Pulmonary Alveoli: Structure, Cellular Composition, and Gas Exchange Function

Alveoli are the terminal gas-exchange units of the lung. ~300–500 million per adult; total surface area ~70–100 m².

Histologic Features

• Type I pneumocytes (95% surface): Squamous, ultra-thin (0.1 µm), gas-permeable; cannot divide.
• Type II pneumocytes (5% surface, 60% cells): Cuboidal, produce surfactant (dipalmitoylphosphatidylcholine + SP-A/B/C/D); progenitors for type I.
• Capillary endothelium: Continuous, fenestrated; basement membrane fused with type I (blood-gas barrier 0.3–0.5 µm).
• Alveolar macrophages (dust cells): Phagocytose debris, pathogens; secrete cytokines.
• Porores of Kohn: Interalveolar connections for collateral ventilation.

Surfactant Function

• Reduces surface tension (LaPlace’s law: P = 2T/r) → prevents collapse at low volume (atelectasis).
• Deficiency → neonatal RDS (hyaline membrane disease).
• Inactivation → ARDS, pneumonia.

Gas Exchange

• O₂ diffuses down gradient (PAO₂ ~100 → PaO₂ ~95 mmHg).
• CO₂ diffuses out (PaCO₂ ~40 → PACO₂ ~40 mmHg).
• Perfusion-limited for O₂ (normally); diffusion-limited in disease/exercise.

Pathology

• Emphysema: Alveolar wall destruction → ↓ surface area.
• Fibrosis: Thickened barrier → diffusion defect.
• Edema: Fluid-filled alveoli → shunt.
• Cancer: Adenocarcinoma (type II origin).

Alveolar integrity is essential for life; surfactant and capillary apposition are evolutionary marvels.