Histology IX Respiratory System PDF

RESPIRATOR Y SYSTEM BLESYLDA TATAD BUSTO, MD  Outline I. Nasal Cavity II. Pharynx III. Larynx IV. Trachea V. Bronchi VI. Bronchioles VII. Alveolar Ducts VIII. Alveoli Respiratory System General Components Respiratory system provides exchange of O2 and CO...

RESPIRATOR Y SYSTEM BLESYLDA TATAD BUSTO, MD  Outline I. Nasal Cavity II. Pharynx III. Larynx IV. Trachea V. Bronchi VI. Bronchioles VII. Alveolar Ducts VIII. Alveoli Respiratory System General Components Respiratory system provides exchange of O2 and CO2 to and from the lungs Structures can be classified anatomically or functionally o ○ Anatomical: upper and lower respiratory tract structures ○ Functional: conducting portion, respiratory portion, and the ventilating mechanism Respiratory System Conducting portion Cleans and dehumidifies air and provides conduit for air Consists of (from exterior to inside of the lungs) nasal cavity, mouth, nasopharynx, pharynx, larynx, trachea, primary bronchi, secondary bronchi (lobar bronchi), tertiary bronchi (segmental bronchi), bronchioles, and terminal bronchioles ○ Luminal diameter of each succeeding tubule continues to decrease, but total cross-sectional diameter of the various branches increases at each level of branching decreasing velocity of inspired air as it proceeds toward respiratory portion Respiratory System Conducting portion A combination of cartilage, elastic and collagen fibers, and smooth muscle provides this with rigid structural support and the necessary flexibility and extensibility to ensure an uninterrupted supply of air Two main functions: ○ To provide a conduit through which air moves to and from the lungs ○ To condition the inspired air by filtering, moistening, and warming it before it enters the lungs Respiratory System Respiratory portion Where gas exchange occurs Consists of respiratory bronchioles, alveolar ducts, and alveoli ○ ALVEOLI – saclike structures that make up the greater part of the lungs; site of gas exchange Respiratory System Ventilating mechanism Creates a pressure difference responsible for inspiration and expiration (ex. Diaphragm, abdominal muscles, elastic CT) Two types of respiration: ○ Internal respiration Between blood and cell Occurs in different parts of the body ○ External respiration between blood and external air Occurs in the lungs between blood capillaries and air sacs RESPIRATORY PORTION RESPIRATORY PORTION Epithelium Respiratory epithelium Lines most of the conducting portion Wall of the respiratory tract is composed of three layers: ○ Tunica mucosa – most superficial layer made up of the epithelium and the lamina propria. Mucus cells and cartilage can be found in the lamina propria ○ Tunica muscularis – layer under then tunica mucosa composed of smooth muscles which regulate luminal diameter ○ Tunica adventitia – deepest layer containing elastic and collagen fibers Epithelium Respiratory epithelium Main type of epithelium in the respiratory tract is ciliated pseudostratified columnar epithelium containing cilia and goblet cells When luminal diameter is decreased, branching occurs From the upper to lower respiratory tract, there is a disappearance of cilia and an appearance of mucus glands Respiratory Epithelial Cell Types Ciliated columnar cells Most abundant Each with about 300 cilia on its apical surface Carry a continuous carpet of secreted mucus toward the pharynx for elimination from the respiratory system Goblet cells Also abundant in some areas of the respiratory epithelium Filled in their apical portions with granules of mucin glycoproteins Secretes approximately 1 liter of mucus per day that entraps particulate matter Respiratory Epithelial Cell Types Brush Cells Much more sparsely scattered and less easily found in columnar cell types Has a small apical surface bearing a tuft of many short blunt microvilli Express some signal transduction components like those of gustatory cells Have afferent nerve endings on their basal surfaces Considered to be chemosensory receptors Respiratory Epithelial Cell Types Small granule cells (Kulchitsky’s cells) Difficult to distinguish in routine preparations Possess numerous dense core granules (100-300 nm in diameter) Like brush cells, they represent about 3% of the total cells Part of the diffuse neuroendocrine system Have the ability to monitor the oxygen and carbon dioxide levels in the lumen of the airway Closely associated with naked sensory nerve ending with which they make synaptic contact and together with these nerve fibers they are referred to as pulmonary neuroepithelial bodies Respiratory Epithelial Cell Types Small granule cells (Kulchitsky’s cells) Contain numerous granules in their basal cytoplasm that houses pharmacological agents such amines, peptides, acetylcholine, and ATP Under hypoxic conditions, these agents are released not only into the synaptic