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The Respiratory System Lecture 1 The Respiratory System The complex of organs and tissues which are necessary to exchange blood carbon dioxide (CO2) with air oxygen (O2) is called " respiratory system". It consists of: 1. Conductive portion (upper respiratory tract): formed of a series of transport passages by which air pass from the atmosphere to the lung. It consist of (nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles & terminal bronchioles). 2. Respiratory portion (lower respiratory system): which is responsible for gaseous exchange consist of (respiratory bronchioles, alveolar ducts & alveoli). The alveoli are specialized sac like structures forming the greater part of the lung & they are the main sites for exchange of O2 & CO2 between the air & the blood. 1 The Respiratory System Lecture 1 The conducting portion of the respiratory system has many functions: 1. It is a system of channels through which air will reach the lungs and to ensure adequate & continuous supply of air to both lungs a combination of cartilages, elastic fibers, collagen fibers & smooth muscle fibers provides the conducting part with rigid structural support and with the necessary flexibility and extendibility. 2. While passing through the conducting part the air will be cleaned (by removal of foreign particles), moistened & warmed, to carry out these functions, the mucosa of the conducting part is lined with a specialized epithelium called respiratory epithelium. 3. In addition the proximal part of the respiratory tract has a specialized structures that are involved in the perception of smell & flavor (Olfactory mucosa) & the production of sound (Larynx). Cartilages in the respiratory system: Cartilages are mainly hyaline in type, found in the lamina propria, they have various forms ranging from small plaques in the bronchi to irregular rings & in the trachea C-shaped cartilages. The cartilages generally produce support to the wall of the conducting part to prevent collapse of the lumen during expiration & to ensure continuous air entry to the lungs. Elasic fibers in the respiratory system: found in both conducting and respiratory parts to provide these areas with flexibility and to allow them to return to their original shape after 2 The Respiratory System Lecture 1 inspiration.The concentration of elastic fibers is inversely proportionate to the diameter of the conducting tubule (ie. the smallest bronchioles have the highest amount of elastic fibers in their lamina propria). Smooth muscles in the respiratory system: bundles of smooth muscles encircle the tube from the trachea to the alveolar ducts. Contraction of those smooth muscles reduces the diameter of the conducting tubules to regulate the air flow during inspiration and expiration. The conducting part of the respiratory system gradually undergoes transition into the respiratory part. The ciliated epithelium, goblet cells & cartilages will gradually decrease in the wall of the system while the content of smooth muscles & elastic fibers gradually increase. Respiratory Epithelium: Most of the conducting portion of the respiratory system is lined with pseudostratified ciliated columnar epithelium that contains large number of mucous secreting cells called goblet cells. As the bronchi subdivide into bronchioles some structural changes occure are: 3 The Respiratory System Lecture 1 pseudostratified epithelium will change to simple columnar epithelium which is further reduced to simple cuboidal epithelium in the terminal bronchioles. goblet cells gradually decrease in number in the smaller bronchi & are completely absent from the epithelium in the terminal bronchioles. ciliated cells still present in the terminal bronchioles without the goblet cells in order to prevent accumulation of mucous in the respiratory portion of the system because the cilia help to move this mucous upward toward the mouth where it is either swallowed or expectorated. Typical respiratory epithelium: consists of five cell types: 1. Ciliated columnar cells: most numerous type of cells, each cell has about 300 cilia in its apical surface, called immotile cilia (due to deficiency of the protein Dynein) those cells become shorter with successive branching of the bronchial tree until they become cuboidal in the most peripheral branches. 2. Mucous secreting goblet cells: the second most numerous type of cells, they are scattered in between the ciliated columnar cells , their cytoplasm is filled with mucous droplets and their number gradually decrease until they are absent in the terminal bronchioles, they increase in some chronic respiratory diseases. 3. Basal cells: small rounded cells lie on the basement membrane, they are not in contact with the lumen, they form stem cells from which other cell types develop. 4 The Respiratory System Lecture 1 4. Brush cells: columnar cells with many microvilli on their apical surface, they have afferent nerve endings on their basal surface & are considered to be sensory receptors. 