mucous membrane that continues deeper into the
nasal cavity.
Deeper into the skull, beyond the nasal vestibules, is the internal nose. The internal nose is a
large cavity in the frontal part of the skull that lies below the nasal bone and above the mouth.
Anteriorly, the internal nose fuses with the external nose, and posteriorly it connects with the
pharynx through two openings called the internal nares or choanae. The lateral walls of the
internal nose are composed by the ethmoid, maxillae, lacrimal, palatine, and inferior nasal
conchae bones. The ethmoid bone forms the roof of the internal nose and the hard palate,
formed by the maxillary and palatine bones, forms the floor of the nasal cavity and separates
the oral and nasal cavities. This large cavity within the skull, called the nasal cavity, forms most
of the nose and is divided into two regions-the large inferior respiratory region and the small
superior olfactory region. Like the nasal vestibules, the nasal cavity is also divided into right and
left halves by a nasal septum. The ventral part of the nasal septum is formed by the septal nasal
cartilages, which attaches to the vomer bone and perpendicular plate of the ethmoid bone.
These bones form the dorsal part of the nasal septum. A deviated septum can result from a
strong impact to the nasal region. The fragile nasal septal bones can break or the cartilage
portion of the nasal septum can be separated from the bony portion and become displaced to
one side during the healing process leading to a narrowing of one side of the nasal cavity. This
makes it more difficult to breathe through that side of the nose.
Three shelves formed by projections of the superior, middle, and inferior nasal conchae extend
out of each lateral wall of the nasal cavity. The conchae, almost reaching the nasal septum,
subdivide each side of the nasal cavity into the superior, middle, and inferior meatuses. These
are narrow grooves rather than open passageways, and the incoming air bounces off the
conchal surfaces and churns around like water flowing over rapids. Mucous membrane lines the
cavity and its shelves so this turbulence serves a purpose, as the air eddies and swirls, small
airborne particles come in contact with the mucus. The turbulence also allows extra time for
warming and humidifying the incoming air.
The olfactory receptors lie in the membrane lining the superior nasal conchae and adjacent
nasal septum. This region is called the olfactory epithelium and inferior to it is a mucous
membrane containing capillaries and pseudostratified ciliated columnar epithelium with many
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goblet cells. Blood circulating in the capillaries warms inhaled air whirling around the conchae
and meatuses. Mucus secreted by the goblet cells traps dust particles and moistens the air. The
cilia move the mucus and its contents toward the pharynx, at which point they can be removed
from the respiratory tract.
The Pharynx
The nose, mouth and throat connect to each other by a common passageway or chamber called
the pharynx. The pharynx is a funnel-shaped tube shared by the digestive and respiratory
systems. It extends between the internal nares and the entrances to the trachea and
esophagus. Its wall is lined with mucous membrane and composed of skeletal muscles. The
veins of the pharynx drain into the pterygoid plexus and the internal jugular veins. Most of the
muscles of the pharynx are innervated by nerve branches from the glossopharyngeal (IX) and
vagus (X) nerves. The pharynx serves as a passageway for air and food, provides a chamber for
speech sounds, and houses the tonsils, which aid in immunological reactions against foreign
invaders. The curving superior and posterior walls are closely bound to the axial skeleton, but
the lateral walls are rather flexible and muscular. The pharynx is divided into three regions: the
nasopharynx, the oropharynx, and the laryngopharynx.
The Nasopharynx
The nasopharynx, is the superior portion of the pharynx. Its connected to the posterior portion
of the nasal cavity by means of the internal nares and is separated from the oral cavity by the
soft palate. There are five openings in the wall of the nasopharynx: two internal nares, two
openings that lead into the eustachian tubes, and the opening into the oropharynx. The
pharyngeal tonsil is located on the posterior wall of the nasopharynx and the lateral walls
contain the openings contain the openings of the eustachian tubes or auditory tubes. The
nasopharynx exchanges small amounts of air with the eustachian tubes to equalize air pressure
between the pharynx and the middle ear.
