NORMAL AERODIGESTIVE TRACT
ANATOMY
Esophageal & Respiratory Systems
Esophageal System
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The esophagus is a tubular
structure linking the pharyngeal
and gastric cavities.
It passes through the neck, then
the chest, through the diaphragm
to attach to the stomach.
In the neck, the esophagus sits
behind the trachea, sharing a
soft tissue wall.
Esophageal System
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The wall of the
esophagus is composed
of four layers, which
include the tunica
mucosa, the submucosa,
the muscularis (muscle),
and adventitia
(membranous outer
covering).
Esophageal System
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The innermost layer,
or the mucosa of the
esophagus, consists of
a thin layer of nonkeratinized stratified
squamous epithelium.
Esophageal System
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Deep to the mucosa, the
submucosal layer contains
connective tissue, blood
vessels, and the glandular
cells that secrete mucus
into the esophagus.
Near the stomach, the
mucosa of the esophagus
also contains mucous
glands.
Esophageal System
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The muscularis externa of
the upper third of the
esophageal body begins at
the inferior border of the
esophageal border of the
cricopharyngeal muscle,
and it is striated.
Esophageal System
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These striated muscles
have an outer
longitudinal and a
thicker inner circular
layer, oriented in an
oblique or screw-like
course.
Esophageal System
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The distal two-thirds of
the esophagus is
composed of smooth
muscle fibers arranged
in a similar manner.
Esophageal System
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The juncture between
smooth and striated
muscle is not sharply
demarcated, but
consists of fibers,
which interdigitate for
variable lengths of
distance along the midesophagus.
Esophageal System
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The outer layer, the
adventitia, is a loose
connective tissue
layer not covered by
epithelium.
Esophageal System
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The valve at the bottom
of the esophagus is the
lower esophageal
sphincter, or cardia,
marking the boundary
between the esophagus
and the stomach.
Esophageal System
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The LES, like the distal
esophagus, is made of
smooth muscle and is not
composed of a distinct
set of identifiable muscle
fibers like the UES.
Esophageal System
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The LES is a functional
sphincter that keeps food
and secretions, including
stomach acid, in the
stomach.
Esophageal System
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It is tonically contracted and is
aided by the impingement of the
diaphragm to produce a pressure
of approximately 10-40 mmHG
in order to prevent reflux of
gastric contents from the
abdomen (which is under
positive pressure) into the chest
(which is under negative
pressure).
Esophageal System
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After the relaxation of the UES and pharyngeal
clearance of the bolus into the proximal esophagus,
a reflexive peristaltic “primary stripping wave” is
initiated that propels the bolus distally.
The amplitude of the peristaltic wave is normally
higher in the distal esophagus than the proximal
and it travels at approximately 3-5 cm per second.
Esophageal System
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As soon as the primary stripping wave is initiated
from the oropharyngeal swallow, the LES relaxes to
approximately less than 10 mmHG and remains open
until the stripping wave has arrived at the LES and the
bolus has passed into the stomach.
Secondary stripping waves which are coordinated
peristaltic waves that originate within the proximal
esophagus rather than from an oropharyngeal
swallow, may propel any residual bolus into the
stomach.
Esophageal System
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The esophagus does not produce digestive enzymes
and does not carry on absorption.
It secretes mucus and transports food to the
stomach by involuntary muscular movements
called peristalsis.
Peristalsis is a function of the muscularis and is
controlled by the medulla.
The Respiratory System
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The respiratory system is
composed of the upper and
lower airways.
The upper airway consists of
the nose, mouth, pharynx, and
larynx.
The lower airway consists of
the tracheo-bronchial tree and
the lungs.
The tracheo-bronchial tree
consist of a system of
connecting tubes that conduct
airflow in and out of the lungs
and allow for gas exchange.
The Respiratory System
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The trachea is situated
anteriorly to the
esophagus, beginning
at the cricoid cartilage
and extending
inferiorly to the carina,
where it bifurcates into
the right and left main
stem bronchi.
The Respiratory System
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The trachea is
composed of c-shaped
cartilaginous rings
joined by connective
tissue.
These cartilaginous
rings assist in keeping
the trachea open during
breathing.
The Respiratory System
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The lungs are situated in
the thoracic cavity,
enclosed by the rib cage
and diaphragm, the major
muscle of ventilation,
which separates the
thoracic cavity from the
abdominal cavity.
The Respiratory System
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The lungs consist of
five lobes.
The right lung has three
lobes: the upper,
middle, and lower
lobes.
The left lung has two
lobes: the upper and
lower lobes.
Respiratory Physiology
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The primary function
of the lungs is gas
exchange in the form
of absorption of
oxygen and
elimination of carbon
dioxide.
Respiratory Physiology
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The respiratory system delivers
O2 from the atmosphere to the
blood and into the cells and
transports CO2 from the cells to
the blood and back into the
atmosphere.
The overall exchange of gases
between the atmosphere, blood,
and cells is called respiration.
Respiratory Physiology
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Three basic processes are involved in respiration:
ventilation, external respiration, and internal
respiration.
Ventilation is the mechanical movement of air into
and out of the lungs in a cyclic fashion and involves
the processes of inhalation and exhalation.
It is accomplished by the action of the respiratory
muscles on the thorax.
Respiratory Physiology
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Inhalation is the inward flow of air, whereas
exhalation is the outward flow of air from the
respiratory tract.
Maintenance of adequate ventilation is controlled
partly by chemoreceptors in the central nervous
system that respond to gas levels in the blood and
inform the brainstem to activate the muscles of
inspiration
Respiratory Physiology
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External respiration is the
exchange of O2 and CO2
between the alveoli of the
lungs and pulmonary blood
capillaries.
It is a passive two-way
transfer of respiratory gases
between the alveoli of the
lung, blood plasma, and the
hemoglobin molecules.
Respiratory Physiology
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Again O2 is diffusing from the blood capillaries,
which have a high PO2 (partial pressure of O2
concentration), to the tissue cells, which have a
low partial pressure of O2 concentration.
Oxygen diffuses from the oxygenated blood
through extracellular fluid and into tissue cells
until equilibrium is reached.
Respiratory Physiology
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In the cells, O2 and other nutritive substances are
used in the process of metabolism.
Cellular metabolism results in the production of
energy, water, and CO2.
While O2 diffuses from the blood capillaries into
the tissue cells, CO2 diffuses in the opposite
direction.
Respiratory Physiology
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CO2 diffused from the tissue cells into the venous
blood is carried in three forms.
Some is physically dissolved in plasma, and is
reflected in blood gas analysis.
Another portion is bound to proteins.
Some diffuses from the plasma into the alveolus
and is then eliminated via ventilation
Respiratory Control
Breathing is controlled by
feedback mechanisms
between receptors in the lung
and periphery, and the
respiratory center in the
brainstem.
 The respiratory center is
located in the medulla of the
brainstem, with contributions
from the pons.
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Respiratory Control
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Within the medulla are the
dorsal respiratory group
(DRG) and the ventral
respiratory group (VRG).
The DRG sends stimuli to
the muscles of inspiration:
the diaphragm and the
external intercostal
muscles.
Respiratory Control
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The VRG sends stimuli to
the muscles of expiration:
the internal intercostals and
abdominal muscles.
These muscles act only in
forced expiration so the
VRG is only active during
forced expiration.
Respiratory Control
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The DRG, however, is
active in both quiet
and forced respiration.
The medulla controls
the rhythmcity of
respiration through
interaction neurons
that fire either during
inspiration or
expiration.
Respiratory Control
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I neurons fire from the
DRG to stimulate the
muscles of inspiration
to bring about
inspiration.
E neurons fire to inhibit
the muscles of
inspiration and to bring
about expiration.
