07_Respiratory

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Respiratory System MEDL 102: Histology - Theory Medical Laboratory Technology Continuing Competencies Medical Laboratory Technology Post Graduate Refresher

1 MEDL 102: Histology - Theory, 07_Respiratory System Rev. Sep/20 © 2019, Southern Alberta Institute of Technology Respiratory System Rationale All body systems depend on the respiratory system for the transportation of oxygen and carbon dioxide to and from body cells. There are a number of diseases and infections that can affect the respiratory system, consequently these tissues are commonly received in the Histopathology Laboratory. Learning Outcome Differentiate the function and microscopic appearance of the respiratory system components. Introduction The respiratory system consists of the paired lungs, which provide an expansive interface for gas exchange between air and blood, and a branching system of airways for conducting air to and from the lungs. Within the lung the air passages branch into increasingly smaller tubes until the smallest air spaces are reached, called alveoli. The principle functions of the respiratory system are: air conduction, air filtration, and gas exchange (respiration). This module describes the function and histological features of the respiratory system, which are important for microscopic identification.

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2 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology Describe the function and microanatomical features of the conducting portion of the respiratory system. The Respiratory System The respiratory system has two functional components, a conducting system responsible for supplying and cleaning the air, and a respiratory portion, that provides an interface for the exchange of gases between the atmosphere and blood. The Conducting Portion The conducting system begins essentially as a single tube which divides repeatedly to form airways of ever decreasing diameter. The nasal cavities, sinuses and nasopharynx clean, warm and moisten the air. The main conducting airways, the trachea and bronchi, have walls that are reinforced with either bone or cartilage to help keep them open. Their mucosal lining consists of pseudostratified columnar ciliated epithelium with goblet cells, which is adapted to cleaning and conditioning the air on its way into the lungs. The incoming air passes through the conducting portion, which consists of the nasal cavities, nasopharynx, larynx, trachea, bronchi, and bronchioles. From here, it enters into the respiratory portion, which consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. The portions of the conducting system which we will discuss microanatomically are the bronchi and the bronchioles. a. Bronchi The bronchi are typically greater than 1 mm in diameter and are lined with respiratory epithelium with goblet cells. The supporting lamina propria contains a large amount of elastin. It is separated from the submucosa by a discontinuous layer of smooth muscle. The submucosa contains mucoserous glands. The cartilage is now comprised of plates or islands of hyaline cartilage. These are all supported by a connective tissue adventitia. b. Bronchioles These are airways of less than one millimeter in diameter and have little or no cartilaginous support. The epithelium is simple, columnar, and ciliated and may also contain a few goblet cells. Goblet cells and cartilage are absent beyond the terminal bronchioles. The smooth muscle layer is the most prominent feature of the bronchiole and circles about the diameter of the bronchiole. Small lymphoid aggregates may be found in the adventitia as part of MALT or mucosa- associated lymphoid tissue.

3 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology Describe the function and microanatomical features of the respiratory portion of the respiratory system. Activity: 1. Complete the Self-Test and check your answers at the end of the module. The Respiratory Portion This is the portion of the respiratory system where gaseous exchange occurs. The respiratory portion consists of respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli . These components do not have much in the way of wall structure, so instead of tubular walls, what is seen is air spaces of varying size and shapes. The only bronchiole that exists here, with any kind of wall structure is the respiratory bronchiole. Respiratory bronchioles Their walls contain a small number of single alveoli. The epithelium is devoid of goblet cells and consists of ciliated, cuboidal cells and smaller numbers of non-ciliated cells called Clara cells . Clara cells are thought to produce a component of surfactant, act as stem cells, and contain an enzyme system which can detoxify noxious substances. Each respiratory bronchiole divides into alveolar ducts and these open along their length into alveolar sacs and alveoli. Alveolar ducts, sacs, and alveoli In tissue sections, all that can be seen of the walls of the alveolar ducts are small aggregations of smooth muscle cells and associated collagen and elastic fibres which form rings surrounding the alveolar ducts and the openings of the alveolar sacs and alveoli. The smooth muscle of the respiratory bronchioles and the alveolar ducts regulates alveolar air movements. The alveolar ducts lead into alveolar sacs, distended spaces each of which gives rise to several alveoli. Each alveolus consists of a pocket, open at one side and lined by extremely flattened epithelial cells. Surrounding each alveolus is a rich network of pulmonary capillaries supplied by pulmonary vessels which follow the general course of the airways. Between each adjacent alveolus, the wall or alveolar septum consists of the flattened epithelial lining cells of each alveolus separated by capillaries and delicate reticular and elastic supporting elements. The alveolar septa contain small openings called alveolar pores which are thought to enable equalization of pressure between alveoli. In general, the alveolar wall consists of three tissue components: surface epithelium, connective tissue, and blood vessels.