clefts but also into the connective tissue spaces of the lamina propria where they act as paracrine hormones or may enter the vascular supply to act as hormones Respiratory Epithelial Cell Types Basal cells Small rounded cells on the basement membrane Not extending to the luminal surface Stem cells that give rise to the other cell types Epithelium Olfactory epithelium Contain olfactory receptors Specialized region of the mucous membrane covering the superior conchae of the nasal cavity Epithelium - Olfactory Cell Types Olfactory cells Bipolar neurons (chemoreceptors) with a bulbous apical/dendritic projection (olfactory vesicle) from which several modified cilia extend Olfactory cilia (olfactory hairs) – receptors for odor; different axoneme pattern (proximal 1/3 – typical 9 +2 axoneme pattern, distal 2/3 – 9 peripheral singlet microtubules around a central pair of microtubules) Distinguished from supporting cells by the position of their nuclei, which lie between those of the supporting cells and the basal cells Epithelium - Olfactory Cell Types Olfactory cells These receptors respond to odoriferous substances by generating an AP along the basal axons of these neurons, which leave the epithelium and unite in the lamina propria as very small unmyelinated nerves that pass through foramina in the cribriform plate of the ethmoid bone to the brain There, they form cranial nerve I (olfactory nerve) and eventually synapse with other neurons in the olfactory bulb Live for less than three months in a healthy person Epithelium - Olfactory Cell Types Supporting cells (sustentacular cells) Columnar with broad, cylindrical apices and narrower bases and apically located nuclei With microvilli on free surfaces Apical cytoplasm has secretory granules housing a yellow pigment (characteristic of the olfactory mucosa) Display a prominent terminal web of actin microfilament (believed to provide physical support, nourishment, and electrical insulation for the olfactory cells Life span: less than a year Epithelium - Olfactory Cell Types Basal cells Small rounded stem cells that give rise to the other two cell types NASAL CAVITY NASAL CAVITY Air-filled space which receives air from the nostrils or nares Composed of an external, dilated vestibule and an internal nasal cavity further differentiated into a respiratory membrane and an olfactory portion ○ Vestibule Most anterior and dilated portion of the nasal cavity Has small stiff hairs called vibrissae that filter inspired air. Anteriorly lined by nonkeratinized epithelium containing sweat and sebaceous glands and posteriorly lined with respiratory epithelium (first portion of the nasal cavity where epithelial lining becomes nonkeratinized) NASAL CAVITY Dermis is anchored by numerous collagen bundles to the perichondria of the hyaline cartilage segments that form the supporting skeleton of the ala Within the vestibule, the epithelium loses its keratinized nature and undergoes a transition into typical respiratory epithelium before entering the nasal tissue Has a lamina propria that is vascular (many venous plexuses) and contains seromucous glands CONCHAE Three bony shelf-like projections extending from each lateral wall (superior, middle, and inferior nasal concha) ○ The middle and inferior conchae are covered with respiratory epithelium ○ The superior conchae is covered with a specialized olfactory epithelium A meatus (narrow passage) is present in between the conchae to improve the conditioning of the inspired air by increasing the surface area of moist, warm respiratory epithelium and by slowing and increasing turbulence in the airflow causing increased contact between air streams and the mucous layer CONCHAE Swell bodies – large venous plexuses within the lamina propria of the conchae; every 20-30 mins, the swell bodies on one side become temporarily engorged with blood, resulting in distention of the conchal mucosa and a concomitant decrease in the flow of air (during this time, most of the air is directed through the other nasal fossa, allowing the engorged respiratory mucosa to recover from dehydration OLFACTORY PORTION Found in the most superior part of the nasal cavity and lined by olfactory epithelium Also found in the olfactory mucosa is the fila olfactoria (CN I), which transmits olfactory impulses to the brain Lamina propria ○ Glands of Bowman – produce a thin, watery secretion (containing mucin, lysozymes, and IgA) released onto the olfactory epithelium through narrow ducts (odorous substances dissolved in this watery material are detected by olfactory cilia); secretion also flushes the epithelial surface -> prepares the receptors to receive new odorous stimuli OLFACTORY PORTION Respiratory membrane Comprises the rest of the nasal cavity not occupied by the vestibule and the olfactory portion Lined with respiratory epithelium containing cilia and goblet cells PARANASAL