The Oropharynx
The intermediate portion of the pharynx, the oropharynx, extends between the soft palate and
the base of the tongue at the level of the hyoid bone. The posterior portion of the oral cavity
communicates directly with the oropharynx, just as the posterior and inferior portions of the
nasopharynx do. In addition to communicating upward with the nasopharynx and downward
with the laryngopharynx, it has an anterior opening, the fauces, which is the passageway
between the oral cavity and the oropharynx. The oropharynx is lined with nonkeratinized
stratified squamous epithelium because it is subject to abrasion by food particles. Two pairs of
tonsils, the palatine and lingual tonsils are also found in the oropharynx.
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The Laryngopharynx
The laryngopharynx, or hypopharynx, is a the most inferior part of the pharynx, and like the
oropharynx it is lined by nonkeratinized stratified squamous epithelium that can resist abrasion,
chemical attack, and pathogenic invasion. The laryngopharynx is narrow and it includes the
region of the pharynx lying between the hyoid bone and the entrance to the esophagus.
The Lower Respiratory System
The Larynx
The larynx, or voice box, is a short passageway that connects the laryngopharynx with the
trachea. The larynx is essentially a cylinder whose cartilaginous walls are stabilized by ligaments
or skeletal muscles or both. It lies in the midline of the neck anterior to the fourth through sixth
cervical vertebrae (C4-C6).
Cartilages of the Larynx
The wall of the larynx is composed of nine pieces of cartilage. Three occur singly (thyroid
cartilage, epiglottis, and cricoids cartilage), and three occur in pairs (arytenoid, cuneiform, and
corniculate cartilages). The arytenoids cartilages are the most important of the paired cartilages
because the positions and tensions of the vocal cords.
The epiglottis is a large, leaf-shaped piece of elastic cartilage that folds back during swallowing
to prevent the entry of liquids or solid food into the respiratory passageways. The narrowed
passageway through the larynx is called the glottis. The glottis is surrounded and protected by
the larynx. It consists of a pair of folds of mucous membrane, the vocal cords in the larynx, and
the space between them called the rima glottidis.
The thyroid cartilage, or Adam’s apple, is the largest laryngeal cartilage and forms most of the
anterior and lateral walls of the larynx. It consists of two fused plates of hyaline cartilage that
form its anterior wall and give it a triangular shape. The thyrohyoid membrane is the ligament
that connects the thyroid cartilage to the hyoid bone.
The thyroid cartilage sits superior to the cricoid cartilage. The cricoid cartilage is a ring of
hyaline cartilage that forms the inferior wall of the larynx. The cricotracheal ligament attaches
the cricoids cartilage to the first ring of cartilage of the trachea. The thyroid cartilage is
connected to the cricoids cartilage by the cricothyroid ligament. The cricoid cartilage is the
landmark for making an emergency airway called a tracheotomy.
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The superior surface of the cricoid cartilage articulates with the small paired arytenoid
cartilages. The arytenoids cartilages are triangular pieces of mostly hyaline cartilage that
articulate with the superior border of the enlarged portion of the cricoid cartilage.
The corniculate cartilages articulate with the arytenoid cartilages. The corniculate and
arytenoid cartilages are involved with the opening and closing of the glottis and the production
of sound. The cuneiform cartilages are club-shaped elastic cartilages anterior to the corniculate
cartilages and support the vocal folds and lateral aspects of the epiglottis.
Structures of Voice Production
The mucous membrane of the larynx forms two pairs of folds: a superior pair called the
ventricular folds (false vocal folds) and an inferior pair called the vocal cords (true vocal cords).
The rima vestibuli is the space between the ventricular folds. When the ventricular folds are
brought together, they function in holding the breath in the thoracic cavity against pressure,
such as might occur when a person strains to lift a heavy object.
Laryngeal Muscles
The larynx is associated with two different groups of muscles, the intrinsic laryngeal muscles
and the extrinsic laryngeal muscles. The intrinsic laryngeal muscles have two major functions.
One group regulates tension in the vocal cords, while the second set opens and closes the
glottis. The extrinsic laryngeal muscles position and stabilize the larynx. During swallowing, both
extrinsic and intrinsic muscles work together to prevent food or drink from entering the glottis.
