The histology of Respiratory system Anatomy of the Respiratory System The respiratory tract has upper and lower parts. Histologically and functionally, the
respiratory system has a conducting portion, which consists of all the components that condition air and bring it into the lungs, and a respiratory portion, where gas exchange actually occurs, consisting of respiratory bronchioles, alveolar ducts, and alveoli in the lungs.
Anatomy of the Respiratory System The conducting portion serves two main functions: to provide a conduit through which air moves to and from the lungs and to condition the inspired air To ensure an uninterrupted supply of air, a
combination of cartilage, elastic and collagen fibers, and smooth muscle provides the conducting portion with rigid structural support and the necessary flexibility and extensibility. Respiratory Epithelium Most of the conducting portion is lined with
ciliated pseudostratified columnar epithelium known as respiratory epithelium . This epithelium has at least five cell types, all of which touch the thick basement membrane. Ciliated columnar cells are the most abundant, each with about 300 cilia on its apical surface
2- Goblet cells are also abundant in some areas of the respiratory epithelium, filled in their apical portions with granules of mucin glycoproteins 3- Basal cells, small rounded cells on the basement membrane and not extending to the
luminal surface, are stem cells that give rise to the other cell types. Nasal Cavities The left and right nasal cavity each has two components: the external vestibule and the internal nasal cavities. The vestibule is the most anterior and dilated portion of each
nasal cavity. Skin of the nose enters the nares (nostrils) partway up the vestibule and has sweat glands, sebaceous glands, and short coarse vibrissae (hairs) that filter out particulate material from the inspired air. Within the vestibule, the epithelium loses its keratinized nature and undergoes a transition into typical respiratory epithelium before entering the nasal fossae.
Larynx The larynx is a rigid, short passage for air between the pharynx and the trachea. Its wall is reinforced by Hyaline cartilage (in the thyroid, cricoid, and the inferior arytenoid cartilages). Smaller Elastic cartilages (in the epiglottis, cuneiform,
corniculate, and the superior arytenoid cartilages), all connected by ligaments. In addition to maintaining an open airway, movements of these cartilages by skeletal muscles participate in sound production during phonation and the epiglottis serves as a valve to prevent swallowed food or fluid from entering the trachea.
Trachea The trachea is 12-14 cm long and lined with a typical respiratory mucosa . In the lamina propria numerous seromucous glands produce watery mucus and in the submucosa 1620 C-shaped rings of hyaline cartilage keep the tracheal lumen open .
The open ends of the cartilage rings are on the posterior surface, against the esophagus, and are bridged by a bundle of smooth muscle (trachealis muscle) and a sheet of fibroelastic tissue attached to the perichondrium. The entire organ is surrounded by adventitia. Trachea
The wall of the trachea is lined by typical respiratory epithelium (E) underlain by connective tissue (CT) and seromucous glands (G) in the lamina propria. The submucosa contains C-shaped rings of hyaline cartilage (C) covered by perichondrium (P). Bronchial Tree & Lung
Bronchial Tree & Lung The trachea divides into two primary bronchi that enter the lungs at the hilum, along with arteries, veins, and lymphatic vessels. After entering the lungs, the primary bronchi course downward and outward, giving rise to three secondary (lobar) bronchi in the right lung and two in the left lung. These lobar bronchi again divide, forming tertiary (segmental) bronchi.
Each of these tertiary bronchi, together with the smaller branches it supplies, constitutes a bronchopulmonary segmentapproximately 10 12% of each lung with its own connective tissue capsule and blood supply. The existence of such lung segments facilitates the specific surgical resection of diseased lung tissue without affecting nearby healthy tissue.
Bronchi Each primary bronchus branches repeatedly, with each branch becoming progressively smaller until it reaches a diameter of about 5 mm. The mucosa of the larger bronchi is structurally similar to the tracheal mucosa except for the organization of cartilage and smooth muscle.
