Trachea:
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Is a thin walled tube about 10 cm long & 2-3 cm. in diameter its wall contain about 15-20 incomplete circular rings of hyaline cartilage, a small narrow area in the posterior wall of the trachea is devoid of cartilage, this gab is bridged by dense fibrous ligament & bundles of smooth muscle called trachealis muscle, contraction of this muscle will produce narrowing of the tracheal lumen as a part of the cough reflex, this narrowing leads to increased velocity of expired air which aids in the cleaning of the air passges. The trachea bifurcates to give rise to the main bronchi, their histological structure corresponds largely to that of the trachea. The trachea is lined by:
typical respiratory epithelium (pseudostratified ciliated columnar cells with goblet cells) the number of goblet cells is variable depending on physical or chemical irritation of the epithelium which will increase the goblet cell number.
lamina propria consists of loose connective tissue with many elastic fibers.subepithelial sero-mucous tracheal glands supplement the secretions of goblet cells in the epithelium, these glands are particularly numerous in the posterior band of the trachea devoid of cartilage.
hyaline cartilage surrounded by perichodrium present in the submucosa.
adventitia containing blood vessels, lymphatics & adipose tissue.
Bronchial tree:
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The trachea divide into 2 main bronchi (called right & left primary bronchi) that enter the lung through the hilum and divide to give secondary bronchi (lobar bronchi) one for each lobe of the lung. These lobar bronchi divide repeatedly giving rise to smaller & smaller bronchi whose terminal branches are called bronchioles & each bronchiole then divide to form 5-7 terminal bronchioles. Bronchial branches are accompanied by branches of the pulmonary artery, nerves and lymph vessels. Throughout their coarse the bronchi have a similar structure to that of the trachea but with few variations only.
Bronchi:
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The primary bronchi generally have the same histological appearance as the trachea so the basic structure of the bronchi is:
a pseudostratified columnar ciliated epithelium with goblet cells, this epithelium become less columnar in the smaller branches, the epithelium contains columnar ciliated cells, goblet cells, basal cells, brush cells & neuroendocrine cells.
a subepithelial fibrocollagenous lamina propria containing variable quantities of seromucous glands which empty their secretions into the lumen by short ducts. Other deeper glands with long ducts are located in between & beneath the cartilaginous plates of the bronchi.
the lamina propria contains variable amounts of smooth muscle fibers and elastic fibers that are arranged spirally around the bronchus.
neumerous lymphocytes are present in the lamina propria & lymphatic nodules are found mainly at the branching points of the bronchial tree.
the bronchial cartilages become more irregular than that found in the trachea, in the smaller bronchi the cartilaginous rings will incompletely encircle the lumen, as the bronchial diameter decrease the cartilagenious rings are replaced by isolated plates of hyaline cartilage.
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Bronchioles
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Are the smallest parts of the conducting portion of the respiratory system, their diameter is 5 mm. or less. They have no cartilage & no glands in their walls, and with absence of the cartilage the smooth muscle fibers become the major component of their wall.
The larger bronchioles are lined with ciliated columnar epithelium, these cells become low columnar and even cuboidal in the smaller terminal bronchioles.
Goblet cells are only found occasionally in the wall of the bronchioles and there are no seromucous glands.
A special type of cells found in the bronchiolar wall called Clara cell which are more numerous in the terminal bronchioles.These cells are non-ciliated, have many secretory granules in their cytoplasm with many mitochondria and smooth endoplasmic reticulum near the luminal surface of the cell that bulge above the level of adjacent ciliated cells, they have many functions:
protection against the effect of inhaled toxins & carcinogenous substances.
play a role in surfactant production.
possibly act as stem cells capable of producing other types of cells.
The lamina propria of the bronchioles composed mainly of smooth muscle and elastic fibers which are under the effect of both parasympathetic (by Vagus nerve) and sympathetic innervations, thus stimulation of Vagus nerve produces contraction of these muscle & leads to decrease in the bronchiolar diameter, while the stimulation of the sympathetic nervous system produces the opposite effect.
The final bifurcation of the bronchiole leads to the formation of terminal bronchioles which are the smallest bronchioles concerned only with air conduction.
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Distal respiratory tract :
Respiratory Bronchioles:
The first part of the distal respiratory tract which is concerned mainly with the gaseous exchange, each terminal bronchiole divides into 2 or more respiratory bronchioles which is considered as a region of transition between the conducting part and the respiratory part of the system.
The mucosa of the respiratory bronchioles is structurally identical to that of the terminal bronchioles (low cuboidal cells plus Clara cells) except that their walls are interrupted by numerous sac-like alveoli where gaseous exchange occur.
The wall of the respiratory bronchiole is lined with ciliated cuboidal epithelial cells and Clara cells, but near the opening of the alveoli this epithelium become continuous with the cells that line the alveolar wall (i.e squamous alveolar lining cells- type I alveolar cells)
Beneath the epithelium the connective tissues of the respiratory bronchiolar wall contain smooth muscles & elastic fibers.
Proceeding distally along the respiratory bronchioles, the alveoli increase greatly in number & the distance between them is markedly reduced.
Alveolar ducts:
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Proceeding distally along the respiratory bronchioles, the number of alveolar openings into the bronchiolar wall become more and more until the wall consist only of the openings of the alveoli, and this tube is now called alveolar duct.Both the alveolar ducts and alveoli are lined with extremely thin squamous alveolar cells. The lamina propria is only formed of a thin network of smooth muscle cells and it will appear as knobs between adjacent alveoli, these alveolar ducts will lead to the alveolar sacs.
Elastic and reticular fibers form a complex network of fibers that surround the alveolar sacs and alveoli. The elastic fibers enable the alveoli to expand with inspiration & contract passively with expiration, while the reticular fibers serve as a support that prevent over distention & damage to the delicate capillaries & thin inter-alveolar septa.
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