Respiratory System: Structure and Function

Dr Dalia El Marakby RESPIRATORY SYSTEM • • The respiratory system consists of 2 lungs + many tubes. The respiratory system is divided into: I- Conducting portion. I- Conducting Portion: It has two functions: II- Respiratory portion. 1- Conduction of air: 2- Conditioning of inspired air: - provided with cartilage + collagen fibers to: Clean, moist & warm by: * Prevent collapse. -The pseudostratified columnar ciliated with goblet * Ensure continuous supply of air. cells. - provided with elastic fibers + smooth muscles for: -The vascular network, serous & mucous glands in the flexibility and extensibility C.T. under epithelium • It includes nasal cavity, nasopharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles. II -Respiratory Portion: • Functions: It is the site of gas exchange between blood and inspired air. • It includes respiratory bronchioles, alveolar ducts, alveolar sacs, alveoli. 1 I. CONDUCTING PORTION NASAL CAVITIES • The nasal cavity consists of the external vestibule and the internal nasal fossae. 1- The Vestibule 2- The Nasal Fossae * Is the anterior dilated portion of * They are two chambers separated by bony nasal septum. the nasal cavity. * Lined with hairy keratinized * Extending From the lateral wall of each nasal fossa 3 bony shelf-like projections (superior, stratified squamous epithelium, middle and inferior conchae). the epithelium has thick short hairs * The middle and inferior conchae are covered with respiratory epithelium. (vibrissae) that filter out large * The superior conchae are covered with olfactory epithelium. particles from the inspired air. * The lamina propria of the conchae: loose C.T. contains large venous plexuses (swell *Deeper epithelium changed to bodies), mucous & serous glands. non-keratinized stratified * Function: improve the conditioning of the inspired air by increasing the surface area of squamous. respiratory mucosa (epithelium & underlying C.T.). N.B. - Allergic reactions and inflammation can cause abnormal congestion of swell bodies of both nasal fossae, limiting air flow. - Epistaxis: Hemorrhage from the nose, arising in the cavity or in sinuses leading into it. The epithelium that lines most of the conducting portion is pseudostratified columnar ciliated with goblet cells (Respiratory Epithelium). Epithelial cell types: Five cell types are recognized. 2 1- Ciliated columnar cells: 2- Mucous goblet cells: The most abundant cells. *Columnar ciliated cells (300 motile cilia/cell). *The apical cytoplasm contains basal bodies of cilia & many mitochondria which supply energy for ciliary beating. The next most numerous. *Their apical portions are filled with mucin granules. *They push mucus with bacteria & dust in one direction outwards *They secrete mucus that covers the epithelium and traps bacteria & dust particles. 3- Brush cells: 4- Basal cells: 5-Small granule cells: * Columnar cells with abundant apical microvilli. *They have basal afferent nerve. *They act as sensory receptors. * Small, rounded cells on the basal lamina but do not reach the lumen. *They have numerous basal dense granules. *They are stem cells that replace other cell types. *They are neuroendocrine cells belonging to APUD. *Secrete hormones like serotonin and catecholamines that control diameter of airways & blood vessels and regulate mucous and serous secretions. • N.B. In smokers, there is accumulation of mucus in the small airways. This is occurring either due to reversed ratio between ciliated cells and goblet cells or paralysis of the cilia. Olfactory Area • • It covers the roof & superior conchae of the nasal cavities. It is lined by olfactory mucosa= olfactory epithelium + lamina propria of C.T. A- Olfactory epithelium: • It is the neuroepithelium responsible for smell sensation. • It is a modified pseudostratified columnar ciliated epithelium but with no goblet cells. • It is much thicker than respiratory epithelium. Formed of 3 types of cells 3 Dendrite Olfactory cilia Sustentacular cell Olfactory receptor cell Microvilli A B Axon Junctional complex Basal cell Diagram of olfactory epithelium. A. shows details of area squared in B. 1- Olfactory neurons *Bipolar nerve cells. 