Airway Anatomy: From Trachea to Alveoli for Gas Exchange

The conducting airways form a branching tree that delivers air to the respiratory zone where gas exchange occurs.

Structural Pathway

1. Trachea (Windpipe): 10–12 cm long, 2–2.5 cm diameter; 16–20 C-shaped hyaline cartilage rings (prevent collapse), posterior membranous wall (smooth muscle). Lined by pseudostratified ciliated columnar epithelium with goblet cells (mucociliary escalator).
2. Carina: Tracheal bifurcation at T4/T5 level into right and left main bronchi.
3. Main Bronchi:
   • Right: wider, shorter, more vertical (aspirated objects more likely).
   • Left: narrower, longer, passes under aortic arch.
4. Lobar (Secondary) Bronchi: 3 right (upper, middle, lower), 2 left (upper, lower).
5. Segmental (Tertiary) Bronchi: 10 per lung (bronchopulmonary segments – surgically resectable units).
6. Bronchioles: <1 mm diameter; no cartilage; smooth muscle regulates airflow. Terminal → respiratory bronchioles (alveolar outpouchings begin).
7. Alveolar Ducts → Alveolar Sacs → Alveoli: ~300–500 million alveoli; total surface area ~70 m². Walls: type I pneumocytes (gas exchange), type II (surfactant), capillary endothelium, fused basement membrane (0.5 μm barrier).

Gas Exchange

O₂ diffuses from alveolar air (PAO₂ ~100 mmHg) → capillary blood (PaO₂ ~40 mmHg). CO₂ diffuses opposite (PACO₂ ~40 mmHg → PaCO₂ ~45 mmHg). Driven by partial pressure gradients; perfusion-limited for O₂, diffusion-limited in disease.

Clinical Relevance

• Asthma/COPD: Bronchiolar smooth muscle constriction, inflammation, mucus → airflow obstruction.
• Pneumonia: Alveolar filling (consolidation) impairs gas exchange.
• ARDS: Diffuse alveolar damage, surfactant loss, hyaline membranes.
• Intubation: Tube passes trachea → main bronchus (risk of right mainstem intubation).