Respiratory System Lab

Learning Objectives

  • Describe the changes in the type of epithelium throughout the respiratory system
  • Explain how the structure of different segments of the respiratory airways reflect the functional roles that these airways play in air movement and gas exchange
  • Distinguish the trachea, bronchi, terminal bronchioles, bronchioles, alveolar ducts, alveolar sacs, and alveoli based on key structural features
  • Identify the different types of pneumocytes and their functions
  • Recognize key pathological conditions associated with the respiratory tract


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Pre-Lab Reading


The respiratory system consists of two divisions with distinct structural elements that reflect their unique functions. These include:

  • The conducting airways, which serve to conduct, clean, warm, and moisten the air. This portion is composed of the nose, pharynx, larynx, trachea, bronchi, and bronchioles.
  • The respiratory airways, which facilitate gas exchange. These are located entirely within the lung and are represented by respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Conducting Airways

The epithelium lining the respiratory tract from the nasal fossa through the bronchi is called the respiratory mucosa and is characterized by a pseudostratified ciliated epithelium with abundant non-ciliated cells known as goblet cells. In the lamina propria there are mixed seromucous (protein- and mucous-secreting) glands, lymphatic tissue, and broad veins.

The conducting airways are divided into two main sections:

  • Extrapulmonary air conduits are located outside of the lungs and begin with the nose, pharynx and larynx. The trachea is continuous with the larynx above and the two primary bronchi below. It is the supporting framework for 16-20 C-shaped hyaline cartilages. These cartilage "bracelets" are open on the posterior wall of the trachea adjacent to the esophagus. A bundle of smooth muscle fibers bridges the gap between the two ends of the cartilage.
  • Intrapulmonary air conduits extend from the intralobar bronchi to the terminal bronchioles. When the bronchi enter the lung, the C-shaped cartilages that characterize the trachea and primary bronchi are replaced by irregular plates or cartilage that completely surround the cylindrical muscular airway tube. Cartilage disappears in the terminal bronchioles, which have narrowed to a diameter of 1 millimeter. The terminal bronchioles initially have a ciliated columnar epithelium that soon transitions to a low cuboidal epithelium. Mucous and seromucous glands and diffuse lymphatic tissue are associated with smaller bronchi but are not found distal to the region where there is a loss of cartilage plates.

Respiratory Airways

The respiratory airways extend from the respiratory bronchioles to the alveoli.

  • The respiratory bronchioles have a diameter of 0.5 millimeters and feature a few alveoli scattered along their walls. The epithelium here remains low cuboidal. Each respiratory bronchiole branches into between 2 and 11 alveolar ducts that still contain smooth muscle fibers in their walls. Along these walls, the alveolar ducts give rise to single alveoli and to numerous alveolar sacs, which are associated with 2 to 4 alveoli. The space at the entrance from the alveolar duct to an alveolar sac is referred to as the atrium.
  • Alveoli can be studied most easily in preparations of expanded lung, especially in those areas in which erythrocytes have been retained in the capillaries. Alveoli have a distinct cup shape separated by loop- or crescent-shaped walls known as interalveolar septa. The interalveolar septa contain myriad capillaries.

The interface between the capillary lumen and the alveolar epithelium is known as the air-blood barrier. The barrier consists of the endothelium of the capillary, the epithelium of the alveolus, and their shared basement membrane.

The surface epithelium of the alveoli contains two developmentally related but functionally distinct cells, known as pneumocytes. Type I pneumocytes are attenuated vesicle-studded cells that line the alveolar walls near the capillaries. Only their flattened nuclei can be recognized with certainty by light microscopy. Type II pneumocytes are cuboidal and occur singly or in small clusters between type I cells. They contain 0.2 to 1 micron wide multilamellar bodies that contain a high content of phospholipid that is the precursor to pulmonary surfactant, which interferes with the surface tension in the alveoli that would otherwise cause them to collapse. Club (Clara) cells are also thought to participate in the synthesis of surfactant. Type II cells serve as precursors to type I cells.

Where there are no capillaries, the alveolar septum contains fibroblasts, collagen, elastic fibers, smooth muscle cells, and macrophages known as dust cells. Also notable are alveolar pores, which equalize air pressure between the alveoli.

