Download Olfactory Epithelium

Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
Olfactory Epithelium
The respiratory system has two major portions, the conducting portion, situated both outside
and within the lungs, conveys air from the external milieu to the lungs and the respiratory
portion, located strictly within the lungs, functions in the actual exchange of oxygen for carbon
dioxide (external respiration).
The roof of the nasal cavity, the superior aspect of the nasal septum, and the superior concha are
covered by an olfactory epithelium. The underlying lamina propria houses serous fluid–
secreting Bowman’s glands, a rich vascular plexus, and collections of axons that arise from the
olfactory cells of the olfactory epithelium. The olfactory epithelium is composed of three types
of cells: olfactory, sustentacular, and basal cells.
Olfactory cells are bipolar neurons whose apical aspect, the distal terminus of its slender
dendrite, is modified to form a bulb, the olfactory vesicle, which projects above the surface of
the sustentacular cells. Six to eight long, nonmotile olfactory cilia extend from the olfactory
vesicle and lie on the free surface of the epithelium. The basal region of the olfactory cell is its
axon, which penetrates the basal lamina and joins similar axons to form bundles of nerve fibers
that synapse with secondary neurons in the olfactory bulb.
The tall, columnar sustentacular cells have secretory granules housing a yellow pigment
characteristic of the color of the olfactory mucosa. These cells are believed to provide physical
support, nourishment, and electrical insulation for the olfactory cells.
Basal cells have considerable proliferative capacity and can replace both sustentacular and
olfactory cells. In a healthy person, the olfactory and sustentacular cells have a life span of less
than a year.
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
Figure (1) the olfactory epithelium, displaying basal, olfactory, and sustentacular cells.
Trachea
The trachea is a tube that begins at the cricoid cartilage of the larynx and ends when it bifurcates
to form the primary bronchi. The wall of the trachea is reinforced by 10 to 12 horseshoe-shaped
hyaline cartilage rings (C-rings). The open ends of these rings face posteriorly and are connected
to each other by smooth muscle, the trachealis muscle. The trachea has three layers: mucosa,
submucosa, and adventitia.
The mucosal lining of the trachea is composed of pseudostratified ciliated columnar epithelium,
the subepithelial connective tissue (lamina propria), and a relatively thick bundle of elastic fibers
separating the mucosa from the submucosa. The lamina propria of the trachea is composed of a
loose, fibroelastic connective tissue. It contains lymphoid elements (e.g., lymphoid nodules,
lymphocytes, and neutrophils) as well as mucous and seromucous glands, whose ducts open onto
the epithelial surface. A dense layer of elastic fibers, the elastic lamina, separates the lamina
propria from the underlying submucosa.
The tracheal submucosa is composed of a dense, irregular fibroelastic connective tissue housing
numerous mucous and seromucous glands. The short ducts of these glands pierce the elastic
lamina and the lamina propria to open onto the epithelial surface.
The adventitia of the trachea is composed of a fibroelastic connective tissue that anchors the
trachea to adjoining structures. The most prominent features of the adventitia are the hyaline
cartilage C-rings and the intervening fibrous connective tissue.
Figure (2) Light photomicrograph of the trachea in a monkey . There are numerous cilia (Ci) as
well as goblet cells (GC) in the epithelium. Also observe the mucous glands (MG) in the
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
subepithelial connective tissue and the hyaline C-ring (HC) in the adventitia. L, lumen; PC,
perichondrium.
Bronchi and Bronchioles
The bronchial tree (conducting portion) begins at the bifurcation of the trachea, as the right and
left primary bronchi, which arborize. The bronchial tree is composed of airways located outside
the lungs, the primary bronchi, and airways located inside the lungs, the intrapulmonary bronchi,
bronchioles, and terminal bronchioles.
Primary bronchi are identical to the trachea, except that bronchi are smaller in diameter and
their walls are thinner.
Intrapulmonary bronchi are similar to primary bronchi, except that the cartilages C-rings are
replaced by irregular plates of hyaline cartilage that completely surround the lumina of the
intrapulmonary bronchi. The smooth muscle is located at the interface of the fibroelastic lamina
propria and submucosa as two distinct smooth muscle layers spiraling in opposite directions.
Elastic fibers radiate from the adventitia to connect with elastic fibers arising from other parts of
the bronchial tree.
Each bronchiole supplies air to a pulmonary lobule. Their epithelial lining ranges from ciliated
simple columnar with occasional goblet cells in larger bronchioles to simple cuboidal (many
with cilia) with occasional Clara cells and no goblet cells in smaller bronchioles.
Terminal bronchioles are lined by Clara cells and cuboidal cells. The thin lamina propria
consists of fibroelastic connective tissue and is surrounded by one or two layers of smooth
muscle cells. Elastic fibers radiate from the adventitia and bind to elastic fibers radiating from
other members of the bronchial tree.
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
Figure (3) the respiratory system, displaying bronchioles, terminal bronchioles, respiratory
bronchioles, alveolar ducts, alveolar pores, and alveoli.
Respiratory Portion
The respiratory portion of the respiratory system is composed of respiratory bronchioles,
alveolar ducts, alveolar sacs, and alveoli.
