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AN24.1-6 | Lungs & Trachea — Part 1
CLINICAL SCENARIO
A 45-year-old bus driver from Chennai presents to casualty with sudden onset breathlessness and chest pain after a road traffic accident. On examination, trachea is deviated to the right, breath sounds absent on the left, and neck veins are distended. Blood pressure 90/60 mmHg and falling.
What is the diagnosis? What anatomy explains the tracheal deviation and absent breath sounds? Why is this a surgical emergency?
Tension pneumothorax — the anatomy you learn today will make you the doctor who diagnoses this in thirty seconds.
WHY THIS MATTERS
The lungs and trachea are your most-examined structures in Year 1 — and the most life-saving to understand clinically. In Indian practice:
- Pulmonary tuberculosis — India carries 26% of the global TB burden. Anatomy of apical segments explains why TB favours the upper lobes
- Pneumonia — knowing segmental anatomy predicts consolidation patterns on CXR and CT
- Pneumothorax and haemothorax — pleural recess anatomy guides chest drain insertion
- Bronchogenic carcinoma — India's most common thoracic malignancy, understood through hilum and segmental anatomy
- Bronchoscopy — requires precise knowledge of bronchial tree anatomy
Cross-reference PY: The pulmonary circulation (low pressure, high volume), surfactant (type II pneumocytes), and ventilation-perfusion ratios all depend on this anatomical foundation.
RECALL
Before we begin, recall:
- The thoracic cage is formed by 12 thoracic vertebrae, 12 pairs of ribs, and the sternum
- The mediastinum separates the two pleural cavities
- The trachea begins at C6 (lower border of cricoid) and enters the thorax through the superior mediastinum
- The sternal angle (T4/T5) is where the trachea bifurcates at the carina
Trachea (AN24.1)
The trachea is a fibrocartilagenous tube, 10–11 cm long, 2 cm diameter, extending from C6 (lower border of cricoid cartilage) to T4/T5 (carina).
Structure: 16–20 C-shaped hyaline cartilage rings — open posteriorly. The gap is bridged by the trachealis muscle (smooth muscle) — allows the oesophagus to bulge anteriorly during swallowing.
Relations in the neck (cervical trachea):
- Anterior: isthmus of thyroid (over rings 2–4), strap muscles
- Posterior: oesophagus
- Lateral: lobes of thyroid, carotid sheaths
Relations in the thorax (thoracic trachea):
- Anterior: arch of aorta, brachiocephalic artery, left common carotid artery
- Posterior: oesophagus (slightly right of midline)
- Right: azygos arch, right vagus
- Left: arch of aorta, left recurrent laryngeal nerve (in tracheo-oesophageal groove)
Bifurcation at the carina (T4/T5):
- The carina is the internal ridge at the bifurcation — its widening on CXR (>70°) suggests subcarinal lymphadenopathy (TB, lymphoma, malignancy)
- Right main bronchus: wider (2.5 cm diameter), shorter (2.5 cm), more vertical (25°) → foreign bodies go right
- Left main bronchus: narrower (2 cm), longer (5 cm), more oblique (45°)
Blood supply: Inferior thyroid artery (cervical part); bronchial arteries (thoracic part).
Nerve supply: Recurrent laryngeal nerves (motor, sensory); sympathetic (trachealis muscle).
Clinical:
- Tracheostomy is performed between rings 2–4 (below the cricoid), avoiding the isthmus — or between rings 1–2 for emergency (cricothyrotomy)
- Intubation check: right mainstem intubation is the most common error (right bronchus wider/more vertical) — causes left lung collapse
Bronchial Tree & Bronchopulmonary Segments (AN24.2, AN24.3)
Divisions of the Bronchial Tree:
Trachea → Main (primary) bronchi → Lobar (secondary) bronchi → Segmental (tertiary) bronchi → bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveoli
RIGHT LUNG (3 lobes, 10 segments):
| Lobe | Lobar bronchus | Segments |
|---|---|---|
| Upper | Right upper lobe bronchus | Apical (S1), Posterior (S2), Anterior (S3) |
| Middle | Right middle lobe bronchus | Lateral (S4), Medial (S5) |
| Lower | Right lower lobe bronchus | Superior (S6), Medial basal (S7), Anterior basal (S8), Lateral basal (S9), Posterior basal (S10) |
LEFT LUNG (2 lobes, 8–10 segments):
| Lobe | Segments |
|---|---|
| Upper | Apicoposterior (S1+2), Anterior (S3), Superior lingula (S4), Inferior lingula (S5) |
| Lower | Superior (S6), Anteromedial basal (S7+8), Lateral basal (S9), Posterior basal (S10) |
Note: The left lung has no middle lobe — the lingula (part of the upper lobe) is its functional equivalent.
Bronchopulmonary Segment: Each is a wedge-shaped, anatomically and functionally independent unit. It has its own:
- Segmental bronchus
- Segmental branch of pulmonary artery
- Visceral pleura on its surface
The pulmonary veins run between segments (intersegmental) — this allows surgical resection (segmentectomy) of a single segment without removing the whole lobe.
Why TB favours the apex: The apical and posterior segments of the upper lobe (S1, S2) have the highest O₂ tension (ventilation exceeds perfusion at the apex) — Mycobacterium tuberculosis is an obligate aerobe → it thrives at the apex. Cross-ref PY respiratory physiology: V/Q ratio is highest at the apex.
SELF-CHECK
A. Right upper lobe — apical segment
B. Right lower lobe — superior segment (S6)
C. Left upper lobe — apicoposterior segment
D. Right middle lobe — lateral segment
Reveal Answer
Answer: .
When supine, the superior segment of the right lower lobe (S6) is the most dependent posterior segment. Aspiration in the supine position preferentially affects S6 right lower lobe. When erect, the basal segments are most dependent.
Hilum of the Lung (AN24.4)
The hilum is the triangular depressed area on the mediastinal surface of each lung through which its root passes.
Structures at the hilum (lung root):
1. Main bronchus
2. Pulmonary artery
3. Pulmonary veins (superior and inferior — 2 veins each side)
4. Bronchial arteries and veins
5. Lymphatic vessels
6. Autonomic nerve plexus
Arrangement at the hilum — a key exam fact:
Right hilum (anterior to posterior):
- Pulmonary veins (anterior), Pulmonary artery (posterior)
- Superior to inferior: Upper lobe bronchus → Pulmonary artery → Middle + Lower lobe bronchi
Left hilum:
- Pulmonary artery arches over the left main bronchus (left PA is superior to the bronchus)
- Mnemonic for left hilum: "VEAL" — Veins anteriorly, artery Above the bronchus, Lymphatics, nerve plexus
Hilum relations:
- Right hilum: azygos vein arches over it; right phrenic nerve in front
- Left hilum: aortic arch + ligamentum arteriosum above; left phrenic nerve in front
Clinical: Hilar lymphadenopathy (bilateral) on CXR in a young adult in India = sarcoidosis or primary TB until proven otherwise. Unilateral hilar enlargement = malignancy until proven otherwise.