Overview
The bronchial tree is the system of air-conducting passages within the lung. It begins at the tracheal bifurcation and divides progressively until it terminates in respiratory bronchioles and alveoli.
The trachea divides at the carina (at the level of the lower border of the 4th thoracic vertebra / sternal angle of Louis) into two principal bronchi.
Levels of Division
| Level | Structure | Number | Notes |
|---|---|---|---|
| 1st | Principal (primary) bronchi | 2 (one per lung) | Right and left |
| 2nd | Lobar (secondary) bronchi | 5 (3 right + 2 left) | One per lobe |
| 3rd | Segmental (tertiary) bronchi | 10 right + 10 left = 20 | One per bronchopulmonary segment |
| 4th onwards | Subsegmental → terminal bronchioles | Many | No cartilage in terminal bronchioles |
| Terminal | Respiratory bronchioles | — | Gas exchange begins here |
| Final | Alveolar ducts → atria → alveolar sacs → alveoli | — | Main site of gaseous exchange |
Lobar Bronchi
Right Lung (3 lobar bronchi)
- Right superior lobar bronchus (also called eparterial bronchus — lies above the pulmonary artery)
- Right middle lobar bronchus
- Right inferior lobar bronchus
Left Lung (2 lobar bronchi)
- Left superior lobar bronchus
- Left inferior lobar bronchus
The left superior lobe is larger than its right counterpart because it corresponds to both the right superior and right middle lobes. Accordingly, the left superior lobar bronchus is larger and divides into an upper division and a lingular division.
Segmental Bronchi
Each lobar bronchus divides into segmental (tertiary) bronchi — one for each bronchopulmonary segment:
Right Lung
- Right superior lobar bronchus → 3 segmental bronchi (apical, posterior, anterior)
- Right middle lobar bronchus → 2 segmental bronchi (lateral, medial)
- Right inferior lobar bronchus → 5 segmental bronchi (superior/apical, medial basal, anterior basal, lateral basal, posterior basal)
Left Lung
- Left superior lobar bronchus (upper division) → 3 segmental bronchi (apical, posterior, anterior)
- Left superior lobar bronchus (lingular division) → 2 segmental bronchi (superior lingular, inferior lingular)
- Left inferior lobar bronchus → 5 segmental bronchi (superior/apical, medial basal, anterior basal, lateral basal, posterior basal)
Note: In the left lung, the medial basal bronchus is usually a small branch of the anterior basal bronchus — they may arise from a common stem called the anteromedial basal bronchus.
Terminal Airways
The segmental bronchi divide repeatedly to form:
- Terminal bronchioles — final purely conducting airways (no cartilage, no smooth muscle glands)
- Respiratory bronchioles — begin gaseous exchange; each aerates a pulmonary unit (pulmonary lobule)
- Alveolar ducts → Atria → Alveolar sacs → Alveoli — primary sites of gaseous exchange