Bronchopulmonary segments
Trachea, bronchi and lungs on imaging: lobes, segments, fissures and collapse
Reading a chest film or CT means knowing how the airways divide, which lobe touches which border and where the fissures run. CPSP asks about the bronchial tree, segments and aspiration, the hilum, the silhouette sign and collapse patterns, and the basic HRCT anatomy behind common lung patterns.
One of the most tested topics in past papers
Trachea and main bronchi
- The trachea runs from C6 to the carina at about T4–T5, pushed slightly right by the aortic arch. Cartilage disappears at the bronchioles; pores of Kohn allow collateral air drift.
- The right main bronchus is wider, shorter and more vertical, so foreign bodies enter it. Its upper lobe bronchus arises about 2 cm below the carina, so a right-sided double-lumen tube easily blocks it. A tracheal bronchus from the right tracheal wall can supply the apical segment.
- A broad, blunt carina suggests enlarged subcarinal nodes. A sabre-sheath trachea (coronal diameter about half the sagittal) affects only the intrathoracic part and is strongly associated with COPD: high intrathoracic pressure during forced expiration and coughing squeezes the cartilage rings from the sides.
- The cervical pleura (cupola) rises above the clavicle into the neck and is roofed by the suprapleural membrane (Sibson fascia), which stops it bulging upwards on coughing.
Airway and alveolar structure
| Level | Key features |
|---|---|
| Bronchi | Cartilage plates, glands, respiratory epithelium |
| Segmental (tertiary) bronchus | Aerates one bronchopulmonary segment, which has its own artery and can be resected alone |
| Terminal bronchiole | No cartilage or glands; club (Clara) cells; last purely conducting airway |
| Respiratory bronchiole and alveoli | Gas exchange |
- Type I pneumocytes form the thin gas-exchange surface; type II pneumocytes secrete surfactant and are the progenitors that replace type I cells after injury. Alveolar macrophages are the first-line phagocytic defence.
- The air–blood barrier is type I cell cytoplasm, a fused basement membrane and capillary endothelium.
Hilum
| Side | Highest structure | Other relations |
|---|---|---|
| Right | Upper lobe (eparterial) bronchus | Azygos arch passes over the root at T4 to join the SVC |
| Left | Pulmonary artery | Aortic arch over the root; descending aorta behind it |
- Pulmonary veins lie anterior and inferior; the inferior pulmonary vein is the lowest hilar structure, at the top of the pulmonary ligament.
- Pulmonary arteries run with the bronchi in the centre of each segment; pulmonary veins run between segments (the guide in segmentectomy). Bronchial arteries: two left from the descending aorta, the right usually from the 3rd right posterior intercostal artery; right bronchial veins drain to the azygos.
- Lymph drains first to bronchopulmonary (hilar) nodes.
Lobes, segments and aspiration
| Lobe | Segments |
|---|---|
| Right upper | Apical, posterior, anterior |
| Right middle | Lateral, medial |
| Right lower | Superior, medial basal, anterior basal, lateral basal, posterior basal |
| Left upper | Apicoposterior, anterior, superior and inferior lingular |
| Left lower | Superior, anteromedial basal, lateral basal, posterior basal |
- The right lung has 10 segments; on the left some share a bronchus, so the medial basal segment has no separate bronchus. Each segment has its own segmental (tertiary) bronchus and artery, so it can be removed alone.
| Position at aspiration | Segment |
|---|---|
| Supine | Superior segment of right lower lobe (first posterior branch below the middle lobe bronchus) |
| Lying on the right side | Posterior segment of right upper lobe |
| Upright | Posterior basal segment of right lower lobe |
- Middle lobe syndrome: a long narrow bronchus ringed by nodes and poor collateral ventilation lead to recurrent collapse.
Fissures
- Oblique fissure: from the T2–T3 spinous process round to the 6th costal cartilage. Horizontal fissure: 4th costal cartilage to the midaxillary line. On a lateral film the horizontal fissure runs forwards from the oblique.
- On thick axial CT the horizontal fissure lies almost in the scan plane, so it appears as a band without vessels. Below it, lung in front of the right oblique fissure is middle lobe.