2- Sustentacular (supporting) cells *Tall columnar cells with wide cylindrical apexes& narrow bases. *The level of their nuclei is between those of supporting *Apical microvilli submerged in a fluid and basal cells. layer. *Their dendrites end in olfactory swelling (olfactory *Cytoplasm contains apical yellow vesicle) with multiple basal bodies, from which arise lipofuscin granules which cilia. gives olfactory mucosa its yellow *Cilia are few, very long, non-motile. They increase color surface area for odorous substance. * Junctional complexes bind * Axons of the cells pass to the lamina propria where supporting cells to olfactory cells. they form the olfactory nerve fibers. 3- Basal cells *Small pyramidal cells. * They have basophilic cytoplasm (free ribosomes) and round nuclei. * They are stem cells for both sensory and supporting cells. 4 B- Lamina propria: • It is a dense C.T that contains: a- Olfactory nerve bundles. b- Bowman’s glands that secrete serous fluid that passes through ducts to the surface. This fluid act as medium around the olfactory cilia where the odorous substances dissolve. N.B. Olfactory neurons are the only neurons that can highly regenerate due to high capacity of stem cells. Differences between Respiratory & Olfactory Mucosa: 1. Epithelial thickness 2. Goblet cells 3. Cilia 4.Basement membrane 5. Bowman’s glands 6. Pigment. Olfactory Mucosa Respiratory Mucosa Thicker Absent olfactory cells have few long non motile cilia Thin Present Thinner Present Has numerous true cilia Thick Absent Present in sustentacular cells Absent PARANASAL SINUSES • They are paired closed cavities in the frontal, maxillary, ethmoid and sphenoid bones of skull. They are lined with thinner respiratory epithelium with few goblet cells. • N.B. Sinusitis is an inflammation of the sinuses. It may be acute or chronic. NASOPHARYNX • • It is the first part of the pharynx. It is lined with respiratory epithelium+ lamina propria contains mucous glands, pharyngeal tonsils & openings of the 2 Eustachian tubes. 5 LARYNX • It connects the pharynx with the trachea.is lined with: 1- Respiratory epithelium except the true vocal cords and lingual surface of epiglottis which are covered with stratified squamous epithelium. 2- Lamina propria contains laryngeal cartilages. Laryngeal cartilages Large cartilages Thyroid, cricoid, and most of the arytenoids hyaline cartilages Calcify with age Small cartilages Epiglottis, cuneiform, corniculate and the tip of the arytenoids elastic cartilages. Don’t calcify Vocal Cords • The vocal cords include two pairs of folds that extend in the lumen of larynx. False vocal cords (vestibular folds) True vocal cords The upper pair The lower pair Respiratory epithelium. Non keratinized stratified squamous epithelium. 1-lining 2- function Prevent food & fluid from entering larynx. Production of voice Functions: 1-Production of voice by the true vocal cords. 2-Prevent food and fluid from entering the respiratory passages by epiglottis & false vocal cords. 3-Maintain open air way by the laryngeal cartilages. TRACHEA • • • It is about 12 cm long. It extends from the larynx to the bifurcation into 2 primary bronchi. Its wall is formed of 4 layers: 6 1-Mucosa: a) Epithelium: Respiratory epithelium (pseudostratified columnar ciliated with goblet cells) b) Lamina propria: loose C.T with elastic fibers. c) Elastic membrane: condensation of elastic fibers. 2- Submucosa: Loose C.T containing lymphoid nodules and mucous & serous glands. 3-Fibrocartilaginous coat: *Dense C.T contains 20 C- shaped (incomplete) rings of hyaline cartilage (keep the trachea always open). *The gaps between the free ends of the rings face the esophagus posteriorly and are bridged by fibroelastic ligament (prevents overdistension) and by smooth trachealis muscle bundles (constrict the lumen and increase the force of air flow during coughing and forced expiration). 