Histology of Respiratory System

Study Notes

Respiratory bronchioles:
- Similar to terminal bronchioles, but with alveoli interrupting the epithelium
- Epithelium consists mostly of Clara cells
- Lined with cuboidal epithelial cells and have alveolar ducts that open into alveoli
Primary, terminal respiratory bronchioles:
- Branch into alveolar ducts
- Alveolar ducts have almost no walls, only alveoli as their peripheral boundary
- Lined with simple squamous epithelium
- Terminate in clusters of alveolar sacs

Alveolar ducts:
- Arise from respiratory bronchioles
- Walls composed of alveolar sacs and alveoli
Alveolar sacs:
- Clusters of alveoli that open into the lumen of the alveolar duct
- Composed of squamous epithelial cells, basement membrane, and capillaries
- Site of respiratory exchange
Alveolar ducts and sacs:
- Open into atria that communicate with alveolar sacs
- Two or more alveolar sacs arise from each atrium
- Elastic and reticular fibers form a complex network around the openings of atria, alveolar sacs, and alveoli

Alveoli are responsible for the spongy structure of the lungs
Structurally, alveoli resemble small pockets open on one side, where O2 and CO2 are exchanged
Inter-alveolar septum consists of two thin squamous epithelial layers, capillaries, elastic and reticular fibers, and connective tissue matrix and cells
The capillaries and connective tissue constitute the interstitium
The interstitium of the interalveolar septum has the richest capillary network in the body

The blood-air barrier consists of three components:
- Surface lining and cytoplasm of alveolar cells
- Fused basal laminae of closely apposed alveolar and endothelial cells
- Cytoplasm of endothelial cells
The total thickness of these layers varies from 0.1 to 1.5 µm

Type 1 alveolar cells:
- Simple squamous cells
- Adjacent cells joined by tight junction
Type II cells:
- Cuboidal cells scattered among type 1 cells
- Secrete pulmonary surfactant, which lowers surface tension and prevents alveolar collapse
Alveolar macrophage or dust cells:
- Remove debris

The pleura is a serous membrane covering the lung
Consists of two layers, parietal and visceral, that are continuous in the region of the hilum
Both membranes composed of mesothelial cells resting on a fine connective tissue layer containing collagen and elastic fibers
Elastic fibers of the visceral pleura are continuous with those of the pulmonary parenchyma