- The cardiac notch lets a needle in the left 5th space near the sternum reach the pericardium without crossing lung.
| Accessory fissure | Feature |
|---|---|
| Azygos fissure | Curved line to a tear-drop (azygos vein) in the right upper zone; four pleural layers; not a true lobe (no own bronchus) |
| Left horizontal | Separates the lingula from the rest of the upper lobe |
Silhouette sign and collapse
| Border lost | Lobe |
|---|---|
| Right heart border | Right middle lobe (medial segment) |
| Right hemidiaphragm (heart border kept) | Right lower lobe |
| Left heart border | Lingula |
| Medial left hemidiaphragm and descending aorta; retrocardiac triangle | Left lower lobe |
| Aortic knuckle; veil over the left upper zone | Left upper lobe |
- Air bronchogram: open airways seen through airless lung. Spine sign: lower thoracic vertebrae whiter than upper on the lateral film means lower lobe consolidation.
- The direct sign of collapse is fissure displacement; raised diaphragm, shifted trachea and hilum, and crowded vessels are indirect.
- Right upper lobe collapse with a central mass: the horizontal fissure rises, concave laterally but convex medially (reverse S).
- Left upper lobe collapse: a periaortic crescent of lucency from the over-expanded superior segment of the left lower lobe.
- A white hemithorax with shift towards it is main bronchus obstruction; shift away suggests effusion.
| Atelectasis | Mechanism |
|---|---|
| Resorptive | Airway obstruction |
| Passive (relaxation) | Pneumothorax or effusion; airways open |
| Cicatricial | Fibrotic contraction (old tuberculosis): small dense upper lobe, coarse strands, dilated air-filled bronchi, hilum and trachea pulled towards it |
| Rounded | Pleural-based mass with vessels curving in (comet tail); asbestos pleura |
- Check-valve foreign body: the obstructed lung stays large and lucent on expiration.
Lucent lung and air–fluid problems
| Finding | Diagnosis and mechanism |
|---|---|
| Small hyperlucent lung, small hilum, sparse vessels, air trapping on expiration after infant viral bronchiolitis | Swyer–James (MacLeod) syndrome: obliterative bronchiolitis stunts lung and vessel growth |
| Mucus-filled branching "finger-in-glove" bronchus not connected to the central airway, hyperlucent segment beyond (often left upper lobe apicoposterior) | Bronchial atresia: the segment is ventilated by collateral drift and traps air |
| Thin-walled air cysts without fluid levels appearing in staphylococcal pneumonia in a child | Pneumatoceles: check-valve necrosis of bronchiolar walls; they may outlast recovery and can rupture into a pneumothorax |
| Cavity with an air–fluid level and a wavy floating membrane in a shepherd who coughs up salty fluid | Ruptured hydatid cyst (water-lily sign); never aspirate |
| Long curved edge with lung markings beyond it, continuing outside the rib cage, a broad density change rather than a thin white line | Skin fold, not pneumothorax |
| Absent breath sounds, trachea pushed away, hypotension | Tension pneumothorax: immediate needle decompression, not an X-ray |
| Wedge-shaped pleural-based opacity with its apex to the hilum after surgery | Pulmonary infarct (Hampton hump); most emboli do not infarct because of the dual (bronchial) supply |
- After pneumonectomy the space fills slowly with fluid and the mediastinum shifts towards it. A sudden fall in the air–fluid level, coughing up serous fluid and the mediastinum swinging back mean a bronchopleural fistula; lie the patient operated side down to protect the remaining lung.
HRCT anatomy and patterns
- The secondary pulmonary lobule (1–2.5 cm) has a central bronchiole and pulmonary artery; its septa carry veins and lymphatics, so smooth septal thickening means oedema or lymphatic spread.