4- Adventitia: loose C.T. Muco Epithelium Pseudo.str. colu. Cil with goblet. Lamina propria Elastic membrane Submucosa Tracheal glands Lymphatic nodule Fibro cartilagenous layer 7 Bronchi and Bronchioles * The trachea divides into 2 primary or extrapulmonary bronchi which have the same structure as trachea. *Primary bronchi penetrate the hilum of each lung forming intrapulmonary or secondary bronchi that divide giving rise to bronchioles Differences between Extrapulmonary & Intrapulmonary bronchi Extrapulmonary bronchus Lumen Mucosa: Lining Intrapulmonary bronchus Wider & less folded. Narrower & more folded *Lined with pseudostratified columnar ciliated epithelium with many goblet cells. *The same epithelium with less goblet cells. Submucosa *Separates mucosa from submucosa. Present *Absent but lamina propria is rich in elastic fibers & lymphatic follicles. Absent Cartilage C-shaped (incomplete) rings of hyaline cartilage multiple plates of hyaline cartilage in adventitia Smooth muscles Present posteriorly at the ends of C-shaped cartilage. In the submucosa Arranged spirally & encircle the whole lumen elastic membrane Mucoserous glands and lymphatic nodules In the adventitia in between the cartilage plates Differences between Intrapulmonary bronchi &bronchioles Intrapulmonary bronchus Mucosal epithelium: pseudostratified columnar ciliated epithelium with few goblet cells Musculosa: Less developed spirally arranged smooth muscles. Its contraction→ short and less bronchial diameter Bronchiole Simple columnar or cuboidal ciliated with Clara cells (no goblet cells) Cilia gradually disappear as the bronchioles become smaller. More developed, circularly arranged smooth muscles. Its contraction controls the flow of air to sites of gas exchange. 8 Adventitia: Cartilage plates Mucoserous glands Lymphatic nodules Present Present Absent Absent Present Absent N.B. Bronchitis is inflammation of the mucous membrane of the bronchi which appears congested with swollen mucous glands. Clara cells: -L.M. Dome shaped, non-ciliated cells. -E.M. basal r-ER and apical s-ER, mitochondria, Golgi & glycoprotein granules. -Functions: 1- Secretion of a surfactant like material which reduces surface tension of bronchioles and maintains their patency (sER& golgi). 2-Protection of bronchiolar epithelium via glycoproteins in their granules (rER & Golgi). 3- Protection against emphysema by inhibiting macrophage- secreted proteases & elastase. 4-Degradation of toxins in the inhaled air (sER). 5- Act as stem cells for other types. N.B. Bronchial Asthma is an allergic condition, in which spasm of bronchiolar smooth muscle causes constriction of bronchioles and difficulty during respiration. II. RESPIRATORY PORTION The wall of this part enables the gas exchange. It includes: 9 1.Respiratory bronchioles: 2. Alveolar ducts: *The region of transition between the conducting portion and respiratory portion. * The free terminations of respiratory bronchioles. * Group of adjacent alveoli opens into a common central space. * The structural and functional units of gas exchange. * Open into alveolar sacs, alveolar ducts & respiratory bronchioles. * Lined Completely by openings of alveoli & alveolar epithelium *Lined with alveolar epithelium. * Lined with the alveolar epithelium. *Between their walls there are alveolar pores (of Kahn). These pores equalize pressure between alveoli & provide collateral circulation of air when a bronchiole is obstructed. * Smooth muscles are present at the openings of alveoli. *No muscle. Mucosa: * Epithelium: simple cubical ciliated with Clara cells, cilia absent in distal portion. * C.T. corium: rich in elastic fibers Musculosa: thin layer of smooth muscle fibers. Adventitia: loose C.T. *The wall is interrupted with the openings of some alveoli, where gas exchange occurs. 3. Alveolar sacs: 4. Alveoli: Alveolar epithelium if formed of 2 types of cells: type I & II pneumocytes 10 Type I pneumocvtes (squamous alveolar cells): Type II pneumocytes (great alveolar cells): *Percentage 97% *LM: *Flat squamous cells *Cuboidal cells bulging in air spaces. *Flat nuclei. *Central round nucleus. * Little cytoplasm. * Foamy cytoplasm. * Few organelles around the nucleus. * Rich in mitochondria, ribosomes, rER and *Small pinocytic vesicles (turnover of pulmonary well-developed Golgi body “secretory cells". surfactant. * Membrane bound multilamellar - Tight junctions with type I& II. bodies (cytostomes) *Free surface with short microvilli. *EM: 3% 1-Provide a very thin membrane for gas exchange. 