Pitch is controlled by the tension on the vocal cords. If they are pulled firmly by the muscles,
they vibrate faster, resulting in a higher pitch. Decreasing the muscular tension on the vocal
cords produces lower-pitch sounds. Sounds originate from the vibration of the vocal cords, but
other structures are needed for turning sound into recognizable speech. The pharynx, mouth,
nasal cavity, and paranasal sinuses all act as resonating chambers that give the voice its
individual and human quality. Whispering is accomplished by closing all but the posterior
portion of the rima glottidis. Since the vocal cords do not vibrate during whispering, there is no
pitch to this form of speech.
The Trachea
The epithelium of the larynx is continuous with that of the trachea, or windpipe. The trachea is
a tough, flexible tubular passageway for air and it is located anterior to the esophagus. It
extends from the larynx to the superior border of the fifth thoracic vertebrae where it divides
into the right and left primary bronchi.
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The layers of the tracheal wall, from deep to superficial, are: the mucosa, the submucosa, the
middle tunic, and the adventitia. The mucosa of the trachea consists of an epithelial layer and
an underlying layer of lamina propia. The lamina propia is a layer of loose connective tissue
which separates the respiratory epithelium from underlying cartilages. The submucosa consists
of areolar connective tissue that contains seromucous glands and their ducts. The middle tunic
contains horizontal rings of hyaline cartilage that resemble the letter C. The rings are stacked
one above the other and are connected to each other by dense connective tissue. The open
part of each C-shaped cartilage ring faces posteriorly toward the esophagus and is spanned by a
fibromuscular membrane. Within this membrane are transverse smooth muscle fibers, called
the trachealis muscle, and elastic connective tissue that allow the trachea to change in
diameter during inhalation and exhalation. This feature is important for maintaining efficient air
flow. The adventitia of the trachea consists of areolar connective tissue that joins the trachea to
surrounding tissues.
The Primary Bronchi
The trachea branches within the mediastinum, dividing into the right and left primary bronchi.
Like the trachea, the primary bronchi consist of incomplete rings of cartilage. The internal ridge
where the trachea divides into right and left primary bronchi is called the carina. The mucous
membrane of the carina is one of the most sensitive areas of the entire larynx and trachea for
triggering a cough reflex.
The primary bronchi divide to form smaller bronchi once they enter the lungs, called the
secondary (lobar) bronchi, one for each lobe of the lung. The secondary bronchi continue to
branch, forming even smaller bronchi, called tertiary (segmental) bronchi. These then divide
into bronchioles. Bronchioles in turn branch repeatedly, and the smallest ones branch into still
smaller tubes called terminal bronchioles. This widespread branching is commonly referred to
as the bronchial tree due to its resemblance of an inverted tree.
During exercise, activity in the sympathetic division of the autonomic system increases and
causes the release of the hormones epinephrine and norepinephrine, both of which cause
relaxation of smooth muscle in the bronchioles, which dilates the airways. This results in
improved lung ventilation because it allows air to reach the alveoli quicker. The
parasympathetic division of the autonomic nervous system and mediators of allergic reactions
such as histamine cause contractions of the bronchiolar smooth muscle and result in
constriction of distal bronchioles.
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The Lungs
The left and right lungs are located in the left and right pleural cavities in the thoracic cavity.
Each lung is like a blunt cone with the tip, or apex, pointing superiorly. The broad concave
inferior portion, or base, of each lung rests on the superior surface of the diaphragm. Each lung
is surrounded by a protective, double-layered serous membrane called the pleural membrane.
The superficial layer is called the parietal pleura and it lines the wall of the thoracic cavity. The
deep layer is called the visceral pleura and it is attached to the lungs themselves. The pleural
cavity is a small space between the visceral and parietal pleurae. Inflammation of the pleural
membrane, called pleurisy or pleuritis, may cause pain in its early stages due to friction
between the parietal and visceral layers of the pleura. If the inflammation persists, excess fluid
builds up in the pleural space, a condition known as pleural effusion. Removal of this excess
fluid can be accomplished without injuring lung tissue by inserting a needle anteriorly through
the seventh intercostals space, a procedure termed thoracentesis.
