T.S. of Lung – Mammal
Histology of Lung
✓ The lungs are paired lobed organs situated within the thoracic cavity on each side of centrally
placed mediastinum.
✓ Each lung is enclosed and protected by a double-layered serous membrane called the pleural
membrane; parietal (outer) and visceral pleura (inner).
✓ Between the visceral and parietal pleurae is a small space, the pleural cavity, which contains a
small amount of lubricating fluid (reduces friction between the membrane layers).
✓ On entering the lungs, the primary bronchi divide to from secondary bronchi one for each lobe of
the lung. The secondary bronchi continue to branch forming still smaller bronchi called tertiary
bronchi that divide into bronchioles.
✓ Bronchioles in turn branch repetitively to terminal bronchioles and respiratory bronchioles.
✓ As the passageway enters the lungs, the bronchi undergo extensive branching and their diameters
become progressively smaller. There is a gradual decrease in the height of epithelium, amount of
cilia and number of goblet cells.
• Pseudostratified ciliated columnar epithelium- primary bronchi, secondary bronchi, and
tertiary bronchi
• Ciliated simple columnar epithelium with some goblet cells - larger bronchioles
• Ciliated simple cuboidal epithelium with no goblet cells - smaller bronchioles
• Nonciliated simple cuboidal epithelium - terminal bronchioles
• Low cuboidal - respiratory bronchioles.
• Simple squamous epithelium - alveoli
✓ Plates of cartilage gradually replace the incomplete rings of cartilage in primary bronchi and finally
disappear in the distal bronchioles.
✓ As the amount of cartilage decreases, the amount of smooth muscle increases. Smooth muscle
encircles the lumen in spiral bands.
✓ Respiratory Bronchioles with alveoli out pocketing are directly connected to the alveolar ducts and
the alveoli.
✓ Alveoli are cup-shaped out pocketing lined by simple squamous epithelium while an alveolar sac
consists of two or more alveoli that shares the common opening.
✓ The simple squamous epithelium facilitates gas exchange between air spaces and capillary walls
which together forms the respiratory membrane.
Functional correlations
✓ The wall of alveoli consists of two types of cells:
Histology of Lung
✓ The lungs are paired lobed organs situated within the thoracic cavity on each side of centrally
placed mediastinum.
✓ Each lung is enclosed and protected by a double-layered serous membrane called the pleural
membrane; parietal (outer) and visceral pleura (inner).
✓ Between the visceral and parietal pleurae is a small space, the pleural cavity, which contains a
small amount of lubricating fluid (reduces friction between the membrane layers).
✓ On entering the lungs, the primary bronchi divide to from secondary bronchi one for each lobe of
the lung. The secondary bronchi continue to branch forming still smaller bronchi called tertiary
bronchi that divide into bronchioles.
✓ Bronchioles in turn branch repetitively to terminal bronchioles and respiratory bronchioles.
✓ As the passageway enters the lungs, the bronchi undergo extensive branching and their diameters
become progressively smaller. There is a gradual decrease in the height of epithelium, amount of
cilia and number of goblet cells.
• Pseudostratified ciliated columnar epithelium- primary bronchi, secondary bronchi, and
tertiary bronchi
• Ciliated simple columnar epithelium with some goblet cells - larger bronchioles
• Ciliated simple cuboidal epithelium with no goblet cells - smaller bronchioles
• Nonciliated simple cuboidal epithelium - terminal bronchioles
• Low cuboidal - respiratory bronchioles.
• Simple squamous epithelium - alveoli
✓ Plates of cartilage gradually replace the incomplete rings of cartilage in primary bronchi and finally
disappear in the distal bronchioles.
✓ As the amount of cartilage decreases, the amount of smooth muscle increases. Smooth muscle
encircles the lumen in spiral bands.
✓ Respiratory Bronchioles with alveoli out pocketing are directly connected to the alveolar ducts and
the alveoli.
✓ Alveoli are cup-shaped out pocketing lined by simple squamous epithelium while an alveolar sac
consists of two or more alveoli that shares the common opening.
✓ The simple squamous epithelium facilitates gas exchange between air spaces and capillary walls
which together forms the respiratory membrane.
Functional correlations
✓ The wall of alveoli consists of two types of cells:
