Part 1 · Anatomy of the Lung · Section 1 of 2

Embryology, Gross Anatomy & Bronchi

The structural foundations that underpin every thoracic operation — from single-lung ventilation to segmentectomy.

Anatomy Surgical relevance

Embryology

Lung development begins at week 3 to 4 of gestation. The respiratory epithelium derives from endoderm; connective tissue, cartilage, and muscle derive from splanchnic mesoderm and neural crest cells.

The lung bud (respiratory diverticulum) develops as an out-pouching of foregut endoderm by the late third or early fourth week. The proximal portion becomes the trachea; the distal bulbous end forms the bronchial buds, which give rise to all lower respiratory structures.

Table 1 · Five stages of lung development
#StageTimelineKey events
1Embryonic0–5 weeksLung bud formation
2Pseudoglandular5–16 weeksFormation of major airways
3Canalicular17–28 weeksPeripheral airway division; epithelial differentiation; vascularisation; air-blood barrier formation
4Saccular28 weeks – birthType I and II pneumocyte differentiation; surfactant production
5Alveolar28 weeks – 8 yearsAlveolar multiplication; extensive increase in surface area

Gross anatomy

Detailed diagram of the lungs showing lobes, fissures and vasculature
Fig. 1. Gross anatomy of the lungs — lobes, fissures, and hilar structures. Illustration: Patrick J. Lynch, medical illustrator; C. Carl Jaffe MD, cardiologist — Yale University School of Medicine. Wikimedia Commons. CC BY 2.5.

Lobes and fissures

At a glance — lobar anatomy
Right lung3 lobes (upper, middle, lower)
2 fissures: oblique + horizontal
Left lung2 lobes (upper, lower) + lingula
1 fissure: oblique only
Oblique fissure (both sides)T3 spinous process → 5th rib mid-axillary line → 6th costal cartilage
Horizontal fissure (right only)4th costal cartilage → mid-axillary line at 6th rib; divides upper and middle lobes

Surface anatomy

The parietal pleura projects superiorly 1 inch (2.5 cm) above the midpoint of the medial third of the clavicle. Both pleural reflections cross the 8th rib at the mid-clavicular line, the 10th rib at the mid-axillary line, and reach the paraspinal region just below the 12th rib posteriorly. The lungs follow but extend two rib spaces less than the pleura below the 6th rib anteriorly.

Surgical relevance

The gap between lung margin and pleural reflection below the 6th rib anteriorly is the safe zone for thoracocentesis in the sitting patient. Intercostal drain insertion must respect these surface markings to avoid lung puncture.

Anatomical variations

  • Horizontal fissure may be incomplete — partial or complete fusion of upper and middle lobes
  • Superior segment of lower lobe may be delineated by a separate accessory fissure
  • Accessory fissures and lobes may occur in one or both lungs
Azygos lobe

A normal variant of the apical segment of the right upper lobe — caused by failure of the azygos vein to migrate, creating a deep fissure invaginated by two layers of pleura. Not a true accessory lobe. Usually incidental on imaging but can cause technical difficulty during surgery.

Bronchi

The trachea bifurcates into right and left main bronchi at the level of T4. The carina (Latin: keel of a ship) is the sharp cartilaginous ridge at this division — the most sensitive area for triggering the cough reflex.

Right vs. left mainstem bronchus
Right mainstem (~2.5 cm)Wider, shorter, more vertical — aspirated foreign bodies lodge here preferentially
Left mainstem (~5 cm)Narrower, longer, more horizontal — passes under aortic arch and anterior to oesophagus
Surgical relevance — lung isolation

The right upper lobe bronchus takes off within 1–2 cm of the carina. A right-sided DLT or bronchial blocker advanced too far will obstruct the RUL. Always confirm position with a fibreoptic bronchoscope after placement and after repositioning.

