Histology of Respiratory System

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Primary Function of Respiratory System:

1. Gas exchange - characteristics that enhance gas exchange include:

a. Large surface area of about 140 m2, about 300 million alveoli.

b. A thin barrier of .2 micrometers between blood and air to allow for quick diffusion.

c. Wet surface to aid in diffusion.

d. Abundant capillary supply.

Secondary functions:

1. Phonation via the larynx.

2. Olfaction.

Conducting portion includes:

Nasal cavities, pharynx, larynx, trachea, bronchi & bronchioles.

Conducting portion conditions the air:

1. Filters out particles.

2. Humidifies air, partially saturates the air with water vapor. In patients with a tracheotomy, the mucosa may dry out and lead to encrustation & infection.

3. Adjusts air temperature.

4. Mucous prevents dehydration of underlying tissues. Mucous covers all the conducting portion & is moved via cilia where it is swallowed or expectorated.

External nasal cavity:

Keratinized stratified squamous epithelium with hair follicles to filter particles. Epithelium also produces water in which cilia beat and moves the mucous mass to the pharynx. Non-keratinized stratified squamous begins at the vestibule.

Respiratory epithelium is first encountered at the back of the vestibule. Sensitive to abrasion or irritation and will change to stratified squamous, squamous metaplasia (metaplasia: abnormal transformation of one cell type to another cell type).

Cell types of respiratory epithelium:

1. Ciliated columnar cell.

2. Neuroendocrine or Kulchitsky cell.

3. Goblet cell.

4. Brush cells.

5. Basal progenitor cell.

Ciliated columnar with 16-20 cilia:

Primary function to move mucous. Cilia beat at 14 cycles/sec. and move mucous at a speed of 1 cm/min. Kartagener's immotile cilia syndrome (dyenin deficient) results in chronic sinusitis, bronchitis & infertility in males.

Goblet cell elaborates material that:

a. Slows evaporation.

b. Lubricates.

c. Sticks to foreign particles & traps them.

Brush cells:

These are progenitor cells can become goblet or columnar ciliated cells.

Neuroendocrine or Kulchitsky cells:

These are basal cells present in large numbers at junctions of airways, (ex. laryngotracheal junction, bronchial divisions). They are innervated by the ANS and release serotonin which contracts smooth muscle to produce vasoconstriction. Lamina propria has large number of lymphocytes, macrophages & mast cells. Sero-mucous tubuloalveolar glands elaborate sticky material that prevents evaporation & humidifies the air.

Swell bodies:

These are large venous plexuses of the middle & inferior conchae that are under ANS control and periodically engorge about every 30 minutes to block air flow to one side allowing it to recover from desiccation. Also engorges during sexual arousal and during a cold.

Olfactory epithelium:

This is located at superior concha & roof of nasal cavity.

Olfactory cells:

These are bipolar neurons with non-motile cilia that emerge from bulbs termed olfactory vesicles & function as dendrites. Axons pierce the cribriform plate to the olfactory bulb & nerve. Olfactory cells are probably not replaced. With advancing age the sense of smell diminishes. Sustentacular cells have MV & often contain lipid & appear yellow. Metabolic & structural support.

Basal, stem cells replace sustentacular cells. Brush cells are involved in general sensation of the olfactory mucosa, not olfaction, and are associated with the trigeminal (V).

Bowman's Glands:

These are branched tubuloalveolar glands that elaborate a watery fluid to dissolve substances so they can be detected by the olfactory cells.

Paranasal Sinuses:

These include frontal, ethmoid, sphenoid & maxillary are lined by RE. Maxillary sinus has drain hole on the top. Nasopharynx is lined by RE and extends from end of nasal cavity to superior part of larynx.

Oropharynx is bounded by soft palate superiorly & epiglottis inferiorly. Epithelium is stratified squamous mucosal because it is subject to abrasion.


Its function is to prevent food from entering larynx during swallowing & is elastic cartilage. False vocal folds are RE. True vocal folds are SSE with parallel elastic fibers to form the vocalis ligament. Mucous collects in the larynx & must be coughed up.

Effects of smoking:

Chronic smokers irritate RE and it changes via squamous metaplasia to SSE and may lead to cancer. In addition, nicotine paralyzes cilia & mucous accumulates and smokers develop the very attractive "liquid cough." Avoid at all costs.

Rings of the Trachea:

Trachea consists of 16-20 C-shaped hyaline cartilage rings. Includes trachealis muscle (smooth), seromucous glands, and lymphatic nodules.

Anatomy of the Trachea:

Two extrapulmonary primary bronchi (complete cartilage ring) branch from the trachea, secondary go to the lobes of the lung & tertiary go to each bronchopulmonary segment. Secondary and tertiary (isolated cartilage plates) are intrapulmonay are lined by RE. Lamina propria with smooth muscle. In asthma attacks, serotonin from mast cells contracts smooth muscle.

Function of Bronchioles:

Bronchioles have smooth muscle but no cartilage or glands. Pre-terminal bronchioles have RE that decreases in height to become simple ciliated columnar without goblet cells. Pre-terminal bronchioles branch to terminal bronchioles whose ciliated epithelium is short.

Function of Clara cells:

Clara cells & ciliated cuboidal epithelium of terminal bronchioles. Function of Clara cells is obscure but have many mitochondria & secretory granules. They may secrete GAGs that are involved in reducing surface tension & may also protect the area.

Respiratory part is the first part where respiration occurs, includes respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli.

Respiratory Bronchioles:

Respiratory bronchioles & two alveolar ducts with smooth muscle. Respiratory bronchioles are a transition zone between conducting & respiring. Lined by simple columnar ciliated with a few alveoli. Smooth muscle & elastic tissue occurs.

Alveolar Ducts:

Alveolar ducts lead to atria that are spaces bounded on one side by the termination of the alveolar duct & on the other sides by the openings of the alveolar sacs which are surrounded by alveoli. Smooth muscle disappears at the ends of the alveolar ducts. Reticular & elastic fibers act as a skeleton around atria.

Pulmonary Emphysema:

Pulmonary emphysema is characterized by destruction of elastic fibers. Smoking destroys the protein alpha 1-antitrypsin which protects elastic fibers from being destroyed by elastase synthesized by dust cells. Reduced elasticity & large fluid-filled spaces occur & decrease gas exchange area.

Lining Cells of Alveoli:

• Type I pneumocytes (squamous, 95% surface area).

• Type II pneumocytes (cuboidal) with surfactant.

• Alveolar macrophage (dust cell).

Blood-Air Barrier:

Surfactant Type I pneumocyte Basal lamina of type I pneumocyte Basal lamina of endothelial cell fused as alveolar-capillary membrane Endothelial cell

Pneumocyte Type II:

Commonly occurs in angles of alveolus. Produces surfactant which is a phospholipid that reduces surface tension along with the surface water. Without surfactant, the alveoli would collapse. Lamellar bodies are released from secretory granules via exocytosis.

Respiratory Distress Syndrome:

Surfactant begins to be produced from 6th month. Hyaline membrane disease leads to death within 2 days. Treat by increasing air pressure, administer cow surfactant. If Mom has a history of premature births she can be given cortisone in 6th to 8th month to promote maturation of type II pneumocytes. Can estimate surfactant level from amniotic fluid.

Cardiac Failure Cells:

In heart failure, lungs get congested with blood & dust cells consume them. Dust cells are the 1st line of defense in the lungs. They also secrete chemotactic factors for neutrophils.


Bronchi associated lymphoid tissue.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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