Lung Anatomy & Bronchopulmonary Segments

The lungs are the paired organs of respiration occupying the lateral compartments of the thoracic cavity, separated by the mediastinum. This topic is a perennial favourite in NEET PG because it integrates pure anatomy (lobes, fissures, hilar structures, bronchopulmonary segments) with high-yield clinical correlates — foreign body aspiration, pleural tap sites, and bronchoscopy orientation.

Surface anatomy & gross features

Each lung is conical, with an apex projecting 2.5–4 cm above the medial third of the clavicle into the root of the neck (cervical pleura, vulnerable in subclavian vein cannulation), a concave base resting on the diaphragm, three borders (anterior, posterior, inferior) and three surfaces (costal, mediastinal, diaphragmatic).

The right lung is shorter (because of the right dome of the diaphragm pushed up by the liver) but broader and heavier. The left lung is longer but narrower (the heart bulges into it) and carries the cardiac notch on its anterior border, below which lies the tongue-like lingula (homologue of the right middle lobe).

Feature	Right lung	Left lung
Lobes	3 (upper, middle, lower)	2 (upper, lower)
Fissures	Oblique + horizontal	Oblique only
Bronchopulmonary segments	10	8 (some say 9–10 by fusion)
Special features	More vertical & shorter main bronchus	Cardiac notch, lingula, lighter
Impressions (mediastinal surface)	SVC, azygos, oesophagus, right atrium	Arch & descending aorta, left ventricle

High-yield: The right lung has 3 lobes and 2 fissures (oblique + horizontal); the left lung has 2 lobes and 1 (oblique) fissure. The cardiac notch and lingula are exclusively LEFT.

Fissures

• Oblique (major) fissure — both lungs. Surface marking: runs along the line of the 6th costal cartilage anteriorly to the spine of T3 (T2) posteriorly. Roughly follows the medial border of the scapula when the arm is fully abducted overhead.
• Horizontal (minor) fissure — right lung only. Runs from the oblique fissure in the midaxillary line forwards along the 4th costal cartilage / 4th rib to the sternum. Separates the right upper lobe from the right middle lobe.

High-yield: A right-sided horizontal fissure is the classic landmark on a frontal chest radiograph; loss/upward displacement suggests right upper lobe collapse.

The lung root and the hilum (structure order)

The root of the lung is the pedicle connecting the lung to the mediastinum; the hilum is the depression where the root structures enter/leave. The root is enclosed in a sleeve of pleura that hangs inferiorly as the pulmonary ligament (allows distension of pulmonary veins and movement during respiration).

Anteroposterior order at the hilum (front → back): Pulmonary vein → artery → bronchus. A useful learning frame is the AVAB sequence (Artery–Vein–Airway/Bronchus) once you fix the vertical relationships, but the most exam-tested arrangement is:

• Anterior to posterior: Pulmonary vein (anterior) → Pulmonary artery (middle) → Bronchus (posterior).
• Superior to inferior (right hilum): Eparterial bronchus (to upper lobe) is the highest structure → pulmonary artery → hyparterial bronchus → inferior pulmonary vein lowest.
• Superior to inferior (left hilum): Pulmonary artery is the highest structure → bronchus → inferior pulmonary vein lowest.

High-yield: On the RIGHT, the bronchus (eparterial) is the most superior hilar structure. On the LEFT, the pulmonary artery is the most superior. Mnemonic: RALS at the level of the carina describes the great-vessel relationship — Right Anterior, Left Superior pulmonary artery to the bronchus. So on the left the artery arches over the bronchus (highest); on the right it lies anterior, leaving the bronchus on top.

Mnemonic for hilum AP order — "VAB": Vein, Artery, Bronchus from front to back. The inferior pulmonary vein is always the lowest structure in both hila.

Structures crossing the roots

• Right root: crossed superiorly by the azygos vein arching forward to the SVC; the right phrenic and vagus nerves descend in front of and behind the root respectively.
• Left root: crossed superiorly by the arch of the aorta, with the ligamentum arteriosum and the recurrent laryngeal branch of the left vagus hooking under the arch. The left phrenic and vagus relations mirror the right (phrenic anterior, vagus posterior).

High-yield: The left recurrent laryngeal nerve hooks under the aortic arch lateral to the ligamentum arteriosum — explains hoarseness (Ortner syndrome) in mitral stenosis with enlarged left atrium / aortic aneurysm / left hilar (bronchogenic) carcinoma.

Bronchial tree & bronchopulmonary segments

The trachea bifurcates at the carina (level of sternal angle / T4–T5, lower border of T4) into right and left principal (main) bronchi.