Divisions of the right mainstem bronchus

Fig. 2 · Right bronchial tree — divisions
Right mainstem bronchus — divisions Right mainstem bronchus wider · shorter · ~2.5 cm · more vertical RUL bronchus right upper lobe Bronchus intermedius BI RB1 RB2 RB3 RML bronchus right middle lobe RLL bronchus right lower lobe RB4 RB5 RB6 RB7 RB8 RB9 RB10 RB1 apical · RB2 posterior · RB3 anterior · RB4 lateral · RB5 medial RB6 superior · RB7 medial basal (cardiac) · RB8 anterior basal · RB9 lateral basal · RB10 posterior basal

Divisions of the left mainstem bronchus

Fig. 3 · Left bronchial tree — divisions
Left mainstem bronchus — divisions Left mainstem bronchus narrower · longer · ~5 cm · more horizontal LUL bronchus left upper lobe LLL bronchus left lower lobe Upper division left upper div. Lingular division lingula LB1/2 LB3 LB4 LB5 LB6 LB7/8 LB9 LB10 LB1/2 apico-posterior (fused) · LB3 anterior · LB4 superior lingular · LB5 inferior lingular LB6 superior · LB7/8 anteromedial basal (fused) · LB9 lateral basal · LB10 posterior basal Note: LB1/2 and LB7/8 are fused segments unique to the left lung

The conducting zone and acinus

Bronchi undergo an average of 23 divisions. The first 16–17 generations form the conducting zone — anatomical dead space, no gas exchange. Beyond a terminal bronchiole lies the acinus — the gas-exchanging unit — comprising respiratory bronchioles, alveolar ducts, and alveoli.

Secondary pulmonary lobule showing terminal bronchiole, respiratory bronchioles, alveolar ducts and alveoli
Fig. 4. The secondary pulmonary lobule — terminal bronchiole, respiratory bronchioles, alveolar ducts, and alveolar sacs constituting the acinus. Illustration: Frank Gaillard. Radiopaedia.org rID: 8760. Via Wikimedia Commons. CC BY-SA 4.0.

Tracheal bronchus (pig bronchus / bronchus suis)

An ectopic bronchus arising from the right lateral wall of the trachea, typically within 2 cm of the carina, supplying segments or all of the right upper lobe. Incidence approximately 0.1–2%, clear right-sided predilection.

Classification

Supernumerary type: accessory bronchus alongside a normal RUL bronchus — normal segmental anatomy preserved.

Displaced type: replaces one or more segments of the upper lobe bronchus — most commonly the apical segment.

  • Intubation risk: ETT or DLT advanced past the orifice may obstruct the tracheal bronchus, causing atelectasis of the supplied segment
  • Lung isolation: bronchial blocker or DLT placement must account for anomalous anatomy — confirm bronchoscopically
  • Recurrent infection: impaired drainage causes recurrent RUL pneumonia — an under-recognised cause
Key surgical principle

In any patient with recurrent right upper lobe infections or difficulty with lung isolation, look specifically for a tracheal bronchus on pre-operative CT before theatre.

Bronchopulmonary segments

A bronchopulmonary segment (BPS) is an anatomical unit of lung supplied by a specific tertiary (segmental) bronchus with an accompanying segmental artery. The general pattern is 10 segments on the right and 8 segments on the left — LB1/2 and LB7/8 are typically fused on the left.

Structure of a BPS

Pyramidal in shape — apex towards the root of the lung, base towards the surface. Each segment has its own arterial supply and segmental bronchus. Pulmonary veins run in intersegmental planes, not alongside bronchi — forming the surgical plane between adjacent segments.

Bronchopulmonary segments of right and left lung — anterior view with all segments labelled
Fig. 5. Bronchopulmonary segments — anterior view. Right lung (10 segments) and left lung (8 segments) with segment boundaries and nomenclature. © Ranjeetha Shenoy. Original illustration. All rights reserved.
Surgical importance of BPS

Each BPS has no communication with adjacent segments — making segmentectomy possible without damaging neighbours. Intersegmental planes are marked by intersegmental veins, which serve as the surgical guide. There is little bleeding or air leak after segmentectomy if the intersegmental plane is correctly developed.

Further reading

All clinical content should be verified against current guidelines before clinical application. This resource is intended for revision and educational purposes only.

Standard textbooks

  • Shields TW, LoCicero J, Reed CE, Feins RH. General Thoracic Surgery. 7th ed. Lippincott Williams & Wilkins.
  • Sellke FW, del Nido PJ, Swanson SJ. Sabiston & Spencer Surgery of the Chest. 9th ed. Elsevier.
  • Pearson FG, et al. Thoracic Surgery. 3rd ed. Churchill Livingstone.

Current guidelines & resources