Airflow Pathway in the Respiratory System: From Nose to Alveoli and Back

Correct Sequence of Air Movement

Air does not go to alveoli first. The pathway is a continuous, bidirectional flow:

1. Nose or Mouth → entry; nasal cavity warms, humidifies, filters.
2. Pharynx (nasopharynx, oropharynx, laryngopharynx) → common airway/food passage.
3. Larynx (voice box) → epiglottis protects airway during swallowing.
4. Trachea (windpipe) → C-shaped cartilage rings keep it patent.
5. Carina → bifurcation into right and left main bronchi.
6. Main Bronchi → right (wider, shorter, more vertical) and left.
7. Lobar (Secondary) Bronchi → 3 right, 2 left.
8. Segmental (Tertiary) Bronchi → 10 right, 8–10 left (bronchopulmonary segments).
9. Bronchioles → <2 mm diameter, no cartilage, smooth muscle.
10. Terminal Bronchioles → last purely conducting generation.
11. Respiratory Bronchioles → first gas-exchange units (alveolar buds).
12. Alveolar Ducts and Sacs → clusters of alveoli.
13. Alveoli → ~300–500 million; site of gas exchange.

Gas Exchange and Return

O₂ diffuses alveolar air → capillary blood; CO₂ diffuses capillary blood → alveolar air. Then air retraces the path backward:

Alveoli → respiratory bronchioles → terminal bronchioles → bronchioles → bronchi → trachea → larynx → pharynx → nose/mouth.

Key Points

• Conducting zone (nose → terminal bronchioles): no gas exchange; warms, humidifies, filters, conducts.
• Respiratory zone (respiratory bronchioles → alveoli): gas exchange occurs.
• Dead space: Conducting zone volume (~150 mL) where no exchange occurs.
• Mucociliary escalator: Cilia move mucus + trapped particles toward pharynx for swallowing.

Understanding this anatomy is fundamental for pulmonology, anesthesiology, critical care, and respiratory therapy.