How Air Enters the Lungs: The Respiratory Pathway

The Journey of a Breath

Air follows a precise anatomical path from atmosphere to alveoli:

1. Nose/Nares (preferred) or Mouth – filtration, warming, humidification.
2. Nasopharynx / Oropharynx – shared with digestive tract.
3. Laryngopharynx → Larynx – past epiglottis (protects airway during swallowing).
4. Trachea – 10–12 cm, C-shaped cartilages, ciliated pseudostratified epithelium (mucociliary escalator).
5. Carina (T4/T5) → Right/Left Main Bronchi.
6. Bronchial Tree: Lobar (3 right, 2 left) → Segmental → Subsegmental → Bronchioles (no cartilage).
7. Terminal Bronchioles → Respiratory Bronchioles (alveoli budding) → Alveolar Ducts → Alveolar Sacs → Alveoli.

Key Numbers

• ~23 generations of branching.
• Alveoli: ~300–500 million; surface area 70–100 m².
• Dead space (conducting airways): ~150 mL.

Gas Exchange

Occurs across alveolar-capillary membrane (Type I pneumocyte + endothelium) by diffusion down partial pressure gradients (O₂ in, CO₂ out).

Clinical Relevance

• Obstruction at any level (foreign body, tumor, edema, bronchospasm) impairs ventilation.
• Intubation, tracheostomy bypass upper airway.
• Aerosol delivery targets specific generations by particle size.