The surface of the lung lying against the rib is called the costal surface. The mediatinal (medial)
surface of each lung contains a region, the hilum, through which bronchi, blood vessels,
lymphatic vessels, and nerves enter and exit. These structures are firmly anchored in a
meshwork of dense connective tissue and constitute the root of the lung. Medially, the left lung
also consists of a concavity, the cardiac notch, in which the heart lies. Due to the space taken
up by the heart, the left lung is smaller than the right lung.
One or two fissures divide each lung into lobules. Both lungs have an oblique fissure, which
extends inferiorly and anteriorly, and the right lung also has a horizontal fissure. The oblique
fissure in the left lung separates the superior lobe from the inferior lobe. In the right lung, the
inferior part of the oblique fissure separates the inferior lobe from the middle lobe, which is
bordered superiorly by the horizontal fissure.
Each lobe has its own secondary (lobar) bronchus. Therefore, the right primary bronchus gives
rise to three secondary bronchi called the superior, middle, and inferior secondary bronchi.
The left primary bronchus gives rise to superior and inferior secondary bronchi. The secondary
bronchi give rise to the tertiary bronchi within the lung. There are ten tertiary bronchi in each
lung. The segment of lung tissue that each tertiary bronchus supplies is called a
bronchopulmonary segment. Each bronchopulmonary segment of the lungs has many small
compartments called lobules. Each lobule is wrapped in elastic connective tissue and contains a
branch from a terminal bronchiole. The terminal bronchioles subdivide into even smaller
branches called respiratory bronchioles. Respiratory bronchioles then subdivide into several
alveolar ducts. The respiratory passages from the trachea to the alveolar ducts contain about
25 orders of branching.
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Surrounding the alveolar ducts are many alveoli and alveolar sacs. An alveolus is a cup-shaped
outpouching lined by simple squamous epithelium and supported by a thin elastic membrane.
An alveolar sac is a common chamber connected to several individual alveoli. The walls of
alveoli consist of two types of alveolar epithelial cells. Type I alveolar cells, the predominant
cells, form a nearly continuous lining of the alveolar wall. These cells are unusually thin and
delicate. They are also the main sites of gas exchange. Type II alveolar cells, also called septal
cells, are fewer in number and are located between type I alveolar cells. These large cells
produce an oily secretion containing a mixture of phospholipids and lipoproteins. This
secretion, known as surfactant, coats the inner surface of each alveolus. Associated with the
alveolar wall are alveolar macrophages, wandering phagocytes that remove dust particles and
other debris in the alveolar spaces. Fibroblasts that produce reticular and elastic fibers are also
present.
The exchange of oxygen and carbon dioxide between the air spaces in the lungs and the blood
takes place by diffusion across the alveolar and capillary walls, which together form the
respiratory membrane. The respiratory membrane consists of four layers: the alveolar wall, an
epithelial basement membrane, a capillary basement membrane, and endothelial cells. The
alveolar wall consists of a layer of type I and type II alveolar cells and associated alveolar
macrophages. The epithelial basement membrane underlies the alveolar wall. The capillary
basement membrane is fused to the epithelial basement membrane. The endothelial cells are
part of the capillary. Despite having several layers, the respiratory membrane is very thin. This
thinness allows rapid diffusion of gases.
The Blood Supply to the Lungs
The lungs receive blood through two sets of arteries: pulmonary and bronchial arteries.
Deoxygenated blood passes through the pulmonary trunk, which divides into a left pulmonary
artery that enters the left lung and a right pulmonary artery that enters the right lung.
Oxygenated blood is returned to the heart by way of the four pulmonary veins, which drain into
the left atrium. Bronchial arteries branch from the aorta and deliver oxygenated blood to the
lungs. Most blood returns to the heart via pulmonary veins but some blood, however, drains
into bronchial veins and returns to the heart via the superior vena cava. The nerve supply of the
lungs is derived from branches of the vagus (X) nerves and sympathetic trunks.
Pulmonary Ventilation (Breathing)
Respiration is the exchange of gases between the atmosphere, blood, and body cells. It takes
place in three simple steps: pulmonary ventilation, external (pulmonary) respiration, and
internal (tissue) respiration.
