Structure
The airway divides repeatedly from the trachea to the terminal bronchioles, with each generation narrower and more numerous than the last; the classic model describes about 23 generations down to the alveolar sacs.
Trachea and carina
The trachea begins at the lower border of the cricoid cartilage at C6 and descends in the midline to divide into the two main bronchi. Its wall is held open by C-shaped rings of hyaline cartilage, closed behind by the smooth muscle of trachealis. The carina is the keel-shaped ridge of cartilage at the bifurcation, seen at bronchoscopy as the sharp dividing line between the two bronchial openings. Its mucosa is very sensitive and triggers coughing.
Main bronchi
| Feature | Right main bronchus | Left main bronchus |
|---|---|---|
| Length | Short (about 2.5 cm) | Longer (about 5 cm) |
| Calibre | Wider | Narrower |
| Angle | More vertical, nearly in line with the trachea | More horizontal |
| Key relations | Azygos vein arches over it; gives the upper lobe bronchus before entering the hilum | Passes under the aortic arch, in front of the oesophagus and descending aorta |
| Lobar bronchi | Upper, middle, lower | Upper (with lingular), lower |
The right upper lobe bronchus arises above the level of the pulmonary artery and is called the eparterial bronchus. Beyond it the right main bronchus continues as the bronchus intermedius, which divides into the middle and lower lobe bronchi.
Segmental bronchi and beyond
Each lobar bronchus divides into segmental bronchi, one for each bronchopulmonary segment. Cartilage becomes irregular plates in the bronchi and disappears in the bronchioles, whose walls are mostly smooth muscle. The terminal bronchioles end the conducting zone; respiratory bronchioles, alveolar ducts and alveolar sacs form the respiratory zone.
Bronchopulmonary segments
A bronchopulmonary segment is the part of a lung supplied by one segmental bronchus and its accompanying segmental branch of the pulmonary artery; it is pyramidal, apex to the hilum, and surgically removable.
The segmental bronchus and artery run centrally in the segment, while the pulmonary veins run in the planes between segments. The right lung has 10 segments. The left lung is usually described with 8 to 10, because the apical and posterior segments share a bronchus, and the anterior and medial basal segments often do too.
| Lobe | Right lung | Left lung |
|---|---|---|
| Upper | Apical, posterior, anterior | Apicoposterior, anterior, superior lingular, inferior lingular |
| Middle | Lateral, medial | None (the lingula is part of the upper lobe) |
| Lower | Superior, medial basal, anterior basal, lateral basal, posterior basal | Superior, anteromedial basal, lateral basal, posterior basal |
Relations
The trachea and main bronchi lie in the mediastinum surrounded by the great vessels, the oesophagus and the lymph nodes that drain the lungs.
- Trachea: the oesophagus lies behind it, with the recurrent laryngeal nerves in the grooves between them. The brachiocephalic trunk and left common carotid artery cross in front, and the aortic arch lies in front and to the left of its lower end.
- Right main bronchus: the azygos vein arches over it to reach the superior vena cava; the right pulmonary artery lies in front of it, below the upper lobe bronchus.
- Left main bronchus: the aortic arch hooks over it, with the left recurrent laryngeal nerve passing below the arch; the oesophagus and descending aorta lie behind.
- Below the carina: the subcarinal lymph nodes and, just below them, the left atrium.
Clinical relevance
The shape and relations of the bronchial tree predict where inhaled material goes, where tubes end up, and how tumours and enlarged nodes show themselves.
- Foreign bodies: usually enter the right main bronchus because it is wider and more vertical.
- Aspiration pneumonia: in a supine patient, material runs into the posterior segment of the right upper lobe and the superior segment of the right lower lobe; in an upright patient, into the basal segments of the right lower lobe.
- Endobronchial intubation: an endotracheal tube passed too far usually enters the right main bronchus, collapsing the left lung and sometimes the right upper lobe.
- Widened carina: subcarinal lymphadenopathy or an enlarged left atrium splays the angle of the carina, visible on radiograph or bronchoscopy.
- Lung resection: segmentectomy is possible because each segment has its own bronchus and artery.
- Bronchial carcinoma: spreads through hilar and mediastinal nodes and may compress the recurrent laryngeal nerve, causing hoarseness.