Feature	Right main bronchus	Left main bronchus
Length	Shorter (~2.5 cm)	Longer (~5 cm)
Width	Wider	Narrower
Direction	More vertical	More horizontal
Aspiration	More common	Less common

Branching flow: Trachea → Right main bronchus → upper lobe (eparterial), then continues as intermediate bronchus → middle + lower lobe bronchi → segmental bronchi → conducting bronchioles → terminal bronchioles → respiratory bronchioles → alveolar ducts → alveolar sacs → alveoli.

A bronchopulmonary segment is the anatomical, functional and surgical unit of the lung — the portion supplied by a tertiary (segmental) bronchus and its accompanying segmental branch of the pulmonary artery. Key features:

1. Pyramidal, apex toward the hilum, base toward the surface.
2. Has its own segmental bronchus and pulmonary artery (artery is intersegmental → central, runs with the bronchus).
3. Pulmonary veins are intersegmental — they lie in the connective tissue planes between segments and drain adjacent segments. This is the surgical plane.
4. Each is independently resectable (segmentectomy) without sacrificing neighbours.

High-yield: In a bronchopulmonary segment, the bronchus and artery are central (segmental/intrasegmental), while the veins are peripheral/intersegmental. The vein marks the dissection plane.

Segment list (must memorise)

Right lung (10 segments):

• Upper lobe (3): Apical, Posterior, Anterior.
• Middle lobe (2): Lateral, Medial.
• Lower lobe (5): Superior (apical), Medial basal, Anterior basal, Lateral basal, Posterior basal.

Left lung (8–10 segments):

• Upper lobe: Apicoposterior (apical + posterior fused), Anterior, Superior lingular, Inferior lingular.
• Lower lobe: Superior, Anteromedial (anterior + medial basal fused), Lateral basal, Posterior basal.

Mnemonic for right lower lobe basal segments (anticlockwise): "Medial Anterior Lateral Posterior" → think MALP after the superior segment.

High-yield: The superior (apical) segment of the lower lobe is the commonest site for aspiration in a supine/recumbent patient and for lung abscess in such patients.

Clinical anatomy — the heavily tested correlates

1. Foreign body / aspiration patterns

Because the right main bronchus is wider, shorter and more vertical, aspirated material preferentially enters the right lung. The destination within the right lung depends on body position at the time of aspiration:

• Upright / sitting patient → posterior basal segment of the RIGHT LOWER LOBE (gravity-dependent when erect).
• Supine (lying on back) → superior (apical) segment of the RIGHT LOWER LOBE.
• Lying on the right side → posterior segment of the right upper lobe or superior segment of right lower lobe.

High-yield (one of the most asked PG questions): In an upright patient, an aspirated foreign body lodges in the posterior basal segment of the right lower lobe; in a supine patient it lodges in the superior (apical) segment of the right lower lobe. Right lower lobe is the overall commonest destination.

2. Pleura, pleural recesses & tap sites

Two layers — visceral (over lung, autonomic innervation, insensitive to pain) and parietal (lines cavity; somatic innervation — costal & peripheral diaphragmatic by intercostal nerves, central diaphragm & mediastinal by phrenic nerve). The potential space between is the pleural cavity with a thin film of serous fluid.

Pleural reflections (lower borders) — the "2-4-6-8-10-12 rule":

Structure	Midclavicular line	Midaxillary line	Scapular / paravertebral line
Lung lower border	6th rib	8th rib	10th rib (T10)
Pleura lower border	8th rib	10th rib	12th rib (T12)

The 2-rib gap between the lung edge and the pleural reflection forms the costodiaphragmatic recess — the deepest recess, where pleural fluid first collects and where a thoracocentesis is performed.

• Costodiaphragmatic recess: between costal and diaphragmatic parietal pleura. Site of pleural effusion accumulation and pleural tap.
• Costomediastinal recess: between costal and mediastinal pleura, deepest on the left behind the cardiac notch.

High-yield (procedure): A pleural tap (thoracocentesis) is done in the 9th intercostal space in the midaxillary line, OR in the scapular/posterior approach in the 7th–8th intercostal space. Always insert the needle along the UPPER border of the lower rib to avoid the intercostal vein–artery–nerve (VAN) bundle that runs in the costal groove of the upper rib.

High-yield: A chest tube (ICD) for pneumothorax / effusion is inserted in the "triangle of safety" — bordered by the lateral border of pectoralis major, anterior border of latissimus dorsi, and a horizontal line at the level of the nipple (5th intercostal space), in the midaxillary line.

3. Bronchoscopy orientation

On bronchoscopy the carina is a sharp sagittal ridge; a widened/splayed carina suggests subcarinal lymph node enlargement (e.g., bronchogenic carcinoma metastasis). The operator uses the carina and the more vertical right main bronchus to orient. The right upper lobe (eparterial) bronchus arises very early/high — first branch encountered on the right.