Respiratory Control
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The pons sends stimuli to
the medulla to regulate the
rate and depth of
respiration.
The pneumotaxic center
located in the upper pons
increases the respiratory
rate by shortening
inspirations.
Respiratory Control
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The apneustic center
located in the lower
pons increases the
depth of respiration and
reduces the rate by
prolonging inspirations.
Input to the respiratory
centers come from
chemical and
mechanical signaling.
Chemical Signaling
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Peripheral
chemoreceptors are
located in the aorta and
carotid arteries.
They are sensitive to
changes in CO2 and O2
levels in the blood.
Chemical Signaling
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Central chemoreceptors
are located in the medulla
and are sensitive to the
level of carbon dioxide in
the blood.
Increased CO2 levels
stimulate chemoreceptors
more than decreased O2
levels.
Chemical Signaling
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If O2 levels fall or CO2 levels vary too greatly
from the set point, a negative feedback
mechanism will increase respiratory rate.
For example, under normal circumstances, arterial
blood PC02 is 40 mm Hg.
If there is even a slight increase in PCO2, a
condition known as hypercapnia, the
chemosensitive area in the medulla, and the
chemoreceptors in the carotid and aortic arteries are
stimulated.
Chemical Signaling
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This stimulation causes the inspiratory area to
become highly active, and the rate and depth of
respiration increases.
This increased rate, hyperventilation, allows the
body to expel more CO2 until PCO2 are lowered
to normal.
If the reverse is true, that PCO2 is lower than 40
mm Hg, a condition called hypocapnia, the
chemoreceptors are not stimulated, and stimulatory
impulses are not sent to the inspiratory area.
Chemical Signaling
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The inspiratory area just keeps its own moderate
pace until CO2 accumulates and the PCO2 rises to
40 mm Hg.
The O2 chemoreceptors are sensitive only to large
decreases in PO2 since hemoglobin remains about
85% or more saturated at PO2 values all the way
down to 50 mm Hg.
If arterial PO2 falls from a normal of 105 mm Hg to
about 50 mm Hg, the O2 chemoreceptors become
stimulated and send impulses to the inspiratory area
and respiration increases.
Chemical Signaling
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But if the P02 falls much below 50 mm Hg, the
cells of the inspiratory area suffer O2 starvation
and do not respond well to any chemical receptors.
They send fewer impulses to the inspiratory
muscles, and respiration rate decreases or
breathing ceases altogether.
A fall in the blood pH also stimulates the
respiratory center.
The acid-base balance, or pH level in the blood, is
a measure of alkalinity and acidity.
Chemical Signaling
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The pH of the blood is maintained in a very narrow
range (pH = 7.35 to 7.45) by a buffer system provided by
the respiratory system and the renal system.
The respiratory system responds to changes in pH by
increasing ventilation (hyperventilation) or decreasing
ventilation (hypoventilation), which decreases or
increases CO2 levels, respectively.
In other words, in response to hyperventilation, CO2
levels decrease.
In response to hypoventilation, CO2 levels increase.
Mechanical Signaling
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Located in the walls of the bronchi and bronchioles
throughout the lungs are stretch receptors.
When the receptors become overstretched, nerve
impulses are sent along the X vagus nerve to the
inspiratory and apneustic areas.
In response, both the inspiratory and apneustic areas
are inhibited, and expiration follows.
Mechanical Signaling
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As air leaves the lungs during expiration, the lungs
deflate and the stretch receptors are no longer
stimulated.
Thus, the inspiratory and apneustic areas are no
longer inhibited, and a new inspiration begins.
This reflex is referred to as the Hering-Breuer
(inflation) reflex.
It is a protective mechanism for prevention of over
inflation of the lungs rather than a key component in
the regulation of respiration.
Mechanical Signaling
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When you exercise there is a direct stimulus to the
respiratory center from active muscles and joint
receptors.
These stimuli increase respiration before blood
chemistry actually changes enough to demand it.
