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Larynx
Air
enters the lower respiratory system through an opening called
the glottis. The larynx is a cylindrical structure
that surrounds, protects and controls the glottis. |
The
larynx is a complex structure made up of pieces of cartilage
held together by muscles and ligaments. There are three
unpaired cartilages:
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1. Thyroid cartilage (shield-shaped).
The thyroid cartilage is the largest
cartilage and forms anterior and lateral walls
of the larynx. Like a shield, it is open in the
back and on the anterior surface has a prominent
ridge called the laryngeal prominence
(Adam’s apple). |
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2. Cricoid cartilage (ring-shaped).
This cartilage is inferior to thyroid and
forms a complete ring of which the posterior
portion is greatly expanded (like a signet
ring). Ligaments attach the inferior surface to
the first cartilage of the trachea. |
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3. Epiglottis
The epiglottis is a shoehorn-shaped
structure that is supported by the epiglottic
cartilage. The cartilage is attached to the
anterior and superior borders of the thyroid and
the hyoid bone. It projects superior to
glottis (Hence, “epi” glottis). During
swallowing the epiglottis folds over the glottis
and prevents the entry of liquids and solids.
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There are
three paired cartilages:
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1. Arytenoid cartilages (ladle-shaped).
This cartilage articulates with the
superior border of the cricoid cartilage. |
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2. Corniculate cartilages (horn-shaped).
The corniculates articulate with the
arytenoids and together these two pairs of
cartilage control the opening and closing of the
glottis. |
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3. Cuneiform cartilages (wedge-shaped).
These are cartilages that lie in the
aryepiglottic fold that extends from the
arytenoids to the epiglottis. |
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Laryngeal
Ligaments
The intrinsic ligaments bind the cartilage of the
larynx together and the extrinsic ligaments
attach thyroid to hyoid and cricoid to the trachea. The
vestibular and vocal ligaments extend
between the thyroid and arytenoids. Both of these
intrinsic ligaments are covered by epithelium and form
the:
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vestibular folds that are
superior to the vocal folds and protect the
glottis; |
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vocal folds that form the boundaries
of the glottis. |
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The vocal folds contain elastic tissue that vibrate to
produce sound and are also called the true vocal
folds. The vestibular ligaments are stiffer and the
vestibular folds are also called the false vocal
folds. |
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Laryngeal
Musculature
The larynx has both intrinsic laryngeal muscles
that regulate tension on vocal cords opens and closes
glottis and extrinsic laryngeal muscles positions
and stabilizes larynx.
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Trachea
The
trachea is a tough, flexible tube that is supported by 15 to 20
C-shaped tracheal cartilages which are attached by
annular ligaments. The tracheal cartilages are open in
the back where the esophagus is located. This allows the trachea
accommodate the expansion of the esophagus as food passes
through it. The ends of the C-shaped cartilages are spanned by
the trachealis m. Contraction and relaxation of this
smooth muscle is under autonomic control and results in a change
in the diameter of the tracheal lumen. |
The
trachea is lined by typical respiratory epithelium, PCCE. The
thick layer of connective tissue under the mucosa called
the submucosa often contains mucous glands that are
connected by ducts to the surface.
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Primary Bronchus
The
trachea divides into right and left primary bronchi
which are similar to the trachea in design. At the point of
separation there is an internal ridge called the carina
(“keel”). The primary bronchi are also called extrapulmonary
bronchi because they are still outside the lungs. The
primary bronchi enter each lung at an indentation called the
hilus and they enter the lung with blood vessels nerves and
lymphatics. This complex of structures that arises from the
mediastinum with supportive connective tissue is called the
root of the lungs.
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Superficial Anatomy of Lungs
The right
and left lungs are situated in the pleural cavities and are
shaped like blunt cones with a blunt superior apex within
the base of the neck and a concave base on the surface of
the diaphragm. |
Lobes of lungs:
Each lung is divided into distinct lobes. The
right lung has three lobes, a superior, middle
and inferior lobe. The left lung has only
two lobes, a superior and inferior lobe. |
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Lung surfaces:
The curving anterior portion of lungs underneath the rib
cage is the costal surface. The medial or
mediastinal surface presses against the
structures in the mediastinum and as a consequence has
an irregular surface. |
The heart and the great vessels are mostly on the left
side and press into the mediastinal surface of the left
lung. As a result, the medial border of the left lung
has a deep indentation called cardiac notch. |
The surface of the lung is divided in lobules by
connective tissue septa (“walls”) that are
extensions of the connective tissue of the visceral
pleura.
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Internal Structure of the Lungs
Bronchi
After the primary bronchi enter the lungs they
immediately branch into smaller and smaller passage ways
giving rise to what is called the bronchial tree.
The bronchi within the lungs are now referred to as
intrapulmonary bronchi. |
The primary bronchi first divide into the secondary
or lobar bronchi that supply the lobes of each
lung. The right lung has three lobes ands as a
consequence has a superior lobar bronchus, middle
lobar bronchus and inferior lobar bronchus.
The left lung has two lobes and a superior lobar
bronchus and a inferior lobar bronchus. |
Inside the lobes of the lungs, the lobar bronchi
divide into tertiary or segmental bronchi
that provide the bronchopulmonary segments within
each lung. Each bronchopulmonary segment has its own
blood supply and drainage in addition to its own
segmental bronchi. Each lung has 10
bronchopulmonary segments. |
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Bronchioles
As the tertiary bronchi branch within a segment
and become smaller, the cartilaginous plates that
supported the wall disappear and the outer wall is
dominated by smooth muscle. These passageways are
now called bronchioles (“little bronchi”). |
The bronchioles continue to divide and give rise to
smaller bronchioles until the bronchiole is supplying
air to a pulmonary lobule. At this point the
bronchiole is called a terminal bronchiole and
this marks the end of the conducting system
of the respiratory tract. The terminal bronchioles
still have smooth muscle in their walls that control the
flow of air into the lobule. Contraction of the smooth
muscle is called bronchoconstriction which causes
the opening to narrow. Relaxation is called
bronchodilation which causes the opening to widen. |
Within the lobule the terminal bronchiole divides into
bronchioles whose walls become thinner and develop thin
pouches called alveoli. These bronchioles are now
called respiratory bronchioles and the alveoli
are line by simple squamous epithelium across
which gas exchange occurs. The respiratory bronchioles
are the beginning of the respiratory portion
of the respiratory tract. |
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Alveolar Ducts and
Alveoli
The respiratory bronchioles divide into multiple linear
passageways lined by alveoli that end in sacs
surrounded by alveoli. The linear passageways are
alveolar ducts and the sacs are alveolar sacs.
The alveolar walls are associated with capillaries
and elastic fibers that enable the alveolar ducts
and sacs to maintain their relative positions after
expansion. |
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Alveolus and Respiratory
Membrane
The alveolar epithelium is primarily simple
squamous epithelium. The alveolar epithelium
consists of:
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1. Squamous epithelial cells also
called Type I cells or respiratory
epitheliocytes; |
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2. Large cells called septal
cells, surfactant cells, Type II cells or
large alveolar cells. This cell (of many
names!) produces an oily secretion called
surfactant. Surfactant consists of a mixture
of phospholipids that reduce the surface tension
of the fluid in alveoli; |
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3. Alveolar macrophages that
phagocytize any particulate matter or pathogens
that managed to get through the defenses of the
respiratory tract. |
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The respiratory membrane consists of:
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1. alveolar epithelium; |
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2. capillary endothelium; |
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3. fused basal laminae of the
alveolar and endothelial cells. |
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The respiratory membrane is thin and permits
rapid exchange of the lipid soluble respiratory
gases. |
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