Circulatory System of the Lung

Branches of the pulmonary artery accompany the bronchi to the level of the respiratory bronchioles. From there they branch into an extensive network of capillaries suspended within the alveolar walls. Venules arising from these capillaries join in the intersegmental connective tissue and later empty into the pulmonary veins. Bronchi and connective tissue septa within the lung are vascularized by branches of the bronchial arteries, which are part of the systemic circulatory system. These two systems anastomose at the level of alveoli arising from the respiratory bronchioles.

Pre-Lab Quiz

  1. Briefly describe the structural and functional differences between the following:
    • Respiratory Bronchiole and Terminal Bronchiole
    • Alveolar Sac and Alveolus
    • Type I and Type II Pneumocyte
  2. Trace the path of a molecule of oxygen from the nose to the bloodstream. Make sure to include all major airways, as well as each layer of tissue that must be traversed.
  3. Answer:
  4. Describe the changes in the type of epithelia encountered as the molecule of oxygen in the question above moves from the nose to the alveolus.
  5. Answer:
  6. Name two adaptations that ensure that the airways will remain open under the normal conditions of inhalation and exhalation. How do these work?
  7. Answer:


Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden.

  1. Conducting Airway
  2. Conducting Epithelium
  3. Trachea
  4. Bronchus
  5. Intrapulmonary Air Conduits
  6. Bronchiole
  7. Respiratory Bronchioles
  8. Pneumocytes
  9. Pneumocytes EM
  10. Air-Blood Barrier

Virtual Microscope Slides

  1. Trachea
  2. Begin at medium power by identifying the different layers of the trachea (epithelium, basement membrane, lamina propria, submucosa) starting in the lumen.
  3. Bronchi
  4. Note the close proximity of blood vessels with bronchi and bronchioles.
  5. Alveoli
  6. Find a capillary in the wall of an alveolus. Estimate the distance that separates blood and oxygen.
  7. Pathologic Alveoli
  8. This slide contains lung tissue with unusually large amounts of carbon such as might be found in a smoker, coal miner, or city dweller. Identify macrophages filled with digested carbon. What is the name for macrophages localized to the lung? What are the differences in structure between the alveoli you see in this slide and healthy alveoli?


Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden.

  1. Emphysema
  2. Pulmonary Hypertension
  3. Bronchial Pneumonia


  1. Name the regions of the respiratory system.
  2. Answer: Terminal bronchiole and respiratory bronchiole
  3. What does this cell secrete?
  4. Answer: Surfactant
  5. What is the purpose of this type of epithelia in the respiratory system?
  6. Answer: Cleaning, warming and moisturizing air.
  7. Identify this structure.
  8. Answer: Trachea
  9. Name the structure.
  10. Answer: Small Bronchus
  11. What process does this cell use to accumulate the brown-staining material?
  12. Answer: Phagocytosis (Macrophage)
  13. Name the structures (arrow).
  14. Answer: Cilia
  15. Name 3 layers which molecules in the airspace must pass through to reach the red blood cell.
  16. Answer: A: Alveolar epithelium, B: basement membrane, C: capillary endothelium
  17. In this image, identify A, B, C, and D.
  18. Answer: A: Terminal Bronchiole B: Respiratory Bronchiole C: Alveolar Sac D: Alveolar Duct
  19. Describe the pathological changes that you would expect to see in the lungs of a smoker.
  20. Answer: Histologically, the lungs of a smoke would appear to have many dust cells (macrophages that have taken up carbon particles. You could expect to see damage to the cilia that typically lines the airways, an increase in mucous-producing cells, and even squamous metaplasia, which is a protective adaptation. Metaplastic and dysplastic structural changes could eventually lead to lung cancer.
  21. Premature children do not produce adequate amounts of pulmonary surfactant. Name two cells that are involved and explain why this greatly increases the risk of death. What would you expect to see on a histological preparation of lung tissue from such an infant?
  22. Answer: Type II pneumocytes and Club (Clara) cells are both involved in surfactant production. The lung tissue histology of such an infant would appear with collapsed alveoli.