| Pattern | Meaning |
|---|---|
| Tree-in-bud | Bronchioles plugged with pus (endobronchial tuberculosis) |
| Perilymphatic nodules | Along bronchovascular bundles, fissures and septa: sarcoidosis |
| Random nodules, basal and peripheral | Blood-borne metastases (more flow to the bases) |
| Bronchus wider than its accompanying artery | Bronchiectasis (signet ring) |
| Mosaic lung with air trapping on expiration | Small airways disease |
| Subpleural rows of cysts, basal | Honeycombing of usual interstitial pneumonia |
| Subpleural bullae in a row | Paraseptal emphysema (pneumothorax) |
| Diffuse thin-walled round cysts in a woman | Lymphangioleiomyomatosis |
| Nodule with a ground-glass halo in neutropenia | Haemorrhage around an angioinvasive mould |
| Ground-glass | Partial filling of alveoli or thickened walls, vessels still seen |
| Upper lobe vessels wider than lower | Raised pulmonary venous pressure |
- Sequestration: lung not connected to the bronchi, with a systemic artery from the aorta; the extralobar type has its own pleura.
Remember Find the border that is lost, then name the lobe that touches it.
Exam trap The lingula belongs to the left upper lobe, so it blurs the left heart border, not the diaphragm.
Exam trap A supine patient aspirates into the superior segment of the right lower lobe, not the basal segments.
How FCPS asks about this
- Supine aspiration: superior segment of the right lower lobe; upright aspiration goes to the posterior basal segment.
- Segments of the right middle lobe: lateral and medial.
- Loss of the right heart border: right middle lobe; left heart border, lingula.
- Cells making surfactant: type II pneumocytes.
- Sabre-sheath trachea: COPD.
- Small hyperlucent lung with air trapping after childhood bronchiolitis: Swyer–James syndrome.
- Sudden fall in fluid level after pneumonectomy: bronchopleural fistula.
Test yourself on this topic
5 of the 98 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
Within the lung, each bronchus runs in a bronchovascular bundle beside a vessel of about the same diameter. In bronchiectasis the bronchus becomes clearly wider than this companion vessel. Which vessel is it?
Answer: The accompanying pulmonary artery
Within the lung each bronchus and bronchiole runs with a branch of the pulmonary artery in the bronchovascular bundle, and normally the two have similar diameters. In bronchiectasis the bronchus is permanently dilated, so in cross-section its lumen is clearly larger than the adjacent artery, like a ring with a stone (the signet ring sign on CT). Pulmonary veins run separately in the interlobular septa, and bronchial arteries are tiny vessels in the bronchial wall.
Signet ring sign
- Bronchus runs with the pulmonary artery
- Normally similar diameters
- Veins run separately in septa
- Bronchiectasis: bronchus larger than artery
Remember Bronchus and pulmonary artery travel together; veins travel apart.
Exam trap Bronchial arteries also run with the bronchi but are tiny wall vessels, not the companion seen in the bundle.
Why the other options are wrong
- A mucus plug in a small bronchiole A plug lies inside an airway; it does not accompany each bronchus.
- A pulmonary vein tributary Veins run in the septa, away from the bronchi.
- A peribronchial lymph node Nodes lie at the hila and bifurcations, not beside every bronchus.
- A calcified bronchial cartilage Cartilage forms part of the bronchial wall, not a separate structure.
A 30-year-old shepherd from Balochistan has had a cough for months. He suddenly coughs up a large amount of salty, watery fluid containing small white membranes like grape skins. What is the most likely diagnosis?
Answer: Ruptured pulmonary hydatid cyst
Echinococcus granulosus forms fluid-filled cysts in the lung, the second commonest site after the liver, often in the lower lobes. The cyst has an inner germinal endocyst and laminated membrane, and an outer host pericyst. When it ruptures into a bronchus, clear salty fluid and membranes are coughed up; air enters and the collapsed membrane floats on the remaining fluid, giving the water-lily sign on a radiograph. Shepherds and farmers living with dogs are at risk.
Ruptured lung hydatid
- Echinococcus granulosus
- Coughs up salty fluid and membranes
- Radiograph: water-lily sign
- Contact with dogs and sheep
Remember Salty fluid and membranes coughed up: ruptured hydatid.
Exam trap Rupture can cause anaphylaxis and spread daughter cysts; never aspirate a suspected lung hydatid.
Why the other options are wrong
- Lung abscess Foul purulent sputum, not salty fluid with membranes.
- Aspergilloma in an old cavity A fungal ball in an old cavity; may cause haemoptysis.
