The Lung
The lungs are the organs of gas exchange. Their structure transitions from conducting airways (moistening/filtering air) to the respiratory zone (gas exchange).
Histological Progression
| Structure | Epithelial Transition |
|---|---|
| Trachea/Bronchi | Pseudostratified ciliated columnar with goblet cells. |
| Bronchioles | Transition to simple ciliated cuboidal; goblet cells absent. |
| Alveoli | Simple squamous epithelium (for efficient diffusion). |
Alveolar Cell Types
- Type I Pneumocytes: Occupy ~95% of the alveolar surface; ultra-thin, dedicated to gas exchange.
- Type II Pneumocytes: Cuboidal cells; store and secrete surfactant (reduces surface tension) and serve as stem cells for Type I/II repair.
- Alveolar Macrophages: Derived from monocytes; “dust cells” that phagocytose particles.
High-Yield Exam Pearl:
Surfactant is primarily composed of dipalmitoylphosphatidylcholine (DPPC). Deficiency in neonates leads to Neonatal Respiratory Distress Syndrome (NRDS), characterized by atelectasis (alveolar collapse) and ground-glass opacities on imaging.