5. Neuroendocrine cells: small rounded cells with dark nuclei & cytoplasm filled with endocrine granules that regulate the interaction between mucous and serous secretory functions of the system. In smokers the proportion of ciliated cells to goblet cells is altered so greater number of goblet cells are found to aid in clearing the increased amount of particulates & gaseous pollutants in their respiratory epithelium, while the reduction in ciliated cells caused by excessive intake of CO resulting in decreased movement of the mucous layer leading to congestion of the smoker’s airways. Nasal Cavity: The Nasal cavity is divided into three structurally and functionally different parts. 1. Vestibule: is the most anterior dilated part of the nasal cavity forming the first ~1.5 cm of the conductive portion following the nostrils which are lined with a keratinized stratified squamous epithelium, numerous sebaceous glands & thick short hairs, within the vestibule the epithelium loses its keratinization and undergoes gradual transition into typical respiratory epithelium. 2. Respiratory region: at the transition from the vestibule to the respiratory region of the nasal cavity the epithelium becomes first non keratinized stratified squamous and then pseudostratified ciliated columnar with goblet cells (respiratory epithelium). The 5 The Respiratory System Lecture 1 surface of the lateral parts of the nasal cavity is thrown into folds by bony projections called conchae. These folds increase the surface area of the nasal cavity and they create a turbulence in the stream of the passing air, both functions will facilitate the conditioning (warming, cooling and filtration) of the air. The lamina propria contains mucous and serous glands which supplement the secretions of the goblet cells with a large venous plexuses known as swell bodies, they are important for conditioning of air thus abnormal enlargement of swell bodies with blood occurs in allergic reaction & inflammation so they restrict air flow to the air passages. 3. olfactory region formed by tissues on the superior concha and the nasal septum of the nasal cavity, it is lined with a special type of epithelium called Olfactory mucosa. Olfactory mucosa: Is a special type of mucosa, located in the roof of the nasal cavity & extend for a short distance down the septum and lateral wall it is 6 The Respiratory System Lecture 1 responsible for the sensation of smell & the more sophisticated epithelium aspect of taste. The Olfactory mucosa has a pseudostratified columnar epithelium that is composed of three types of cells which are: 1. Basal cells: are small cone shape cells rest on the basement membrane not in contact with the lumen their nuclei are close to the basement membrane, these cells are regarded as the stem cells from which new olfactory cells can develop. 2. Sustentacular cells (supporting cells): are tall cells their nuclei are closest to the lumen & they have a narrow base in contact with the basement membrane & microvilli on their free surface, they give mechanical & metabolic support to the basal & olfactory cells. 3. Olfactory receptor cells: are bipolar neurons found in between the basal & sustentacular cells they have a central bulge contain the nucleus & from the cell extend 2 processes: a) Dendritic process: extend to the surface epithelium where its tip expand into a club-shaped prominence called Olfactory vesicle which bears cilia that protrude into the nasal cavity these cilia are sensitive to odorous substances. b) Proximal process: very narrow & pass between the basal & sustentacular cells to penetrate the basement membrane & join the other proximal processes of other cells to form what is called fila olfactoria which will then form a synaptic connection with the olfactory bulb to form the first cranial nerve (Olfactory nerve). 7 The Respiratory System Lecture 1 Beneath the epithelium of the olfactory mucosa the connective tissue of the lamina propria contain small serous glands called Bowman's glands their secretion may act as a solvent for the odorous substances, this connective tissue is highly vascular and contain the fila olfactoria. Paranasal sinuses: Are closed cavities in the frontal, maxillary, ethmoid & sphenoid bones. They are lined with thin respiratory epithelium that contains few goblet cells. Their lamina propria contains only few small serous and mucous glands. Those sinuses communicate with the nasal cavity through small openings & the mucous produced in the sinuses is drained into the nasal cavity by the action of the ciliated epithelial cells, these sinuses help to increase the surface area for moistening & warming the inhaled air, also the sinus cavities play a role in the nature of the sounds produced.Sinusitis is an inflammatory process in the sinuses that may persist for long period occur due to obstruction of the sinus openings. 8 The Respiratory System Lecture 1 Pharynx: The pharynx connects the nasal cavity with the larynx. Depending on the extent of abrasive forces on the epithelium, the pharynx is either lined with respiratory epithelium (nasopharynx) or with a stratified squamous epithelium (oropharynx) which also covers the surfaces of the oral cavity and the oesophagus.Lymphocytes frequently accumulate beneath the epithelium of the pharynx.Accumulations of lymphoid tissues surrounding the openings of the digestive and respiratory passages form the tonsils Larynx: The larynx connects the pharynx and trachea. The vocal folds of the larynx control airflow and allow the production of sound. The vocal folds are lined by stratified squamous epithelium and contain the muscle (striated, skeletal) and ligaments needed to control the tension of the vocal folds. 9