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Pulmonary Airway and Alveoli Study Guide
Pulmonary Study Guide: Airways to Alveoli
Big Picture
The respiratory tract changes gradually from the trachea down to the bronchioles, then changes sharply at the alveoli.
Trachea → bronchi → bronchioles: mainly conduct air and help clean, humidify, and protect it.
Respiratory bronchioles + alveoli: begin or specialize in gas exchange.
Conducting Airways: Trachea to Bronchioles
A repeating structural pattern is present through most of the airway except the alveoli:
Ciliated cells move airway surface liquid (ASL) upward, away from the alveoli.
Secretory cells/glands produce ASL.
As the airways get smaller, the epithelial cells become shorter.
Trachea and Bronchi
Lined by ciliated pseudostratified columnar epithelium.
Key cell types:
Ciliated cells: propel mucus/ASL upward.
Goblet cells: secrete mucus.
Basal cells: stem cells; make the epithelium appear pseudostratified.
Supporting structures:
Large submucosa
Many submucosal glands
Smooth muscle
Cartilage
Bronchioles
As the airway narrows:
Cells become shorter
Submucosa shrinks
Smooth muscle decreases
Cartilage disappears
Terminal Bronchioles
Still contain:
Ciliated cells
Club (Clara) cells that secrete airway surface liquid
Respiratory Bronchioles
Still contain:
ASL-secreting cells
Ciliated cuboidal epithelium
Absent here:
Submucosa
Smooth muscle
Cartilage
Alveoli
Alveoli are specialized for gas exchange, not airway conduction.
An alveolus is an air space lined by simple squamous epithelium.
This thin lining is essential because gases must diffuse rapidly.
Alveolar Septum
The thin wall between alveoli is the septum.
It is formed mainly by:
Type 1 pneumocytes
Capillary endothelial cells
Their shared basement membrane
This very thin barrier minimizes diffusion distance.
Other Important Alveolar Cells
Type 2 pneumocytes
Fewer and larger than type 1 cells
Secrete surfactant
Serve as stem cells for type 1 pneumocytes
Alveolar macrophages
Abundant immune cells
Usually found in or near the septum
Not part of the epithelium
Alveolar Connections
Alveoli:
Arise from an alveolar sac
Connect to each other through pores of Kohn
Clinical importance:
If one alveolus is obstructed, neighboring alveoli may still help aerate it through collateral ventilation.
This makes the alveoli of one sac function together as a functional unit.
High-Yield Takeaways
Conducting zone extends to the terminal bronchioles.
Respiratory bronchioles begin the respiratory zone.
Cartilage is present in the trachea/bronchi but absent in bronchioles.
Goblet cells and submucosal glands are prominent in larger airways.
Club cells become important in bronchioles.
Type 1 pneumocytes = gas exchange.
Type 2 pneumocytes = surfactant + regeneration.
Alveolar macrophages = immune defense.
Pores of Kohn allow collateral ventilation.
Quick Compare
Large airways: tall epithelium, glands, submucosa, smooth muscle, cartilage
Small airways: shorter/cuboidal cells, less supporting tissue, no cartilage
Alveoli: very thin simple squamous lining for gas exchange