Trachea to Lungs: Airway Anatomy & Airflow Pathway

Airway Anatomy: Trachea to Alveoli

The trachea (windpipe) is a 10–12 cm tube (D-shaped, C-shaped cartilage rings posteriorly open) conducting air between larynx and bronchi.

Branching Architecture

1. Trachea → bifurcates at carina (T4/T5)
2. Right/Left Main Bronchi: Right wider, shorter, more vertical (aspiration risk)
3. Lobar (Secondary) Bronchi: 3 right, 2 left
4. Segmental (Tertiary) Bronchi: 10 right, 8–10 left (bronchopulmonary segments)
5. Bronchioles: <2 mm, no cartilage, smooth muscle
6. Terminal → Respiratory Bronchioles → Alveolar Ducts → Alveolar Sacs → Alveoli

Key Structural Transitions

• Cartilage rings → plates → absent (bronchioles)
• Pseudostratified ciliated epithelium → simple cuboidal → simple squamous (alveoli)
• Goblet cells → Clara cells → none
• Smooth muscle ↑ distally (bronchomotor tone)

Functional Zones

• Conducting zone (trachea – terminal bronchioles): No gas exchange; dead space ~150 mL
• Respiratory zone (respiratory bronchioles – alveoli): Gas exchange; surface area ~70 m²

Clinical Correlates

• Aspiration → right lower lobe
• Asthma/COPD: Bronchiolar smooth muscle constriction + inflammation
• Foreign body: Right main bronchus
• Carinal tumors: Tracheal/bronchial obstruction

23 generations of branching create a vast, delicate gas-exchange interface.