THE RESPIRATORY SYSTEM

Reading: Junqueira: Respiratory System, 10th Ed., Ch. 17, pp 349-368; Berman: Respiratory System, Atlas.

The primary purpose of the respiratory system is to provide for the gaseous interchanges between the atmosphere and the blood.
 
I. Anatomical organization:
A. CONDUCTING SYSTEM
B. RESPIRATORY SYSTEM
1. Nose and its cavities
1. Respiratory bronchioles
2. Pharynx (oral; nasal)
2. Alveolar ducts
3. Larynx
3. Atria
4. Trachea
4. Alveolar sacs
5. Bronchi
5. Alveoli
6. Bronchioles  
Function: To conduct air to and from the lungs. Function: To provide for the exchange of CO2 & O2 in the blood .
C. GROSS RELATIONSHIP: Fig. 1 & Fig. 2.

Figure 1. Depicts general gross organization of the respiratory system. Figure 2. Frontal sections depicting orientation of lab slides.

 


II. HISTOLOGY OF THE CONDUCTING SYSTEM

A. THE NOSE -- a hollow organ, covered with skin, provided with muscles, supported by cartilage and bone and lined with a mucous membrane.

B. THE NASAL CAVITIES -- (separated by the cartilaginous and bony parts of the nasal septum).

1. The vestibule -- opens to the outside at the anterior nares. The integument continues into the vestibule; Changes from cornified stratified squamous epithelium (associated with hairs, sebaceous and sweat glands) gradually to pseudostratified, ciliated, columnar epithelium in the rest of the nasal cavities (i.e. atrium, superior-, middle-, and inferior meatus; the surface of conchae).

2. The respiratory region -- includes nearly all of the septum and lateral walls. The surface area of the lateral walls is increased by shelf-like projections (supported by bone) called conchae (Figure 3).

a. Epithelium: pseudostratified, ciliated columnar with goblet cells; Cilia beatsbackwards, toward the pharynx; Goblet cells sometimes are concentrated in intraepithelial pits; The basement membrane varies from thin to very thick.

b. Lamina propria: of loose FECT;

i. contains: mixed sero-mucous glands (comp. tubuloalveolar);
ii. contains: a rich cavernous venous plexus, which serves to warm the passing air; upon irritation the plexus can be distended by blood and reduces air flow.

c. Submucosa: lacking, the deepest layer of the lamina propria fuses with the periosteum below;

3. The olfactory region -- located on the superior concha and adjacent septum (dime-size areas).

a. Epithelium: pseudostratified ciliated columnar; composed of olfactory cells, supporting cells, and basal cells. (Figure 4).

i. Olfactory cells: spindle shaped, with round nuclei; (Nuclei are in the middle and deep zone of the epithelium.) the apical part extends to the epithelial surface, ends in a bulbous knob which bears 6-8 olfactory hairs; these hairs or cilia are non-motile, sensory and serve as dendrons; they are embedded in a thick layer of mucus and parallel the surface of the olfactory epithelium. The proximal (basal) part of the cells form a long thin axonal process, which constitute the olfactory nerve fibers; these unmyelinated fibers are connected to the olfactory center of the brain (in the olfactory bulbs). Olfactory cells are modified bipolar neurons.

Functions: the olfactory cilia are stimulated by gaseous, odori-ferous substances dissolved in the secretion of serous glands moistening the epithelium.

ii. Supporting cells: tall slender cells make up the upper third of the pseudostratified epithelium (nuclei in top 2-3 rows). They are attached to the sensory (olf.) cells at the surface of the epithelium by zonula adherens (junctional complex with terminal web). Have a small Golgi complex in the apical cytoplasm and pigment granules (brown). There are numerous slender villi on their apical surface.

iii. Basal cells: constitute a single layer of conical elements a the base of supporting cells (bottom layer of nuclei); have dark nuclei and branching processes.

 

b. Lamina propria: continuous with the dense connective tissue of the underlying periosteum in the adult or with the perichondrium in the fetus. The venous plexus in the lamina propria is continuous with that of the respiratory regions.

A group of branched, tubuloalveolar (mainly serous) olfactory glands called Bowman's glands present in the lamina propria supply the necessary solvents to trap odoriferous substances. The continuous replacement of secretion keeps the receptors ready for new stimuli.



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LABORATORY EXERCISE: Nasal Cavity -- Fetal. Go to the Digital Histology CD then examine the virtual slide entitled nose, olfactory epithelium. Identify the nasal cavity, nasal septum and conchae on CD. What type of cartilage is present? What type of epithelia lines the nasal cavity? Can you identify areas lined with olfactory epithelium on both CD and VS? How can olfactory epithelium be distinguishes from respiratory epithelium? compare the epithelial lining of the developing oral cavity to the epithelium of nasal cavities. Identify olfactory epithelium. Identify the venous plexus in the lamina propria on CD and VS.
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C. THE PHARYNX -- is a flattened conical chamber through which air and food pass. It is divided partly by the soft palate into:

1. Naso-pharynx -- lined with ciliated columnar epithelium with goblets cells;

2. Oro-pharynx -- lined with moist stratified squamous epithelium.

D. THE LARYNX -- is interposed between the pharynx and the trachea. It is supported by cartilages and muscles; and contains the vocal folds (cords). Figure 5.

1. Epithelium -- varies according to wear and tear in the different regions of the larynx.

a. The epiglottis -- which closes that laryngeal aperture like a lid, is covered by moist stratified squamous epithelium;

b. The false vocal cords - (located above the true vocal cords) are covered with pseudostratified, ciliated columnar epithelium with goblet cells.

c. The true vocal cords - are covered with moist stratified squamous epithelium.

d. The rest of the larynx is covered with pseudostratified, ciliated columnar epithelium with goblet cells (including the lateral walls of the laryngeal ventricles between the false and true vocal cords), resting on a thin basement membrane. The cilia beat toward the mouth, moving the mucus and attached particles or bacteria toward the exterior.
  2. Lamina propria -- is rich in elastic fibers.
    a. In the true vocal cord there is an elastic band that constitutes the vocal ligament and it is adjacent and parallel to the vocal muscle in the deeper layers of the lamina propria.
     
    b. The glands in the larynx are tubuloacinar, mixed (sero-) mucous glands, they are absent from the avascular vocal cords.
   
  3. Cartilages -- support the walls of the larynx;
    They are united by ligaments and maintain the larynx as a constantly open tube. Early in life all are of the hyalin type. Latter most of the epiglottis and arytenoid cartilages become elastic and smaller parts of these may become fibrocartilage. In sum, the larger cartilages remain hyalin (e.g., thyroid cart.) while the small ones become elastic.
     
  4. Functional correlations:
   
   

a. The extrinsic muscles of the larynx -- muscles which attach to the cartilages externally -- aid in swallowing (deglutition);
     
 

 

 b. The intrinsic muscles -- muscles which interconnect the cartilages -- function in changing the pitch of the sound (e.g. vocal muscle);
   

 

 

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LABORATORY EXERCISE:
Virtual Slide (Aut_01) Larynx. Vocal Folds. This slide is a longitudinal section of the larynx. Identify the laryngeal and tracheal cartilages with the aid of the Digital Histology CD. Identify the false and true vocal folds, the ventricle, the vocal ligament, and the vocal muscle on the virtual slide and the CD. What types of epithelia are present? Following the epithelium through its transition from stratified squamous to the pseudostratified ciliated columnar epithelium lining the trachea. Observe the distribution of mixed glands, their abundance in the false vocal fold and their absence from the true vocal cords. Identify the dense vocal ligament and the vocalis muscle. Classify the cartilage in the epiglottis and thyroid cartilage. In some of the slides you may observe focal ossification of the thyroid cartilage.
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E. TRACHEA, PRIMARY AND SECONDARY BRONCHI -- thin walled, flexible, extensible tubes.
 

The trachea is supported by C-shaped cartilages, with the open part of the C facing the esophagus and it is spanned by the trachealis muscle and a membrane of D.-FECT. Adjacent cartilages are connected by dense fibro-elastic membranes.

