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The Respiratory System


  1. Observe the changes that occur in the lining of the respiratory tract and correlate these changes with the function of these passageways.
  2. Recognize the various segments of the respiratory system.

The respiratory system can be separated into a conducting and a respiratory portion.The conducting portion consists of the air-transmitting passages of the nose, nasopharynx, larynx, trachea, bronchi and bronchioles. This part of the respiratory system serves to filter, warm and humidify air on its way to the lungs. Specialized portions of the conducting system also serve other functions, e.g., the nose in the sense of smell, the pharynx in alimentation, and the larynx in phonation. The actual exchange of gases occurs in the respiratory portion which consists of the respiratory bronchioles, alveolar ducts and sacs and alveoli.

Gradual changes in the morphology of the conducting and respiratory portions of the respiratory system occur as the diameter of the respiratory tubes becomes smaller. The epithelium gradually changes from pseudostratified ciliated columnar with goblet cells to an extremely flattened squamous epithelium devoid of goblet cells. The lamina propria also decreases in thickness and elastic fibers become relatively more numerous. A layer of smooth muscle becomes a conspicuous component of the wall of the lower part of the conducting portion of the respiratory tree. It becomes less prominent in the respiratory portion and is absent in the alveolar walls. The "C" shaped cartilage rings alter shape and appear as plates in the intrapulmonary branchi. The plates gradually get smaller and finally disappear when the tube reaches about one millimeter in size. At this point the tube is called a bronchiole. Lack of the cartilage plates makes bronchioles able to constrict to the point of closure in some individuals (asthmatics). Use the accompanying chart (last page of this section), diagrams and pictures to identify the various components of the respiratory system.

There are two separate arterial systems in the lungs: (1) the pulmonary arteries which carry deoxygenated blood, follow the respiratory passages and end in capillaries in alveolar walls, and (2) the bronchial arteries which carry oxygenated blood, follow the walls of the air tubes to the level of the respiratory bronchioles and nourish these passages and the accompanying pulmonary vessels. The bronchial arteries also supply the pleura. Most of the venous drainage from the lung is through the pulmonary veins. These vessels initially run alone in the connective tissue septa between lobules and eventually join the pulmonary arteries at the bronchi. These then follow the bronchial tubes to the hilum of the lung.

The lungs are richly supplied with lymphatic vessels, which are organized into two sets - a deep set that accompanies the pulmonary vessels and airways and a superficial set that lies beneath the visceral and parietal pleura. Little or no anastomosis occurs between the two plexi except in the region of the hilum.

Nasal Cavity (Slide #70)

Study the nasal mucosa (slide #70). Identify the pseudostratified cilated epithelium with goblet cells which is characteristic of the conducting portion of the respiratory system. Examine the lamina propria and notice the arteries, and the numerous venous plexi which are often highly engorged with blood. The lamina propria contains mixed mucous and serous glandswhose ducts open on the epithelial surface. The secretions from these glands and the goblet cells not only aid in keeping the epithelium moist but produce a sticky (mucoid) material which traps particulate matter. The particulate material and mucous is moved toward the nasopharynx by ciliary action.


Slide #71 is a longitudinal section of the human epiglottis. The epithelium on the surface facing the tongue (lingual surface) and on one third to one half of the surface facing the larynx (laryngeal surface) is stratified squamous. The epithelium on the remainder of the laryngeal surface is pseudostratified ciliated columnar with goblet cells, and in many cases is damaged due to postmortem changes. The lamina propria contains blood vessels, nerves (best seen on even slide) and serous and mucous glands (including serous demi-lunes). Note the elastic cartilage. The elastic fibers in the cartilage are clearly demonstrated with special stains in the even boxes.


Slide #72 odd is a cross section of trachea only showing the posterior wall; #72, even, which has both a cross section and a longitudinal section is stained to show elastic fibers. Study the trachea using slide #72. Examine its general structure by eye. With the microscope under low power three layers may be seen. Using higher magnification, examine each layer carefully.

  1. The mucosa is composed of a pseudostratified ciliated epithelium with goblet cells (which may be torn in some areas), a prominent basement membrane and a lamina propria. Elastic fibers, form a muscularis mucosa.

