Respiratory System Divisions

Divisions of Respiratory System

• The respiratory system has two divisions: the conducting system and the respiratory portion.
• The conducting system conducts, filters, and conditions inspired air before it reaches the lung.
• The respiratory portion is responsible for the exchange of gases.

Trachea

• The trachea is approximately 12 cm long and extends from the larynx above to the bifurcation into two bronchi in the lower end.
• The trachea is kept patent with 20 C-shaped, incomplete rings of hyaline cartilage.

Microscopic Structure of the Trachea

• Mucosa (epithelium and lamina propria):
  • Epithelium: pseudostratified columnar ciliated cells with goblet cells.
  • Goblet cells secrete mucus.
  • Cilia move up dust particles.
  • Lamina propria: rich in elastic fibers.
• Submucosa:
  • Connective tissue with mixed mucoserous glands.
  • Elastic membrane separating it from the lamina propria.
• The fibrocartilagenous layer:
  • Connective tissue with incomplete C-shaped hyaline cartilage.
  • Deficient posteriorly towards the esophagus.
  • Smooth muscle fibers between the cartilage ends.

Bronchi

• Extrapulmonary bronchi are similar in structure to the trachea.
• The right lung has 3 lobes, while the left lung has 2 lobes.

Microscopic Structure of the Lung

• Intrapulmonary bronchus:
  • Mucosa: folded.
  • Submucosa: continuous thin sheath of smooth muscle.
  • Fibrocartilagenous layer: multiple plates of cartilage and glands.

Bronchioles

• Mucosa:
  • Large bronchioles: simple columnar ciliated epithelium with goblet cells.
  • Small bronchioles: simple cuboidal ciliated epithelium with few Clara cells, no goblet cells.
• Muscle layer: well-developed smooth muscle layer.
• C.T layer: no cartilage or glands.

Respiratory Bronchiole

• Lined with cuboidal cells.
• Numerous alveoli open into it, and a few alveoli open into them.

Alveoli

• Polyhedral sacs with one wall deficient.
• Lined with alveolar epithelium.
• Separated by interalveolar septa, which contain:
  • Elastic fibers.
  • Reticular fibers.
  • Pulmonary alveolar phagocytic cells.
  • Blood capillaries.

Alveolar Epithelium

• Type I alveolar cells:
  • Flattened cells (97% of alveolar surface).
  • Small amount of cytoplasm.
  • Function: provide minimum thickness for diffusion of gases and share in blood-air barrier.
• Type II alveolar cells:
  • Large cuboidal or rounded cells (3% of alveolar epithelium).
  • Apical surface contains microvilli.
  • Vesicular nucleus.
  • Cell organoids.
  • Function: secrete surfactant, which reduces the surface tension of alveoli, preventing them from collapsing at the end of expiration.