The trachea (4.5 inches long and 1 inch in diameter) bifurcates and gives rise to the primary (or main) bronchi which are held open by rings of hyalin cartilage.
     
   

1. Epithelium -- is of the respiratory type, - pseudostratified, ciliated columnar with many goblet cells. It rests upon a very prominent basal lamina (cilia move mucus in an external direction).

   

2. Lamina propria -- is a thin layer, with longitudinal elastic fibers in place of muscularis mucosa in the trachea and with muscularis mucosa (sm.m.) in the bronchi. Reticular fibers are abundant below the basal lamina.
    3. Submucosa -- is a deeper layer with tubuloacinar, mixed (sero-) mucous glands.
    4. Adventitia -- is a dense irregular connective tissue layer surrounding the trachea and the bronchi and contains the cartilages. The cartilaginous rings of primary bronchi (Figure 6) are replaced by irregular cartilaginous plates in the adventitia, after the bronchi enter the substance of the lungs; These bronchi with the cartilaginous plates are the secondary bronchi.
   
    Figure 6. Illustrates the gross and microanatomy of the walls of these air passages

 

 

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LABORATORY EXERCISE:

Virtual Slide 72: Trachea. Study the trachea on the CD first then the virtual slide. Study all three annotations on slide 72 (accessible by searching for trachea on the data base). Identify the four layers of the trachea. What type of cartilage is present? Classify the glands present in the submucosal. Are goblet cells present in the epithelium? What is their function? Observe the unusually thick basement membrane underlying the epithelium.
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F. BRONCHIOLES -- In the lobes of the lung the primary bronchi divide into several (5) short secondary bronchi, which in turn divide to form bronchioles (10 on the right and 9 on the left). The bronchioles are distributed within the right (3) and left (2) lobes of the lung. With further divisions and reduction in the diameter of the bronchioles the conducting airways end with the so called terminal bronchioles (0.5mm in diameter).

   

1. Epithelium -- respiratory; gradually reduces in thickness; the number of goblet cells is reduced and the epithelium becomes simple ciliated columnar without goblet cells in the terminal bronchioles. Goblet cells are replaced by Clara cells which produce a less viscous secretion.
   

2. The lamina propria -- becomes dominated by a spiraling layer of muscularis mucosa in the terminal bronchioles.
   

3. Cartilage and glands disappear at the level of the bronchioles; only a thin layer of adventitia remains in the terminal bronchioles.

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LABORATORY EXERCISE:

Virtual Slide75 Lung. Sudy the secondary bronci on the CD first. Secondary bronchi can be found also on virtual slide 75. Identify, based on the information provided in the lecture syllabus the secondary bronchi. Note that secondary bronchi can be distinguished from the trachea and primary bronchi by their cartilaginous plates in the adventitia.
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II. HISTOLOGY OF THE RESPIRATORY SYSTEM

The respiratory unit of the lung is the primary lobule which consists of the:
          A. Respiratory bronchiole  
          B. Alveolar ducts  
          C. Alveolar sacks  
          D. Alveoli  
         

E. Associated blood vessels, lymphatics, nerves, C.T.

  



 

Figure 7. Illustrates the microanatomy of bronchioles and alveolar sacs.

A. RESPIRATORY BRONCHIOLE -- in adult man they begin with a 0.5mm diameter. A few alveoli bud from their walls.

1. Epithelium -- starts with a simple ciliated columnar epithelium which is reduced in a short distance to a simple (non-ciliated) low cuboidal epithelium.

2. Wall -- composed of collagenous C.T. interlaced with bundles of smooth muscle. (The few alveoli associated with the wall are responsible for the term "respiratory bronchiole.")

B. ALVEOLAR DUCTS -- are the branches (2 to 11) of respiratory bronchioles. These are thin walled tubules. Their wall is interrupted by the opening of many thin walled outpouchings, the alveoli.

1. Epithelium -- not distinguishable.

2. Wall -- composed of strands of elastic and collagenous fibers, and a few smooth muscle cells which are visible around the mouth of alveolar sacks.

C. ALVEOLAR SACKS -- are composed of 2 to 4 or more alveoli. Alveolar sacks open only into the alveolar ducts. The space into which alveolar sacks open in the alveolar duct is called the atrium.