  2. The submucosa contains connective tissue and glands.

  3. The adventitia contains C-shaped hyaline cartilage rings and an external layer of areolar tissue. The cartilage rings (which are incomplete) are surrounded by a perichondrium. The gap in the ring is bridged by a fibroelastic membrane and a thick layer of smooth muscle called the trachealis muscle (not apparent on all sides). Using slide #72 even, the arrangement of the cartilage rings can be seen by holding the slide to the light.


(A) Bronchi: Slide #28 (Verhoeff stain stains elastic fibers black), is a portion of the hilar region of the lung and contains sections of bronchi. Holding the slide to the light, try to identify the bronchi, lymph nodes, portions of pulmonary (elastic) arteries, and a small piece of lung tissue. Both even and odd slides are very useful for examination of these structures under high power. In the lymph node note the cortex, medulla, lymph channels (trabecular and subcapsular) and the presence of carbon (found primarily in macrophages in the medulla). Now carefully examine the structure of the bronchial wall. Note the epithelium, smooth muscle, glands and cartilage. The lumen contains mucous and nuclei of many sloughed cells.

(B) Bronchioles, respiratory bronchioles, alveolar ducts and sacs

(See accompanying chart and slide #73 — both even and odd.

Study sections of the lung (slide #73 — both even and odd). Examine, compare and identify the various components of the respiratory system found on your slides. Note bronchioles lack cartilage and glands whereas the amount of smooth muscle and connective tissue is proportionally increased. Outpockets of alveoli begin at the level of the respiratory bronchioles. Identify respiratory bronchioles, alveolar ducts and alveolar sacs and alveoli.

(C) Alveoli (Slide #28, odd & even)

Examine slide #28 - (both odd and even boxes) again and locate the lung tissue. It is present in very small amounts on the outer edge of the tissue slice. These sections are thin and suitable for the study of alveolar walls. At low power, observe the thickness of the interalveolar septum, the size of the alveoli and the amount of material in the alveolar spaces. These are characteristics which undergo the most obvious changes in lung disease. At high magnification, identify the rich supply of alveolar capillaries, nuclei of surface epithelial cells (difficult to see) and alveolar macrophages. Then examine the scanning and transmission electron micrographs of alveoli and interalveolar septa. Note the ultrastructure of the alveolar epithelial cells and the composition of the interalveolar septum.





Extrapulmonary bronchi

  1. Pseudostratified ciliated columnar epithelium with goblet cells.

  2. Prominent basement membrane.

  3. Relatively thin lamina propria (elastic layer at base)

  4. Submucosa with seromucous glands

  5. "C" shaped hyaline cartilage rings w/ smooth muscle between ends of cartilage

Pulmonary artery and vein accompany bronchi. Bronchial bronchi. Lymphatics present inwalls of bronchi.

Intrapulmonary bronchi

  1. Pseudostratified ciliated columnar changing to ciliated simple columnar in smaller branches. Goblet cells at all levels.

  2. Below lamina propria are interlacing spirals of smooth muscle

  3. Seromucous glands decrease as bronchi get smaller.

  4. Plates of cartilage gradually disappear

Pulmonary artery with each bronchus. Bronchial artery (smaller) supplies bronci. Lymphatics. Bronchial vein 2-3 largest bronchi. Pulmonary vein drains the rest.

Bronchioles (1 mm or less)

  1. Ciliated columnar to ciliated cuboidal

  2. Goblet cells decrease and Clara cells appear

  3. Spirals of smooth muscle relatively heavier than elsewhere (gradually decrease in amount)

  4. No seromucous glands

  5. No cartilage

Pulmonary artery with each bronchiole. Bronchial artery supplies bronchiole. Lymphatics. (pulmonary venules in septa)




Respiratory broncioles

  1. Cuboidal epithelium with some cilia. Clara cells and no goblet cells.

  2. Thin supporting wall of C.T. and an incomplete layer of smooth muscle.

  3. Outpocketings of alveoli, numbers inc. at lower levels.

Pulmonary artery supplies caps to alveoli. Bronchial artery supplies caps to bronchiloe walls. Lymphatics. (Pulmonary venules in septa)

Alveolar ducts

  1. Thin tubes with an almost continuous outpocketing of alveolar sacs and alveoli.

  2. The small areas of wall contain C.T. and smooth muscle covered by simple squamous epithelium.

Pulmonary capillaries. Lymphatics. No bronchial arterioles

Alveolar sacs

  1. Rotunda-like spaces with alveoli opening off of the center space

  2. Little connective tissue and no smooth muscle

Pulmonary capillaries. No Lymphatics