4. Lymphatic drainage & cancer spread

Drainage flows from pulmonary → bronchopulmonary (hilar) → tracheobronchial → paratracheal → bronchomediastinal trunks. Critically, lymph can cross to the opposite side, and the left lung lower lobe drains partly to the RIGHT superior tracheobronchial nodes — explaining contralateral spread of left lower lobe carcinoma.

5. Blood supply distinction

• Pulmonary arteries carry deoxygenated blood for gas exchange (functional circulation).
• Bronchial arteries (from the descending thoracic aorta, usually 1 right, 2 left) supply the bronchial tree down to respiratory bronchioles (nutritive circulation). Bronchial veins drain partly to azygos/hemiazygos and partly to pulmonary veins (a source of physiological shunt).

Investigation & approach (integration)

Stepwise approach to a suspected aspirated foreign body: Clinical (choking, unilateral wheeze, decreased air entry) → Chest X-ray (PA + lateral; look for radiopaque object, air-trapping/obstructive emphysema, or atelectasis) → CT chest if doubtful → Rigid bronchoscopy = diagnostic AND therapeutic (definitive removal).

High-yield: For foreign body removal in a child, RIGID bronchoscopy is the investigation and treatment of choice; flexible bronchoscopy is preferred for diagnostic sampling in adults.

For a pleural effusion: confirm with ultrasound (most sensitive for small effusions and for guiding the tap), then diagnostic thoracocentesis with Light's criteria to classify exudate vs transudate.

	Transudate	Exudate
Pleural/serum protein	< 0.5	> 0.5
Pleural/serum LDH	< 0.6	> 0.6
Pleural LDH	< 2/3 upper normal serum	> 2/3 upper normal serum
Typical cause	CHF, cirrhosis, nephrotic	TB, malignancy, parapneumonic

(Light's criteria — exudate if any one is met.)

Complications & differentials of the anatomical scenarios

• Missed/retained foreign body → recurrent pneumonia, post-obstructive lung abscess, bronchiectasis distal to obstruction, or check-valve obstructive emphysema.
• Pleural tap below the recommended level → injury to liver/spleen/diaphragm; tap along the lower border of a rib → neurovascular bundle injury and haemothorax.
• Iatrogenic apical pleural injury during subclavian/internal jugular cannulation → pneumothorax (apex projects above clavicle).
• Differentials of an opacity tracking to a fissure: loculated effusion ("pseudotumour" in the horizontal fissure in CHF) vs collapse vs mass.

Recently asked / exam angle

• Most-asked one-liner: Aspirated foreign body in an upright patient → posterior basal segment, right lower lobe; in a supine patient → superior (apical) segment, right lower lobe. Repeatedly tested across INI-CET / NEET PG.
• Site of pleural tap and the 2-rib rule (lung ends 2 ribs above pleura at each vertical line).
• Most superior structure at the right hilum = bronchus (eparterial); at left hilum = pulmonary artery (RALS concept).
• Veins are intersegmental, bronchus & artery are intrasegmental in a bronchopulmonary segment — surgical anatomy MCQ.
• Cardiac notch and lingula are LEFT-sided; middle lobe and horizontal fissure are RIGHT-sided.
• Left recurrent laryngeal nerve relations → hoarseness in left hilar carcinoma / aortic aneurysm (Ortner).
• Carina at the sternal angle (T4/T5); widened carina = subcarinal nodal disease.
• Number of bronchopulmonary segments: Right 10, Left 8 (clinically described).

Rapid revision

1. Right lung = 3 lobes, 2 fissures (oblique + horizontal); Left = 2 lobes, 1 oblique fissure.
2. Cardiac notch, lingula → LEFT; middle lobe + horizontal fissure → RIGHT.
3. Hilum AP order: Vein → Artery → Bronchus (VAB); inferior pulmonary vein is always lowest.
4. Most superior hilar structure: RIGHT = bronchus, LEFT = pulmonary artery (RALS).
5. Right main bronchus = wider, shorter, more vertical → commonest aspiration route.
6. Aspiration: upright → posterior basal segment RLL; supine → superior segment RLL.
7. Bronchopulmonary segment: bronchus + artery central (intrasegmental); veins intersegmental = surgical plane.
8. Lung lower border 6/8/10; pleural reflection 8/10/12 (mid-clavicular/mid-axillary/paravertebral) — 2-rib gap.
9. Pleural tap site: costodiaphragmatic recess, ~9th ICS midaxillary line, needle on upper border of lower rib.
10. Chest tube → triangle of safety, 5th ICS, midaxillary line.
11. Bronchial arteries (from descending thoracic aorta) = nutritive; pulmonary arteries = functional/deoxygenated.
12. Carina at sternal angle (T4/T5); widened carina = subcarinal lymphadenopathy; rigid bronchoscopy = treatment of choice for airway foreign body.