Respiratory bronchioles are similar to terminal bronchioles, but their wall is interrupted by
alveoli, where gaseous exchange (O2 for CO2) can occur. Subsequent to several branchings,
each respiratory bronchiole terminates in an alveolar duct
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
Alveolar ducts do not have walls of their own. Each alveolar duct usually ends as a blind
outpouching composed of two or more small clusters of alveoli, in which each cluster is known
as an alveolar sac. These alveolar sacs thus open into a common space, which some
investigators call the atrium.
Alveoli are small air sacs composed of highly attenuated type I pneumocytes and larger type II
pneumocytes.
The region between adjacent alveoli is known as the interalveolar septum. It is occupied by an
extensive capillary bed composed of continuous capillaries.
The thinnest regions of the interalveolar septum where gases can be exchanged are called the
blood-gas barriers The narrowest blood-gas barrier, where the type I pneumocyte is in intimate
contact with the endothelial lining of the capillary and the basal laminae of the two epithelia
become fused, is most efficient for the exchange of O2 (in the alveolar lumen) for CO2 (in the
blood). These regions are composed of surfactant (manufactured by type II pneumocytes), type I
pneumocytes, basal lamina, endothelial cells.
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
Figure (4) A, A respiratory bronchiole, alveolar sac, alveolar pore, and alveoli. B, Interalveolar
septum. C, Carbon dioxide uptake from body tissues by erythrocytes and plasma. D, Carbon
dioxide release by erythrocytes and plasma in the lung.
Respiration
Approximately 200 ml of CO2 is formed by the cells of the body per minute. CO2 enters the
blood stream and is transported in three forms: as a dissolved gas in plasma (20 ml), bound to
hemoglobin (40 ml), and as plasma bicarbonate ion (140 ml). The following sequence of events
occurs:
1. Most of the CO2 dissolved in the plasma diffuses into the cytosol of the erythrocytes.
2. Some of the CO2 binds to the globin moiety of hemoglobin. Although CO2 is carried in a
different region of the hemoglobin molecule, its binding capacity is greater in the absence than in
the presence of O2 in the heme portion.
3. Within the cytosol of the erythrocyte, most of the CO2 combines with water, a reaction
catalyzed by the enzyme carbonic anhydrase, to form carbonic acid, which dissociates into
hydrogen ion (H+) and bicarbonate ion (HCO3–). The hydrogen ion binds to hemoglobin, and
the bicarbonate ion leaves the erythrocyte to enter the plasma. To maintain ionic equilibrium,
chloride ion (Cl–) enters the erythrocyte from the plasma; this exchange of bicarbonate for
chloride ions is known as the chloride shift.
Figure (5) A, A
respiratory
bronchiole, alveolar
sac, alveolar pore,
and
alveoli.
B,
Interalveolar
septum. C, Carbon
dioxide uptake from
body tissues by
erythrocytes
and
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
plasma. D, Carbon dioxide release by erythrocytes and plasma in the lung.
Respiration (cont.)
he bicarbonate-rich blood is delivered to the lungs by the pulmonary arteries. Because the level
of CO2 is greater in the blood than in the lumina of the alveoli, CO2 is released (following the
concentration gradient). The mechanism of release is the reverse of the previous reactions. The
following sequence of events occurs:
Bicarbonate
of
Cl–
ions
from
enter
the
red
the
blood
erythrocytes
cells
into
(with
the
a
plasma,
consequent
known
release
as
the
chloride shift).
Bicarbonate ions and hydrogen ions within the erythrocyte cytosol combine to form carbonic
acid.
In the lung, the combining of O2 with hemoglobin makes the hemoglobin more acidic and
reduces its ability to bind CO2. Additionally, the excess hydrogen ions released because of the
greater acidity of hemoglobin become bound to bicarbonate ions, forming carbonic acid.
Carbonic anhydrase catalyzes the cleavage of carbonic acid to form water and CO2.
CO2 dissolved in the plasma, bound to hemoglobin, and cleaved from carbonic acid follows the
concentration gradient to diffuse across the blood-gas barrier to enter the lumina of the alveoli.
Hemoglobin also has two types of binding sites for nitric oxide (NO), a neurotransmitter
substance that, when released by endothelial cells of blood vessels, causes relaxation of the
vascular smooth muscle cells with a resultant dilation of the blood vessels. Hemoglobin, Snitrosylated (binding site 1) by nitric oxide manufactured by blood vessels of the lung, ferries
bound nitric oxide to arterioles and metarterioles of the tissues, where NO is released and causes
vasodilation. In this fashion, hemoglobin not only contributes to the modulation of blood
pressure but also facilitates the more efficient exchange of O2 for CO2. Moreover, once O2
leaves the heme portion of hemoglobin to oxygenate the tissues, NO takes its place on the iron
atoms (binding site 2) and is transported into the lungs, where it is released into the alveoli to be
exhaled along with CO2.
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Dr.Hazem……………….……...…….Respiratory system…………………………..2nd stage
Figure (6) A, A respiratory bronchiole, alveolar sac, alveolar pore, and alveoli. B, Interalveolar
septum. C, Carbon dioxide uptake from body tissues by erythrocytes and plasma. D, Carbon
dioxide release by erythrocytes and plasma in the lung.
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