- Cavitating squamous carcinoma Haemoptysis and weight loss in a smoker, no membranes.
- Tuberculous cavity Blood-stained sputum with night sweats; no membranes.
Twelve days after a right pneumonectomy, a man develops fever and suddenly coughs up a large amount of serous fluid, the same as that filling the pneumonectomy space. He becomes breathless when lying on his left side. What is the most likely cause?
Answer: Bronchopleural fistula
After pneumonectomy, the space normally fills slowly with fluid over weeks and the mediastinum shifts towards the operated side. Breakdown of the bronchial stump creates a bronchopleural fistula: air enters the space and fluid from it is coughed up, especially when the patient lies with the healthy side down, so it spills into the remaining lung. On a radiograph the fluid level falls suddenly. Infection of the space (empyema) often follows.
Bronchopleural fistula
- Normal: space fills slowly with fluid
- Fistula: stump breaks down
- Coughs up space fluid
- Risk of aspiration into the remaining lung
Remember Coughing up pleural fluid after pneumonectomy: stump leak.
Exam trap Lie the patient with the operated side down to stop fluid spilling into the remaining lung.
Why the other options are wrong
- Normal post-pneumonectomy course Fluid accumulates silently; it is not coughed up.
- Haemorrhage into the space Blood would fill the space quickly, not be coughed up as serous fluid.
- Chylothorax Chyle accumulates after feeding; it is not coughed up.
- Phrenic nerve palsy Causes a raised hemidiaphragm, not expectoration of fluid.
An erect chest radiograph of a 65-year-old man with an old myocardial infarction shows that the upper lobe vessels are now wider than the lower lobe vessels, the reverse of normal. There is no oedema yet. What does this upper lobe diversion indicate?
Answer: Raised pulmonary venous pressure
In the upright lung, gravity normally makes the lower lobe vessels larger than the upper lobe vessels. When left atrial and pulmonary venous pressure rise, as in left heart failure or mitral stenosis, fluid starts to collect in the basal interstitium and local hypoxia constricts the basal vessels. Blood is redirected to the upper lobes, which become wider, an early radiographic sign before interstitial and alveolar oedema develop.
Upper lobe diversion
- Normal erect: lower vessels larger
- Raised pulmonary venous pressure
- Basal vasoconstriction
- Early sign of left heart failure
Remember Big upper lobe veins on an erect film: rising venous pressure.
Exam trap On a supine film upper and lower vessels are equal, so diversion cannot be judged.
Why the other options are wrong
- Pulmonary arterial embolism Causes reduced vessels (oligaemia) beyond the clot, not upper lobe diversion.
- Normal supine appearance Applies only to supine films; this is erect.
- Low cardiac output alone Not by itself a cause of redistribution.
- Raised alveolar oxygen at the bases Alveolar oxygen is not higher at the bases.
On a lateral chest radiograph of a man with pneumonia, a dense opacity lies in front of the right oblique fissure and above the horizontal fissure, which runs forwards from the oblique fissure to the front of the chest. Which lobe is consolidated?
Answer: Right upper lobe
On the lateral view, the oblique fissure runs from about T4–T5 posteriorly down to the front of the diaphragm, separating the lower lobe behind from the lobes in front. On the right, the horizontal fissure runs forwards from the oblique fissure, dividing the anterior lung into the upper lobe above and the middle lobe below. An opacity in front of the oblique and above the horizontal fissure is therefore in the right upper lobe, mainly its anterior segment.
Lobes on the lateral view
- Behind oblique fissure: lower lobe
- Front, above horizontal: upper lobe
- Front, below horizontal: middle lobe
- Horizontal fissure meets the oblique fissure
Remember Lateral film: the fissures box in each lobe.
Exam trap Both oblique fissures overlap on the lateral film; the right one is identified by the horizontal fissure joining it.
Why the other options are wrong
- Right middle lobe Lies below the horizontal fissure, in front of the oblique fissure.
- Right lower lobe Lies behind the oblique fissure.
- Lingula Part of the left lung, which has no horizontal fissure.
- Left upper lobe Lies on the left; the right horizontal fissure is the landmark here.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Thorax, Trachea, bronchi, lungs: lobes, segments and fissures on imaging