D. THE ALVEOLUS -- is a thin walled polyhedral sack which opens on one side only into the alveolar sack or individually into an alveolar duct ( Figure 8.)

 

Figure 8. Shows the microanatomy of the alveolus.
 

1. The walls of alveoli contain a dense network of anastomosing capillaries; reticular fibers and elastic fibers form the supporting framework of the wall. There are small openings in the wall (alveolar septum) between adjacent alveoli; these are called alveolar pores (7-9mm diameter).

2. The epithelial lining of the alveolus:

a. it is of endodermal origin;

b. constitutes a thin cellular covering (clearly visible only with the electron microscope); separated from the endothelium of capillaries by a continuous basal lamina.

c. The cells composing this layer are:

i. the squamous pulmonary epithelial cells (Type I pneumocytes);

ii. the rounded great alveolar cells (Type II pneumocytes).

iii.Function: a. Type II pneumocytes secrete pulmonary surfactant (rich in phospholipids) in

the form of multilamellar bodies.

b. the faulty production or absence of surfactant in newborns results in the fetal distress syndrome (hyalin membrane disease).

c. Type I pneumocytes function as lining cells of alveoli and thought to have the potential of differentiating into Type II pneumocytes.
 

3. Alveolar phagocytes (Pulmonary or lung macrophages, also known as dust cells).

a. Originate from blood borne monocytes;

b. Migrate into interstitium (C.T.) of lung and from there into the alveoli;

c. Similar to other macrophages;

d. Function in the removal of cell debris and foreign material (e.g. dust) from the lung.

Blood supply of the lung: via large elastic pulmonary arteries whose branches accompany the bronchi and its branches as far as the respiratory bronchioles. Alveoli are supplied by capillaries from branches at the level of the alveolar duct. Venules arise from the alveolar septa and from the pleura, run independently of arterioles and form the pulmonary veins which return the blood to the heart. The bronchi are supplied with blood independently.
 

The Pleura -- consists of a thin layer of collagenous connective tissue interspersed with layers of elastic fibers, and a layer of mesothelial cells. The visceral pleura covers the surface of the lung and the parietal pleura forms the lining of the thoracic cavity.

 

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LABORATORY EXERCISE:
Virtual Slide 74, Lung ( Bronchioles and alveoli). Study the CD first then identify the same structures on the virtual slide 74. Identify bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. What is the difference between terminal and respiratory bronchioles? How can you distinguish alveolar ducts? Identify Type I and Type II pneumocytes. Find and study the morphology of alveolar macrophages and macrophages in the interstitium. Why are pulmonary phagocytes (macrophages) sometimes called dust cells?
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RESPIRATORY SYSTEM LAB REQUIREMENTS:
Structures to be identified:

nasal septum
classify cartilage
identify perichondrium
identify conchae
Junction of respiratory and olfactory epithelium
classification of these epithelia
olfactory cells
supporting cells
basal cells
Bowman's glands
olfactory nerves

Identify and/or classify:

laryngeal and tracheal cartilages
false vocal fold
true vocal fold
laryngeal ventricle
classify epithelial covering of above
distribution of glands (i.e., in falls and true vocal folds)
vocal ligament (id and classify both)
vocalis muscle
classify cartilage in epiglottis and thyroid cartilage

Identify and/or classify:
Four layers of trachea (what are they?)
I. mucosa: 1. classify and identify epithelium, Goblet cells, basement membrane; 2. lamina propria;
II. muscularis mucosae;
III. submucosa, classify the glands;
IV. adventitia, cartilage

Identify and/or classify:
secondary bronchi
mucosa
muscularis mucosae
submucosa
glands
adventitia
cartilaginous plates
ordinary bronchiole and its layers
terminal bronchiole
respiratory bronchiole
alveolar duct
alveolar sacs
alveolusalveolar wall and components under (oil) 100X objective.
Squamous alveolar epithelial cell or Type I ep. cell
Great alveolar cell or Type II ep. cell
capillaries
basement membranes
alveolar macrophages (dust cells, heart failure cells, why?)