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Pulmonary ventilation, or breathing, is the first process, and it consists of inhalation and
exhalation of air. It is the exchange of air between the atmosphere and the air spaces of the
lungs. External respiration is the exchange of gases between the air spaces of the lungs and
blood in pulmonary capillaries across the respiratory membrane. The blood gains oxygen and
loses carbon dioxide during this process. Internal respiration is the exchange of gases between
blood in systemic capillaries and tissue cells. The blood loses oxygen and gains carbon dioxide
during this process.
Inhalation
Breathing in is called inhalation or inspiration. For inhalation to occur, the lungs must expand.
This involves the contraction of the main muscles of inhalation-the diaphragm and/or external
intercostals.
The diaphragm is the most important muscle of inhalation and it forms the floor of the thoracic
cavity. During inhalation, the diaphragm contracts and flattens, increasing its diameter. At the
same time the diaphragm contracts, the external intercostals contract. When these muscles
contract, the ribs are pulled superiorly and the sternum is pushed anteriorly, therefore
increasing the diameter of the thoracic cavity. During deep, forceful inhalations, accessory
muscles of inspiration also participate in increasing the size of the thoracic cavity. Some of
these muscles include the sternocleidomastoid muscles which elevate the sternum, the scalene
muscles which elevate the first two ribs, and the pectoralis minor muscles which elevate the
third through fifth ribs.
Exhalation
Breathing out is called exhalation or expiration. Normal exhalation depends on two factors: the
recoil of elastic fibers that were stretched during inhalation and the inward pull of surface
tension due to alveolar fluid. Exhalation begins when the inspiratory muscles relax. As the
external intercostals relax, the ribs move inferiorly and the diaphragm relaxes. As the
diaphragm relaxes, its dome moves superiorly. These movements decrease the diameters of
the thoracic cavity and air then flows outwards into the atmosphere.
Respiration allows humans to express certain emotions such laughing, sighing, and sobbing.
Furthermore, respiratory air can be used to expel foreign matter from air passages through
sneezing and coughing.
Although breathing can be voluntarily controlled for short periods of time, the nervous system
usually controls it automatically to meet the body’s demand without conscious effort. The
respiratory center consists of a widely dispersed group of neurons that send nerve impulses to
respiratory muscles. This area can be divided into three areas: the medullary rhythmicity area in
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the medulla oblongata, the pneumotaxic area in the pons, and the apneustic area also in the
pons.
The medullary rhythmicity area functions to control the basic rhythm of respiration and within
this area are inspiratory and expiratory neurons. Nerve impulses generated in the inspiratory
area establish the basic rhythm of breathing. While this area is active, it sends out nerve
impulses. These nerve impulses eventually reach the diaphragm and external intercostals
muscles, causing these muscles to contract, and inhalation occurs. The neurons of the
expiratory area remain inactive during quiet breathing but are activated by nerve impulses
from the inspiratory area during forceful breathing. Impulses from the expiratory area cause
contraction of the abdominal muscles and internal intercostal muscles, which decreases the
size of the thoracic cavity and causes forceful exhalation.
The pneumotaxic area in the upper pons transmits inhibitory impulses to the inspiratory area
and helps coordinate the transition between inhalation and exhalation. The primary effect of
these nerve impulses is to help shut off the inspiratory area before the lungs become too full of
air. When the pneumotaxic area is more active, breathing rate is much faster.
The apneustic area in the lower pons also coordinates the transition between inhalation and
exhalation. This area sends stimulatory impulses to the inspiratory area to prolong inhalation.
When the pneumotaxic area is active, it overrides the signals from the apneustic area.