1- Secretion of pulmonary surfactant, 2-Prevent leakage of tissue fluid into the alveolar 2- Stem cells for both types. cavity by tight junction. N.B. Respiratory Distress Syndrome: Deficiency of pulmonary surfactant is associated with premature labour leading to respiratory distress. Interalveolar walls (septa): These are delicate septa separating adjacent alveoli. *Formed of loose C.T. containing: • have the richest capillary network in the body. • For gas exchange • Elastic fibers • to allow expansion of the lung during inspiration. • Reticular fibers • to support lung tissue and prevent over-expansion that may injure the delicate blood capillaries. • Extravasated leucocytes especially monocytes • monocytes act as alveolar phagocytes. *Functions: 11 N.B. Emphysema is abnormal permanent enlargement of the alveoli. It results from the destruction of the inter-alveolar septa by proteases and elastases secreted by lung macrophages (dust cells). It is Elastic fibers mainly caused by cigarette smoking that inhibits the protein responsible Blood-air for destruction of those proteases and elastase. Endothelium barrier Blood-air barrier: • It represents the structures through which gas exchange occurs. • It comprises four layers: 1– Film of pulmonary surfactant on the alveolar surface. 2– Cytoplasm of type I pneumocytes. 3– Fused basal lamina type I pneumocytes and capillary endothelial cells. 4– Cytoplasm of endothelial cells of blood capillaries. Alveolar Phagocytes: Dust cells: Origin Site Function Heart failure cells: Blood monocytes bulging from the interalveolar wall or free inside alveoli. Engulf dust Engulf RBCs in congestive heart failure of blood capillary Type I pneumocyte Blood capillary Reticular fibers Alveolar macrophage Type II pneumocyte Components of the interalveolar wall and blood-air barrier. brick red color Cytoplasm dust particles Hemosiderin granules Vital stain e.g. trypan blue Stain Fate a-Migrate into bronchioles to be coughed in the sputum. b-Exit the lung through lymphatic drainage. c-Remain in the interalveolar septa. 12 Pleura: •Serous membrane which has 2 layers: visceral pleura: covering the lungs parietal pleura: lining thoracic cavity • Both layers are lined by: flat mesothelial cells+ loose C.T. rich in elastic fibers. • The 2 layers are separated by a thin film of fluid to: 1Lubricate the sliding movement. 2Reduce friction between the lung surfaces and the thoracic walls during respiratory movements. Fetal lung: •The lung has no function in fetus, so it is collapsed in the intrauterine life. • It is characterized by: 1– Its lobes and lobules are clear due to presence of thick C.T. septa. 2- The fetal lung is similar to gland (branches of bronchiolar tree are similar to ducts and alveoli are similar to acini). 3-The alveoli are collapsed and lined by simple cubical epithelium. 4-Its bronchi and bronchioles are folded. 5- The cartilage plates are present around the bronchi, and this differentiates fetal lung from a gland. 6- pulmonary blood vessels are congested. 7- Lung sinks in water & this has a medicolegal importance. Structural Changes in the respiratory airway: 1) Epithelial changes: decrease gradual in thickness. 2) The lamina propria decreases gradual in thickness. 3) Cilia and Goblet Cells: decreases gradual. goblet cells are stopped before cilia as the produced mucus should be cleared by the cilia. 4) Serous & mucous glands decreases gradually. 5) Cartilage changes: decrease gradually then disappear. 6) Smooth Muscle changes: In trachea: Trachealis at the free ends of C-shaped cartilage In intra pulmonary bronchi: spirally arranged. In bronchioles: circularly arranged. In alveolar ducts: at the openings of alveoli. In alveolar sacs: completely absent. 13 14