In the primary bronchi most cartilage rings completely encircle the lumen, but as the bronchial diameter decreases, cartilage rings are gradually replaced with isolated plates of hyaline cartilage. Bronchi Abundant mucous and serous glands are also
present, with ducts opening into the bronchial lumen. In the bronchial lamina propria is a layer of crisscrossing bundles of spirally arranged smooth muscle , which become more prominent in the smaller bronchial branches. Contraction of this muscle layer is responsible for the folded appearance of the bronchial mucosa
observed in histologic section. Tertiary (segmental) bronchus. Bronchioles Bronchioles are the intralobular airways with diameters of 5 mm or less, formed after about the tenth generation of branching, and have
neither cartilage nor glands in their mucosa . In the larger bronchioles, the epithelium is still ciliated pseudostratified columnar, but this decreases in height and complexity to become ciliated simple columnar or cuboidal epithelium in the smaller terminal bronchioles. Goblet cells disappear during this transition, but the epithelium of terminal bronchioles instead contains other numerous columnar cells:
the exocrine bronchiolar cells, commonly called Clara cells . These mitotically active cells secrete surfactant components and have various important defensive roles. Respiratory Bronchioles Each terminal bronchiole subdivides into two or more respiratory bronchioles that serve as regions of transition between the
conducting and respiratory portions of the respiratory system . The respiratory bronchiolar mucosa is structurally identical to that of the terminal bronchioles, except that their walls are interrupted by the openings to saclike alveoli where gas exchange occurs. Portions of the respiratory bronchioles are lined with ciliated cuboidal epithelial cells and Clara cells, but at the rim of the alveolar openings the bronchiolar epithelium becomes continuous
with the squamous alveolar lining cells. Diagram shows the branching relationship, as well as the pulmonary blood vessels that travel with the bronchioles and the dense layer of branching capillaries that surrounds each alveolus for gas exchange between blood and air.
The micrograph shows the branching nature of the bronchioles in two dimensions. SEM shows in three dimensions the relationship of alveoli to terminal and respiratory bronchioles Alveolar Ducts
Respiratory bronchioles branch into tubes called alveolar ducts. Both the alveolar ducts and the alveoli are lined with squamous alveolar cells. In the lamina propria surrounding the rim of the alveoli is a thin network of smooth muscle cells, which disappears at the distal ends of alveolar ducts.
A rich matrix of elastic and collagen fibers provides the only support of the duct and its alveoli. Alveolar Ducts Alveoli Alveoli are saclike evaginations of the respiratory bronchioles, alveolar
ducts, and alveolar sacs. Alveoli are responsible for the spongy structure of the lungs. Structurally, alveoli resemble small pockets that are open on one side, similar to the honeycombs of a beehive. Within these cuplike structures, O2 and CO2 are exchanged between the air and the blood. The structure of alveolar walls is specialized to enhance diffusion between the
external and internal environments. Generally, each wall lies between two neighboring alveoli and is therefore called an interalveolar septum Alveoli Air in the alveoli is separated from capillary blood by three components referred to collectively as the
respiratory membrane or blood-air barrier: 1- Surface lining and cytoplasm of the alveolar cells. 2- Fused basal laminae of the closely apposed alveolar cells and capillary endothelial cells. 3- Cytoplasm of the endothelial cells Alveolar walls
Ultrastructure of the blood-air barrier Type I alveolar cells Type I cells cover 97% of the alveolar surface . Organelles such as the ER, Golgi apparatus, and mitochondria are grouped around the nucleus, leaving large areas of cytoplasm virtually
free of organelles and reducing the thickness of the blood-air barrier. The cytoplasm in the thin portion contains pinocytotic vesicles, which may play a role in the turnover of surfactant and the removal of small particulate contaminants from the outer surface. In addition to desmosomes, all type I epithelial cells have occluding junctions that prevent the leakage of tissue fluid into the alveolar air space .
The main role of these cells is to provide a barrier of minimal thickness that is readily permeable to gases. Type II alveolar cells Type II cells are rounded cells that often occur in groups of two or three along the alveolar surface at points where the alveolar walls unite.
These cells rest on the basement membrane and are part of the epithelium, with the same origin as the type I cells that line the alveolar walls. They divide by mitosis to replace their own population and also the type I population. Ultrastructure of type II alveolar cells
Alveolar macrophages Also called dust cells, are found in alveoli and in the interalveolar septum . Tens of millions of monocytes migrate daily from the microvasculature into the lung tissue, where they phagocytose erythrocytes lost from damaged
capillaries and air-borne particulate matter that has entered alveoli . THANK YOU
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