SINUSES Bilateral cavities in the frontal, maxillary, ethmoid, and sphenoid bones of the skull Lined with a thinner respiratory epithelium with fewer goblet cells The lamina propria contains only a few small glands and is continuous with the underlying periosteum Communicate with the nasal cavities through small openings Mucus produced in the sinuses is move into the nasal passage by the activity of ciliated epithelial cells Clinical Correlation Smoking ○ affects the respiratory epithelium Immobilization of the cilia squamous metaplasia of the epithelium NASOPHARYNX NASOPHARYNX Continuous with the nasal cavity anteriorly and with the oropharynx posteriorly (begins at the choana and extends to the opening of the larynx) The posterior continuation of the nasal cavities becomes continuous with the oropharynx at the level of the soft palate Lined with respiratory epithelium Mucosa of the nasopharynx contains the medial pharyngeal tonsil and bilateral openings of the auditory tubes connected to the middle ear cavity NASOPHARYNX Three subdivisions: 1. Superior nasopharynx 2. Middle oropharynx (posterior part of the oral cavity) 3. Inferior laryngeal pharynx Oral and laryngeal regions are lined by a stratified squamous epithelium Contains the medial pharyngeal tonsil and the bilateral openings of the auditory tubes to each middle ear NASOPHARYNX Lamina propria composed of loose to dense irregular type of vascularized CT housing seromucous glands and lymphoid elements and is fused with the epimysium of the skeletal muscle components of the pharynx Lamina propria of the posterior aspect of the nasopharynx houses the pharyngeal tonsil, an unencapsulated collection of lymphoid tissue LARYNX LARYNX Situated below the tongue and hyoid bone and between the great blood vessels of the neck Lies at the level of C4 to C6 Opens above into the layngopharynx and continuous below with the trachea Covered in front by the infrahyoid strap muscles and at the sides by the thyroid gland LARYNX Epiglottis - provides a cover over the laryngeal aditus (opening) During respiration – vertical position, permitting the flow of air During swallowing of food, fluids, or saliva – positioned horizontally, closing the laryngeal aditus Yet normally, even in the absence of an epiglottis, swallowed materials bypasses the laryngeal opening has two surfaces: ○ lingual surface – covered with stratified squamous epithelium together with the apical portion of the laryngeal surface ○ laryngeal surface – the epithelium undergoes a transition to ciliated pseudostratified columnar epithelium at variable points on this surface LARYNX In general, the larynx is lined by respiratory epithelium except in: anterior surface and upper half of the posterior surface of the epiglottis, hyoepiglottic folds, and vocal cords lined by nonkeratinized stratified squamous epithelium Lamina propria is composed of elastic fibers, glands, cartilage, and muscle The cartilages serve as attachments for the muscles present in the larynx In addition to maintaining an open airway, movements of the hyaline and elastic cartilages by skeletal muscles participate in sound production during phonation LARYNX Along with the cartilage support, the wall of the larynx is also composed of mucosa and striated muscle layers The mucosa of the larynx extends into two pairs of folds bilaterally into the lumen (below the epiglottis) Vestibular folds or false vocal cords ○ Upper pair and immovable ○ Partly covered with typical respiratory epithelium beneath which lie numerous seromucous glands LARYNX Vocal folds or vocal cords ○ Lower pair of folds ○ Covered with stratified squamous epithelium and contain bundles of parallel elastic fibers (vocal ligament) and large bundles of striated vocalis muscles (assist other intrinsic muscles of the larynx in altering the tension on the vocal folds and also regulate the width of the space between the vocal folds (rima glottides), thus permitting precisely regulated vibrations of their free edges by the exhaled air ○ As expelled air is forced between the folds, variable tension in these vocal cords produces different sounds ○ All structures and spaces in the respiratory tract above the vocal folds are involved in modifying the resonance of the sounds LARYNX Because the larynx of a post pubescent male is large than that of a female, men tend to have deeper voices than women The cilia of the larynx beat toward the pharynx, transporting mucus and trapped particulate matter toward the mouth to be expectorated or swallowed TRACHEA TRACHEA Lined with respiratory epithelium with a hugely vascularized lamina propria Walls of the trachea are reinforced by: ○ C-shaped rings – 10-12 horseshoe-shaped hyaline cartilage rings; the open end of these rings face posteriorly and are connected to each other by smooth muscle, the trachealis muscle; prevents tracheal collapse TRACHEA