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Alveoli: How Tiny Air Sacs Enable Efficient Gas Exchange

Alveoli are the terminal gas-exchange units of the lung—~480 million in adults, creating ~100 m² surface area. Their structure is exquisitely optimized for diffusion.

Structural Adaptations for Diffusion

  1. Enormous surface area: 70–100 m² (tennis court) from millions of 200–300 µm sacs
  2. Minimal diffusion distance: Alveolar epithelium (type I pneumocytes) + capillary endothelium + fused basement membranes = 0.2–0.5 µm total
  3. Dense capillary network: Each alveolus wrapped in capillary mesh; RBCs flatten against wall
  4. Surfactant (type II pneumocytes): Dipalmitoylphosphatidylcholine reduces surface tension → prevents collapse (atelectasis) at low volumes
  5. Type I pneumocytes: Cover 95% surface; ultra-thin (0.1 µm) for gas exchange
  6. Type II pneumocytes: Produce surfactant; progenitor cells for repair

Gas Exchange Physics (Fick's Law)

Diffusion rate ∝ (Surface Area × ΔP × Solubility) / (Thickness × √Molecular Weight)

  • O₂: Alveolar PₐO₂ ~100 mmHg → capillary PᵥO₂ ~40 mmHg → ΔP = 60 mmHg
  • CO₂: Capillary PᵥCO₂ ~45 mmHg → alveolar PₐCO₂ ~40 mmHg → ΔP = 5 mmHg
  • CO₂ diffuses 20× faster than O₂ (higher solubility) despite smaller gradient

Hemoglobin's Role

  • Each Hb binds 4 O₂ cooperatively (sigmoidal dissociation curve)
  • Loading in lungs (high PO₂, high pH, low CO₂, low 2,3-BPG)
  • Unloading in tissues (low PO₂, low pH, high CO₂, high 2,3-BPG — Bohr effect)

Ventilation-Perfusion Matching

  • Ideal V̇/Q̇ = 1.0
  • Gravity creates gradients: base better perfused, apex better ventilated
  • Hypoxic pulmonary vasoconstriction redirects flow from poorly ventilated areas

Clinical Correlates

  • ARDS: Surfactant loss, edema, membrane thickening → diffusion impairment
  • Emphysema: Alveolar wall destruction → ↓ surface area, ↑ diffusion distance
  • Pulmonary fibrosis: Interstitial thickening → diffusion block
  • High altitude: Low PₐO₂ → ↓ driving pressure

Alveolar architecture is a masterpiece of evolutionary engineering—maximizing gas exchange while minimizing work of breathing.

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