Certain chemical stimuli determine how quickly and how deeply we breathe. Sensory neurons
that are responsive to chemicals are called chemoreceptors. There are chemoreceptors located
in two places. Central chemoreceptors are located in the medulla oblongata in the central
nervous system and respond to changes in hydrogen or carbon dioxide concentration, or both,
in cerebrospinal fluid. Peripheral chemoreceptors are located in the aortic bodies,
chemoreceptor bundles in the wall of the arch of the aorta, and in the carotid bodies, which
are oval nodules in the wall the left and right common carotid arteries where they divide into
the internal and external carotid arteries. These chemoreceptors are part of the peripheral
nervous system and are sensitive to changes in oxygen, hydrogen, and carbon dioxide in the
blood. If there is even a slight increase in carbon dioxide, central and peripheral
chemoreceptors are stimulated. These in turn send nerve impulses to the brain that cause the
inspiratory area to become highly active, and the respiration rate increases. This allows more
carbon dioxide to be expelled by the body until its levels are lowered to normal.
The Inflation Reflex
Located in the walls of bronchi and bronchioles are stretch-sensitive receptors called
baroreceptors or stretch receptors. Nerve impulses are sent to the inspiratory and apneustic
areas when these receptors become stretched during overinflation. In response, these areas
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are inhibited and expiration begins. The lungs deflate as air leaves the lungs and the stretch
receptors are no longer stimulated. This reflex is referred to as the inflation (Hering-Breuer)
reflex and is essentially a protective mechanism for prevention of excessive inflation of the
lungs.
Exercise and the Respiratory System
The respiratory and cardiovascular systems make adjustments in response to both duration and
intensity of exercise. When muscles contract during exercise, they consume large amounts of
oxygen and produce large amounts of carbon dioxide. Oxygen consumption and pulmonary
ventilation both increase dramatically during vigorous exercise. An abrupt increase in breathing
is caused by neural changes that send excitatory impulses to the inspiratory area in the medulla
oblongata. During moderate exercise the gradual increase in ventilation is due to chemical and
physical changes in the bloodstream. In addition, the depth of ventilation changes more than
the rate of breathing. When exercise is more arduous, the frequency of breathing also
increases. At the end of an exercise session, a sudden decrease in breathing is followed a
gradual decline to the resting level.
Development of the Respiratory System
At about four weeks of development, the respiratory system begins as an outgrowth called the
respiratory diverticulum. The endoderm lining the respiratory diverticulum eventually becomes
the epithelium and glands of the trachea, alveoli, and bronchi. Splanchnic mesoderm surrounds
the respiratory diverticulum and eventually becomes the connective tissue, cartilage, and
smooth muscle of these structures. As the respiratory diverticulum lengthens, its distal end
enlarges to form a tracheal bud, which gives rise to the trachea. Not much later, it divides into
bronchial buds, which repeatedly branch out and develop with the bronchi. By the 24 th week,
respiratory bronchioles have been formed. During weeks 6 to 16, all main components of the
lungs have developed, except those involved in gaseous exchange (respiratory bronchioles,
alveoli, and alveolar ducts). Fetuses born at this time cannot survive because respiration is not
yet possible at this stage. During the following weeks, lung tissue and alveoli continue to
develop. A fetus born near the end of this period can survive if given intensive care but death
still frequently occurs due to the immaturity of the respiratory and other systems. Infants born
before 26-28 weeks are at high risk for respiratory distress syndrome (RDS), in which the
alveoli collapse during exhalation and must be reinflated during inhalation. However, the
condition can be treated by forcing air into the lungs via respirator.
As the lungs develop, they obtain their pleural sacs. The visceral pleura develops from
splanchnic mesoderm and the parietal pleura develops from somatic mesoderm. The space
between the layers is the pleural cavity. At birth, most of the lungs of the fetus are filled with
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fluid. This fluid is a mixture of amniotic fluid, mucus, and surfactant. Once breathing begins at
birth, some of the fluid is reabsorbed by blood and lymph capillaries and the rest is expelled
through the nose and mouth during delivery.
Aging and the Respiratory System
The airways and tissues of the respiratory tract become less elastic and more rigid with
advancing age. The result is a significant decrease in lung capacity. In fact, in can decrease as
much as 35% by age 70. In addition, a decrease in blood level of oxygen, decreased activity of
macrophages in alveoli, and diminished action of cilia of the epithelium lining the respiratory
tract occur as well. Elderly people are more prone to pulmonary disorders because of all these
age-related factors. It can also contribute to an older person’s reduced ability to exercise
vigorously.
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