Walls of the trachea are reinforced by: ○ Perichondrium of each C-ring is connected to the perichondria lying directly above and below it by fibroelastic connective tissue, which provides flexibility to the trachea and permits its elongation during respiration TRACHEAL LAYERS MUCOSA Composed of pseudostratified ciliated columnar (respiratory) epithelium, the subepithelial connective tissue (lamina propria), and a relatively thick bundle of elastic fibers separating the mucosa from the submucosa The lamina propria 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 into the epithelial surface TRACHEAL LAYERS MUCOSA 6 cell types (5 cells of the typical respiratory epithelium + serous cells). ○ Serous cells – about 3%, columnar cells having apical microvilli with apical granules containing an electron dense secretory product TRACHEAL LAYERS SUBMUCOSA Composed of loose irregular fibroelastic connective tissue housing numerous mucous and seromucous glands Short ducts of these glands pierce the elastic lamina and the lamina propria to open onto the epithelial surface contains mucus and serous glands Lymphoid elements are also present Moreover, this region has a rich blood and lymph supply, the smaller branches of which reach the lamina propria TRACHEAL LAYERS ADVENTITIA Composed of fibroelastic connective tissue The most prominent features of the adventitia are the hyaline cartilage C-rings and the intervening fibrous connective tissue Also responsible for anchoring the trachea to the adjacent structures (i.e, esophagus and connective tissues of the neck) Tracheal Wall BRONCHI PRIMARY BRONCHIOLE BRONCHI Lined with respiratory epithelium Supported by plates of hyaline cartilage; number of plates decrease with each level Levels ○ Primary/extrapulmonary bronchi Structure is identical to that of the trachea, except that the primary bronchi are smaller in diameter and their walls are thinner Lined with pseudostratified columnar epithelium with few seromucus glands Part of bronchi before it enters the lung Accompanied by pulmonary arteries, veins, and lymph vessel BRONCHI Levels ○ Secondary/intrapulmonary bronchi similar epithelial lining as primary bronchi Discontinuous plates of hyaline cartilage completely surrounding the lumina of the intrapulmonary bronchi (as compared to the Crings of the primary bronchi) Smooth muscle is located at the interface of fibroelastic lamina propria and submucosa as two distinct smooth muscle layers spiraling in opposite directions ○ Tertiary/segmental bronchi – goes to each lung lobule and lined with simple columnar epithelium BRONCHI Mucosa of the larger bronchi is structurally similar to the tracheal mucosa except for the organization of cartilage and smooth muscle Most cartilage rings in the primary bronchi completely encircle the lumen, but as the bronchial diameter decreases, cartilage rings are gradually replaced with isolated plates of hyaline cartilage Abundant mucous and serous glands are present, with ducts opening into the bronchial lumen Numerous lymphocytes are found both within the lamina propria and among the epithelial cells BRONCHI The bronchial lamina propria contains elastic fibers and a layer of crisscrossing bundles of spirally arranged smooth muscle o ○ 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 Lymphatic nodules are also present and are particularly numerous at the branching points of the bronchial tree The elastic fibers, smooth muscle, and MALT become relatively more abundant as bronchi become smaller and cartilage and other connective tissue are reduced. BRONCHIOLES Intralobular airways Formed after about the tenth generation of dichotomous branching of the bronchial tree Have neither cartilage nor glands in their mucosa Elastic fibers radiate from the fibroelastic connective tissue that surrounds the smooth muscle coats of bronchioles These elastic fibers connect to elastic fibers ramifying from other branches of the bronchial tree During inhalation, as the lung expands in volume, the elastic fibers exert tension on the bronchiolar walls; by pulling uniformly in all directions, the elastic fibers help maintain the patency of the bronchioles BRONCHIOLES The bronchiolar lamina propria is also composed largely of smooth muscle and elastic fibers Lack mucosal glands and cartilage, but contain dense CT Epithelium composed of simple ciliated columnar to cuboidal as size decreases Terminal bronchioles ○ Lined by simple cuboidal ciliated epithelium, with a thin, incomplete layer of smooth muscle. BRONCHIOLES Respiratory bronchioles Similar to terminal bronchioles but with scattered alveoli and fewer muscle fibers First region of the respiratory system where exchange of gases can occur Regions of transition between the conducting and respiratory portions of the respiratory system Lined by simple cuboidal epithelium consisting of Clara cells and some ciliated cells, except where their walls are interrupted by alveoli, the sites where gas exchange occurs Between alveoli, the bronchiolar epithelium consists of ciliated cuboidal epithelium, although cilia may be absent in more distal portions BRONCHIOLES Respiratory bronchioles Smooth muscle and elastic CT lie between the epithelium of respiratory bronchioles But at the rim of the alveolar openings, the bronchiolar epithelium becomes continuous with the squamous alveolar lining cells (type I alveolar cells) As respiratory bronchioles branch, they become narrower in diameter and their population of alveoli increases Each respiratory bronchiole terminates in an alveolar duct BRONCHIOLES The terminal and respiratory bronchioles both contain Clara cells (exocrine bronchiolar cells) ○ They secrete surfactants, detoxify inhaled xenobiotic substances, secrete antimicrobial peptides and replace cells via injury-induced mitosis Larger bronchioles ○ Epithelium is ciliated pseudostratified columnar ○ Goblet cells are present Smaller terminal bronchioles ○ 1-2 mm in diameters ○ Constitute the terminus of the conducting portion of the respiratory system BRONCHIOLES Smaller terminal bronchioles ○ These structures supply air to the lung acini, subdivisions of the lung lobule ○ Epithelium decreases in height and complexity to become ciliated simple columnar or cuboidal epithelium ○ Goblet cells absent ○ Clara cells present As airways progressively decrease in size, several trends are observed, including: ○ A decrease in the amount of cartilage, the numbers of glands and goblet cells, and the height of epithelial cells ○ An increase in smooth muscle and elastic tissue (with respect to the thickness of the wall)  BRONCHIOLES Clara Cells Mitotically active nonciliated exocrine bronchiolar cells (stem cells for the bronchiolar epithelium) With bulging domes of apical cytoplasm containing granules Secrete surfactant components (reduces surface tension and helps prevent collapse of the bronchioles) Have important defensive roles, producing: ○ Enzymes that help break down mucus locally (the P450 enzyme system of their smooth ER detoxifies potentially harmful compounds in air) ○ Secretory component for the transfer of IgA into the bronchiolar lumen BRONCHIOLES Clara Cells Have important defensive roles, producing: ○ Lysozyme and other enzymes active against bacteria and viruses ○ Several cytokines that regulate local inflammatory responses Neuroepithelial bodies Occur in some bronchioles and at higher levels in the bronchial tree Innervated by autonomic and sensory fibers Some of the cells function as chemosensory receptors monitoring air O2 levels Intrapulmonary bronchiole TERTIARY BRONCHIOLE BRONCHIOLE BRONCHIOLE BRONCHIOLE BRONCHIOLE TERMINAL BRONCHIOLE Structure Bronchi Bronchiole Epithelium pseudostratified ciliated columnar simple ciliated columnar Cartilage extrapulmonary – continuous; absent intrapulmonary – discontinuous Goblet cells present few to absent Glands present absent ALVEOLAR DUCTS ALVEOLAR DUCTS Linear passageways continuous with respiratory bronchioles Completely lined by the openings of alveoli Walls consist of adjacent alveoli, which are separated from one another only by an interalveolar septum (slender connective tissue) Proceeding distally along the respiratory bronchioles, the number of alveolar openings in the bronchial wall slowly increases Both the alveolar ducts and the alveoli are lined with extremely attenuated squamous alveolar cells (type I pneumocytes) and type II pneumocytes 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 ALVEOLAR DUCTS A rich matrix of elastic and collagen fibers provides the only support of the duct and its alveoli. Open into the atria of two or more alveolar sacs Elastic and reticular fibers form a network encircling the openings of the atria, alveolar sacs, and alveoli. The opening of each alveolus to the alveolar duct is controlled by a single smooth muscle cell (smooth muscle “knob”), embedded in type III collagen, which forms a delicate sphincter regulating the diameter of the opening Elastic fibers are responsible for nonforced exhalation, thus enabling the alveoli to expand with inspiration and to contract passively with expiration ALVEOLAR DUCTS Reticular fibers serve as a support that prevents overdistension and damage to the capillaries and thin alveolar sacs Both fibers contribute to the CT housing the network of capillaries around each alveolus ALVEOLI ALVEOLI ALVEOLI ALVEOLI Sac-like evaginations from the bronchioles, alveolar ducts, and alveolar sacs; responsible for the spongy structure of the lungs Structurally resemble small pockets that are open on one side, similar to the honeycombs of a beehive Site of gas exchange (exchange of O2 and CO2 between the air and the blood, respectively) Constitute majority of lung parenchyma Air in the alveoli is separated from capillary blood every three components referred to collectively as the respiratory membrane or blood-air barrier O2 from the alveolar air passes into the capillary blood through the blood through the blood-air barrier; CO2 from H2CO3 is catalyzed by the enzyme carbonic anhydrase ALVEOLI Alveolar walls Structurally specialized to enhance diffusion between the external and internal environments Each wall lies between two neighboring alveoli and is therefore called an interalveolar septum Within this septum, densely anastomosing pulmonary capillaries are supported by the meshwork of reticular and elastic fibers, which are the primary structural support of the alveoli Its interstitium also contains macrophages and other leukocytes Has pores that are 10-15 micrometers in diameter which connects neighboring alveoli opening to different bronchioles, equalize air pressure in the alveoli, and promote collateral circulation of air when a bronchiole is obstructed  ALVEOLI CELL TYPES Capillary endothelial cells Extremely thin Easily confused with type I alveolar epithelial cells The endothelial lining of the capillaries is continuous and not fenestrated Clustering of the nuclei and other organelles allows the remaining areas of the cell to become extremely thin, increasing the efficiency of gas exchange The most prominent feature of the cytoplasm is the flattened portions of the cell called pinocytic vesicles ALVEOLI CELL TYPES Type I Pneumocytes Aka alveolar cells Squamous cells forming the walls of the alveoli; major sites of gas exchange Extremely attenuated cells that line 97% of the entire vascular system across which gas exchange occurs Provide a barrier of minimal thickness that is readily permeable to gases Organelles (ER, Golgi apparatus, and mitochondria) grouped around the nucleus Large areas of cytoplasm virtually free of organelles, reducing the thickness of the blood-air barrier ALVEOLI CELL TYPES Type I Pneumocytes The cytoplasm in the thin portion contains pinocytic vesicles, which plays a role in the turnover of surfactant and the removal of small particulate contaminants from the outer surface Contain desmosomes and occluding junctions, which prevents the leakage of tissue fluid into the alveolar space Not capable of division Main role: to provide a barrier of minimal thickness that is readily permeable to gases  ALVEOLI CELL TYPES Type II Pneumocytes aka type II alveolar cells, great alveolar cells, granular pneumocytes, or septal cells Cuboidal cells that release surfactants (phosphatidylcholine and dipalmitoyl lecithin); much fewer than Type I (interspersed among type I alveolar cells) Large rounded cells that often occur in groups of two or three located near septal intersections Rest on the basement membrane and are part of the epithelium, with the same origin as type I cells Exhibit a characteristic vesticular or foamy cytoplasm in histologic sections caused by the presence of lamellar bodies (give rise to pulmonary surfactant) ALVEOLI CELL TYPES Type II Pneumocytes Can divide and regenerate into both types of alveolar pneumocytes Form desmosomes and tight junctions with adjacent cells (either type I or II) Vesicles in cytoplasm are caused by the presence of lamellar bodies (average 1 to 2 mm in diameter, contain concentric or parallel lamellae limited by a unit membrane and where materials for surfactant are collected) these bodies are released at the apical surface of the cells as pulmonary surfactants ALVEOLI CELL TYPES Pulmonary surfactant ○ Structure – consists of phospholipids, glycosaminoglycans, and at least four different proteins forming tubular myelin (a network-like configuration) when it is first released from lamellar bodies. Spreads to produce a monomolecular film composed mainly of dipalmitoyl phosphatidylcholine and phosphatidylglycerol over the alveolar surface, forming a lower aqueous phase and a superficial lipid phase ○ Function – reduces the surface tension of the alveolar surface, permitting the alveoli to expand easily during inspiration and preventing alveolar collapse during expiration ALVEOLI CELL TYPES Alveolar macrophages (aka dust cells) Principal mononuclear phagocytes of the alveolar surface Found in alveoli and in the interalveolar septum Active macrophages are slightly darker due to their content of dust and carbon from air and hemosiderin (complex iron) from erythrocytes Filled macrophages have various fates Migrate to the bronchioles after becoming filled with debris from there, they are carried via ciliary action to the upper airways, eventually reaching the oropharynx, where they are swallowed or expectorated May also exit by migrating into the interstitium and leaving via lymphatic vessels ALVEOLI CELL TYPES Alveolar macrophages (aka dust cells) Phagocytose particulate matter in the lumen of the alveolus as well as in the interalveolar spaces Assist