4 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology 1. Epithelium It forms a continuous alveolar lining and consists of cells of two types. Most of the alveolar surface area is covered by large, extremely flattened cells called Type I pneumocytes. Type I cells are thought to function in gaseous exchange. The Type II pneumocytes are in greater numbers in the lining epithelium. Type II cells are thought to secrete a surface-active material called surfactant which reduces surface tension within the alveoli. 2. Connective tissue It forms an attenuated, supporting layer beneath the epithelium and surrounding the blood vessels of the alveolar wall. This layer primarily consists of fine reticular, collagenous, and elastic fibers with few fibroblasts. 3. Blood vessels Mainly capillaries form a rich bed around each alveolus. In most of the alveolar wall, the basement membrane which supports the capillary endothelium is directly applied to the basement membrane supporting the surface epithelium; in such sites the two basement membranes are fused and the connective tissue layer is absent. This arrangement provides an interface of minimal thickness between alveolar air and blood. Although the defense mechanisms of the conducting passages filter most particulate matter from inspired air, small particles such as carbon reach the alveoli and are engulfed by phagocytic cells found in the alveolar wall or free in the alveolar space. These phagocytes, known as alveolar macrophages or dust cells , are derived from circulating blood monocytes and are usually recognizable by their content of engulfed, particulate material.

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5 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology Module Self-Test Directions: • Answer the following questions. • Compare your answers to the enclosed answer key. • If you disagree with any of the answers, review learning activities and/or check with your instructor. • If no problems arise, continue on to the next objective or next examination. 1. List the two components of the respiratory system and their function. i. ii. Select the best answer for the following five multiple choice questions. 2. Which of the following is located in the respiratory portion of the respiratory system? a. Alveolar ducts b. Terminal bronchioles c. Nasal Cavity d. Trachea 3. Which of the following is located in the conducting portion of the respiratory system? a. Respiratory bronchioles b. Alveoli c. Alveolar ducts d. Bronchi 4. What is the correct order in which air passes through the respiratory system on inhalation? a. Alveoli, alveolar duct, bronchiole, bronchi, trachea b. Nasopharynx, bronchiole, bronchi, alveolar duct, alveoli c. Nasal cavity, bronchi, bronchiole, alveolar sac, alveoli d. Trachea, Nasopharynx, bronchiole, alveolar sac, alveoli 5. What type of epithelium makes up the mucosa of the main conducting airways? a. Simple squamous b. Simple cuboidal c. Pseudostratified columnar ciliated d. Simple columnar ciliated

6 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology 6. What type of specialized cell is found in the conducting portion of the respiratory system? a. Clara cell b. Goblet cell c. Dust cell d. Pneumocyte 7. Complete the following sentences by filling in the blanks. The trachea and bronchi are reinforced with either or to keep them . Small lymphoid aggregations may be found in the of the conducting portion as part of . 8. Indicate whether the following statements are referring to a bronchi or bronchiole of the conduction portion. Airways are less than 1mm in diameter. The submucosa contains mucoserous glands. The mucosa consists of pseudostratified columnar ciliated and goblet cells. Smooth muscle layer circles the entire diameter. The lamina propria contains large amounts of elastin. The epithelium is simple columnar ciliated. Select the best answer for the following five multiple choice questions. 9. Which of the following does NOT describe the epithelium of respiratory bronchioles? a. A small number of alveoli may be present. b. Numerous goblet cells are present. c. It is ciliated cuboidal. d. There are a small number of non-ciliated Clara cells. 10. What regulates alveolar air movement in the respiratory bronchioles and alveolar ducts? a. Elastin b. Skeletal muscle c. Cartilage d. Smooth muscle 11. What are the main tissue components of the alveolar wall? a. Smooth muscle, surface epithelium, blood vessels b. Surface epithelium, blood vessels, connective tissue c. Blood vessels, connective tissue, smooth muscle d. Connective tissue, smooth muscle, surface epithelium

7 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology 12. Fill in the blanks to complete the following sentences. The walls or, , of the alveoli consist of the epithelial lining cells of each alveolus separated by and delicate and support tissue. The alveolar septa contain small openings called which are thought to between alveoli. 13. Match the cell type with the statement which describes it. P1 – Type I pneumocyte P2 – Type II pneumocyte D – Dust cell C – Clara cell Most numerous cell in the alveolar lining. Extremely flattened cell which functions in gaseous exchange. Known as the alveolar macrophage. Secretes surfactant which reduces surface tension. Covers most of the alveolar surface.

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8 MEDL 102: Histology - Theory, 07_Respiratory System © 2019, Southern Alberta Institute of Technology Module Self-Test Answers 1. i. Conducting portion responsible for supplying and conditioning and cleaning air ii. Respiratory portion provides an interface for gaseous exchange 2. a 3. d 4. c 5. c 6. b 7. The trachea and bronchi are reinforced with either bone or cartilage to keep them open. Small lymphoid aggregations may be found in the adventitia of the conducting portion as part of MALT (mucosa-associated lymphoid tissue). 8. Bronchiole Airways are less than 1mm in diameter. Bronchi The submucosa contains mucoserous glands. Bronchi The mucosa consists of pseudostratified columnar ciliated and goblet cells. Bronchiole Smooth muscle layer circles the entire diameter. Bronchi The lamina propria contains large amounts of elastin. Bronchiole The epithelium is simple columnar ciliated. 9. b 10. .d 11. .b 12. The walls or, alveolar septum, of the alveoli consist of the flattened epithelial lining cells of each alveolus separated by capillaries and delicate reticular and elastic support tissue. The alveolar septa contain small openings called alveolar pores which are thought to equalize pressure between alveoli. 13. P2 Most numerous cell in the alveolar lining. P1 Extremely flattened cell which functions in gaseous exchange. D Known as the alveolar macrophage. P2 Secretes surfactant which reduces surface tension. P1 Covers most of the alveolar surface.

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