The carotid and aortic sinuses also contain
baroreceptors or pressure receptors that detect
changes in blood pressure.
Although mainly concerned with controlling
circulation, they affect respiration as well.
Mechanical Signaling
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A sudden rise in blood pressure decreases the rate of
respiration.
A drop in blood pressure increases the respiratory
rate.
Finally the respiratory center has connections from
the cerebral cortex, which means we can voluntarily
alter our pattern of breathing.
We can even refuse to breathe at all for a short time.
Voluntary control is protective because it enables us
to prevent water or irritating gases from entering the
lungs.
Mechanical Signaling
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However, the ability to stop breathing is limited by
the buildup of CO2 and H+ in the blood.
When PCO2 and H+ increase to a certain level, the
inspiratory areas is stimulated, nerve impulses are
sent to inspiratory muscles, and breathing resumes
whether or not the person wishes.
Respiratory Defenses*
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The basic nature of the respiration system, as well as
the shared nature of the initial anatomical structures
for the passage of food and air, places the airway and
lungs under the constant threat of exposure to a
variety of harmful airborne particles, organisms, and
other substances, as well as aspirated gastric contents
or accidental inhalation of foodstuffs.
Respiratory Defenses*
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A variety of defensive mechanisms have evolved
along with the normal function of the respiratory
system to help protect against such threats.
Airway protection relies upon specialized epithelial
barriers and immune responses.
There are also a variety of highly coordinated neural
reflex responses that help to limit the degree of
potential harm and to remove or expel the harmful
substance from the airways.
Respiratory Defenses*
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The most highly recognized neural response involved
in airway protection is coughing.
Coughing is a reflex-evoked modification of breathing
pattern in response to airway irritation.
Reflex cough occurs when subsets of airway sensory
nerves are activated by inhaled, aspirated, or locally
produced substances.
These afferent nerves provide modifying inputs to the
brainstem neural elements controlling respiration and
help generate the cough respiratory pattern.
Respiratory Defenses*
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In addition, cough can be initiated voluntarily, although
little is known about the cortical pathways responsible
for voluntary coughing.
Airway sensory nerves can be broadly classified as
either primarily mechanically sensitive
(mechanosensors) or primarily chemically sensitive
(chemosensors).
As we have discussed, mechanosensor are activated by
one or more mechanical stimuli, including lung
inflation, bronchospasm, or light touch.
Respiratory Defenses*
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Similarly, chemosensors are typically activated
directly by a wide range of chemicals.
The only exception is that some mechanosensors also
directly respond to chemical stimuli including acid and
ATP.
Mechanosensors located in the larynx, trachea, and
large bronchi are referred to as extrapulmonary
mechanosensors.
They are exquisitely sensitive to touch type
mechanical stimuli but not to tissue stretching,
changes in luminal pressure, or airway smooth muscle
constriction.
Respiratory Defenses*
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They are also activated by rapid changes in pH as
might be expected to occur following aspiration of
gastric contents.
Mechanical irritation and changes in pH are both
stimuli that readily evoke cough in conscious and
anesthetized humans.
Chemically sensitive airway afferent fibers are found
throughout the airways and lungs and are generally
quiescent in the normal airway.
They are recruited during airway inflammation or
irritation.
Respiratory Defenses*
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The basic primary defensive cough, in response to
an acute stimulus, such as aspiration or direct
mechanical probing of the airway mucosa, is
likely mediated primarily by extrapulmonary
cough receptors.
However, cough associated with airway
obstruction or more chronic airway irritation, as
would be expected to occur in airways diseases,
may involve recruitment of both intrapulmonary
mechanosensors and chemosensor pathways from
the nose and esophagus.
Respiratory Defenses*
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As such, inflammatory diseases of the airway, nose,
and/or esophagus, may contribute to hypertussive
states that are non-productive.
* From Mazzone, S. B. (2005). An overview of the sensory
receptors regulating cough. Cough, 1(2), 1-9.