type II pneumocytes in the uptake of surfactant Some reenter the pulmonary interstitium and migrate into lymph vessels to exit the lungs ALVEOLI CELL TYPES Bronchoalveolar fluid Alveolar lining fluids (removed by the conducting passage as a result of ciliary activity ) combined with bronchial mucus Aids in the removal of particulate matter components brought in with inspired for Contains several lytic enzymes (e.g., lysozyme, collagenase, beta-glucuronidase ALVEOLI Regeneration in alveolar lining Inhalation of toxic gases or similar materials can kill type I and type II cells lining pulmonary alveoli Death of type I cells results in increased mitotic activity in the remaining type II cells, the progeny of which become both cell types The normal turnover rate of type Ii cells is estimated to be 1% per day and results in a continuous renewal of both alveolar cells Blood-air barrier (or blood-gas barrier) Components/layers ○ Surfactant and(cytoplasm of) type I pneumocytes \ ○ Fused basal laminae of the closely apposed type I alveolar cells and capillary endothelial cells ○ Cytoplasm of the continuous capillary endothelial cells Structure ○ Thicker regions of the barrier measure about 0.5 micrometers across and have an interstitial area interposed between the two basal laminae which are not fused ○ Thinnest regions of the barrier are 0.2 micrometers or less in thickness and consist of the three layers said above Blood-air barrier (or blood-gas barrier) Function ○ Permits the diffusion of gases between the alveolar airspace and the blood ○ O2 passes from the alveolus into the capillary, and CO2 passes from the capillary blood into the alveolus Capillary endothelial cells Extremely thin and can be easily confused with type I alveolar epithelial cells Continuous and not fenestrated Clustering of the nuclei and other organelles allows the remaining areas of the cell to become extremely thin, increasing the efficiency of gas exchange Most prominent feature of the cytoplasm: pinocytotic vesicles RESPIRATORY PORTION RESPIRATORY PORTION Clinical Correlation Diffuse alveolar damage or adult respiratory distress syndrome injuries to the alveolar epithelial and the capillary endothelial cells Infant respiratory distress syndrome leading cause of death in premature babies, due to incomplete differentiation of type II alveolar cells and a resulting deficit of surfactant and difficulty in expanding the alveoli in breathing. Treatment involves insertion of an endotracheal tube to provide both continuous positive airway pressure (CPAP) and exogenous surfactant, either synthesized chemically or purified from lungs of cattle. PULMONARY VASCULATURE AND VEINS Pulmonary arteries Thin-walled as a result of the low pressures (25 mm Hg systolic and 5 mm Hg diastolic) encountered in the pulmonary circuit Branches within the lung, which accompanies the bronchia tree, with its branches surrounded by adventitia of the bronchi and bronchioles At the level of the alveolar duct, the branches form the capillary network in the interalveolar septum and in close contact with the alveoli The pulmonary artery carries blood to the lungs to be oxygenated Pulmonary veins Venules that originate in the capillary network are found singly in the parenchyma, somewhat removed from the airways, supported by a thin covering of CT After veins leave a lobule, they follow the bronchial tree toward the hilum Nutrient vessels follow the bronchial tree and distribute blood to most of the lung up to the respiratory bronchioles, at which point they anastomose with small branches of the pulmonary artery Lymphatic vessels Originate in the CT of bronchioles Follow the bronchioles, bronchi, and pulmonary vessels and all drain into lymph nodes in the region of the hilum Called the lymphatic deep network to distinguish it from the superficial network of lymphatic vessels in the visceral pleura Both networks drain toward the hilum, either following the entire length of the pleura or after entering the lung tissue via the interlobular septa Clinical Correlation Emphysema chronic lung disease most commonly caused by cigarette smoking involves dilation and permanent enlargement of the bronchioles leading to pulmonary acini and accompanying loss of cells in the alveoli and other parts of the airway walls, leading to an irreversible loss of respiratory function Clinical Correlation Lung cancer Squamous cell carcinoma ○ closely correlated with a history of smoking, ○ arises most often from epithelial cells of segmental bronchi Adenocarcinoma ○ Most common in nonsmokers ○ arises from epithelial cells more peripherally, in bronchioles and alveoli Small cell carcinoma ○ Less common but highly malignant ○ develops after neoplastic transformation of small granule Kulchitsky cells in bronchial respiratory epithelium  Questions? 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