Segmental Bronchi

Etymology

• Derived from the Latin bronchus, meaning “windpipe,” combined with “segmental,” indicating their role in supplying specific bronchopulmonary segments.

AKA and Abbreviation

• AKA: Tertiary bronchi
• Abbreviation: None commonly used

What is it

• The segmental bronchi are the third-order branches of the tracheobronchial tree, branching from the lobar bronchi.
• Each segmental bronchus supplies a specific bronchopulmonary segment.
• They are essential for directing airflow to distinct functional and anatomical lung regions.

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Principles

Parts

• Right lung: 10 segmental bronchi corresponding to 10 bronchopulmonary segments:
  • Upper lobe: Apical, posterior, and anterior segments.
  • Middle lobe: Lateral and medial segments.
  • Lower lobe: Superior, medial basal, anterior basal, lateral basal, and posterior basal segments.
• Left lung: Typically 8–10 segmental bronchi due to segmental fusion:
  • Upper lobe: Apicoposterior (fused apical and posterior) and anterior segments.
  • Lingula: Superior and inferior lingular segments.
  • Lower lobe: Superior, anteromedial basal (fused anterior and medial basal), lateral basal, and posterior basal segments.

Size

• Diameter varies based on lung location and branching:
  • Segmental bronchi are 2–4 mm in diameter proximally.
  • Diameter decreases as they taper distally.
  • Wall  thickness
    • If the diameter of the two walls added together is greater than 50% of the lumen it is considered thickened

Shape

• Tubular, with progressively thinner walls as branching continues.
• Reinforced by discontinuous cartilaginous plates.

Position

• Centrally located within each bronchopulmonary segment.
• Extend distally from lobar bronchi to supply subsegmental and alveolar airways.

Character

• Composed of:
  • Cartilage plates for structural support.
  • Smooth muscle and elastic fibers for flexibility.
  • Mucosa lined with pseudostratified columnar epithelium.

Time

• Develop during embryonic lung branching (4th–6th weeks gestation).
• Continue to mature through childhood.

Blood Supply

• Supplied by bronchial arteries arising from the thoracic aorta.

Venous Drainage

• Drains into bronchial veins, which connect to the azygos and hemiazygos systems.

Lymphatic Drainage

• Drains into regional lymph nodes, including hilar and mediastinal nodes.

Nerve Supply

• Innervated by the vagus nerve (parasympathetic), sympathetic trunks, and pulmonary plexus.

Embryology

• Arise from the lung bud, derived from the ventral foregut during embryogenesis.

Histology

• Lined with ciliated pseudostratified columnar epithelium, with goblet cells for mucus production.

Physiology and Pathophysiology

• Direct airflow to distinct lung regions.
• Participate in mucociliary clearance.
• Pathological changes include inflammation, obstruction, or collapse, affecting ventilation and perfusion.

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Applied Anatomy to Radiology

CXR (Chest X-ray)

• Indirect Visualization:
  • Segmental bronchi are not directly visible due to their size.
  • Pathologies inferred from:
    • Segmental atelectasis: Wedge-shaped opacities corresponding to specific bronchopulmonary segments.
    • Obstruction: Manifesting as distal consolidation or air trapping.
    • Peribronchial cuffing: Indicates interstitial fluid or inflammation around the bronchi.

CT (Computed Tomography)

• Direct Visualization:
  • Segmental bronchi are visible down to the subsegmental level.
  • Pathologies identified include:
    • Bronchial wall thickening: Seen in asthma or chronic bronchitis.
      • Mucus plugging, air trapping, or peribronchial thickening further support the diagnosis.
      • can be overestimated in:
        • Expiratory-phase imaging (due to airway collapse).
        • Patients with inadequate lung inflation during imaging.
      • It is crucial to differentiate true wall thickening from:
        • Peribronchial cuffing (fluid or inflammation in the interstitial tissue surrounding bronchi).
    • Obstruction: Due to tumors, foreign bodies, or mucus plugs.
    • Bronchiectasis: Dilated segmental bronchi with thickened walls.
    • Infective changes: Centrilobular nodules or tree-in-bud appearance in tuberculosis or bronchiolitis.
• 3D Reconstruction and Virtual Bronchoscopy:
  • Enables pre-procedural planning and assessment of airway patency.

MRI (Magnetic Resonance Imaging)

• Limited Role:
  • MRI lacks sufficient resolution to visualize segmental bronchi directly.
  • Advanced techniques like hyperpolarized gas MRI can indirectly evaluate airway function and ventilation.

Bronchography

• Historical Role:
  • Contrast-based imaging of bronchi, now replaced by CT for safety and precision.

PET-CT

• Functional Imaging:
  • Highlights increased metabolic activity in tumors or inflammation involving segmental bronchi.
  • Valuable for staging lung cancers with bronchial involvement.

Endobronchial Ultrasound (EBUS)

• Guided Procedures:
  • Allows real-time imaging and biopsy of structures near segmental bronchi, such as lymph nodes.
  • Essential for diagnosing mediastinal pathology.

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Pathological Implications

• