Trachea and Bronchial Tree

Fibrocartilaginous tube extending from the lower border of the cricoid cartilage (approximately C6 vertebral level) to the tracheal bifurcation at the level of the sternal angle/T4-T5 intervertebral disc (in a person of average build in the supine, resting position, the level can shift with posture and respiration).

• Composed of 15–20 incomplete (C-shaped) cartilaginous rings, open posteriorly, bridged by the trachealis muscle (smooth muscle), which allows for some flattening/narrowing (e.g., during coughing) and accommodates the closely related oesophagus immediately posterior.
• Approximately 10–12 cm in length in an adult, roughly half in the neck and half in the thorax.

• Anterior: In the neck, isthmus of the thyroid gland (crossing anterior to the 2nd–4th tracheal rings), inferior thyroid veins. In the thorax, manubrium, thymic remnants (variable), left brachiocephalic vein, aortic arch and its branches (brachiocephalic trunk crosses anteriorly at a slightly lower level).
• Posterior: Oesophagus, throughout its entire length, the trachealis muscle's posterior positioning allows the oesophagus to bulge slightly into the trachea's posterior wall during swallowing without airway obstruction.
• Lateral: In the neck, common carotid arteries, lobes of the thyroid gland. In the thorax, right and left mediastinal pleura, right vagus nerve, arch of the azygos vein (right side), aortic arch and left recurrent laryngeal nerve (left side).
• Recurrent laryngeal nerves: Ascend in the tracheo-oesophageal groove on both sides, closely applied to the trachea's lateral surface, a critical relation for thyroid and tracheal surgery, given the risk of nerve injury.

• Occurs at the sternal angle/T4-T5 level (a key correlate of this important surface landmark, alongside the aortic arch limits and 2nd costal cartilage).
• The internal ridge at the bifurcation, where the trachea divides into right and left main bronchi, is called the carina.

• A sharp, internal cartilaginous ridge, visible on bronchoscopy, marking the point of tracheal division.
• Richly innervated and highly sensitive: irritation here (by foreign body, mucus, or bronchoscope) triggers a particularly vigorous cough reflex.
• Widening or distortion of the carinal angle on imaging is a clinically important sign, classically associated with subcarinal lymphadenopathy (e.g., from bronchogenic carcinoma metastasis, lymphoma) pushing the two main bronchi apart.

• The right main bronchus additionally gives off the right superior lobar (eparterial) bronchus before it passes below/behind the right pulmonary artery (hence "eparterial", above the artery), a distinguishing anatomical feature not present on the left (where the entire left main bronchus and all its branches lie below/behind the pulmonary artery, i.e., "hyparterial").

• Right side: Three lobar (secondary) bronchi, right superior, middle, and inferior lobar bronchi, corresponding to the three lobes of the right lung.
• Left side: Two lobar bronchi, left superior and inferior lobar bronchi, corresponding to the two lobes of the left lung.

• Each lobar bronchus divides further into segmental (tertiary) bronchi, each supplying an individual bronchopulmonary segment.
• Segmental bronchi continue to divide into progressively smaller bronchioles, terminal bronchioles, respiratory bronchioles, alveolar ducts, and alveoli, the conducting-to-respiratory airway transition, generally covered in histology/physiology rather than gross anatomy in depth.

Right lung (typically 10 segments):

Left lung (typically 8–10 segments, depending on classification: apical and posterior segments, and anterior and medial basal segments, are often fused/combined):

The left lung's fused apicoposterior and anteromedial basal segments reflect its embryological/comparative reduction relative to the right lung's fully separate 10-segment pattern, a commonly tested asymmetry.

• Systemic vessels (from the descending thoracic aorta, or occasionally intercostal arteries) supplying the bronchial tree walls, connective tissue, and visceral pleura down to the level of the respiratory bronchioles (beyond which the pulmonary circulation takes over local tissue nutrition via diffusion).
• Follow the branching pattern of the bronchial tree.

• Carry deoxygenated blood for gas exchange, following the same branching pattern as the bronchial tree (bronchovascular bundles), down to the level of the alveolar capillaries.

• Drain oxygenated blood from the alveolar capillary beds; characteristically run an intersegmental course (between rather than within segments), making them useful surgical landmarks for identifying segmental boundaries during lung resection.

• Follow the bronchial tree centripetally: peribronchial and bronchopulmonary (hilar) nodes → tracheobronchial nodes (grouped around the tracheal bifurcation/carina, superior and inferior tracheobronchial groups) → paratracheal nodes → bronchomediastinal trunks → venous system.
• The tracheobronchial (carinal/subcarinal) node group is a particularly important site for staging bronchogenic carcinoma, given its central position receiving drainage from both lungs.

• Parasympathetic (vagus nerve): Bronchoconstriction, increased mucous gland secretion, vasodilation of bronchial vessels.
• Sympathetic (thoracic sympathetic trunk, via the pulmonary plexus): Bronchodilation, vasoconstriction.
• Both contribute to the pulmonary plexus at the lung root, from which fibres distribute along the bronchial tree alongside the airways and vessels.

Aspirated foreign bodies:

• Preferentially lodge in the right main bronchus, given its shorter length, wider calibre, and more vertical orientation, a near-direct continuation of the tracheal axis compared to the more angled left main bronchus.
• Within the right lung, the exact lodging site depends on patient position at the time of aspiration (supine vs upright) and the size of the foreign body, larger objects may lodge in the main bronchus itself, smaller objects may pass further into a lobar or segmental bronchus.

Bronchoscopy anatomy:

• The carina serves as the primary internal orientation landmark during bronchoscopy, marking the division into right and left main bronchi.
• Systematic examination proceeds from the carina into each main bronchus, then sequentially into each lobar and segmental bronchus, using the predictable branching pattern to identify and biopsy specific segments.
• The right upper lobe (eparterial) bronchus is anatomically distinctive as the first branch encountered on the right, arising very close to the carina, a key orientation point.

Carinal widening:

• A widened, flattened, or distorted carinal angle on chest imaging or bronchoscopy is a classic sign of subcarinal lymphadenopathy, most commonly from metastatic spread of bronchogenic carcinoma to the tracheobronchial/subcarinal lymph node group, though also seen with lymphoma or other mediastinal masses.
• This finding carries significant diagnostic and staging implications in suspected lung malignancy.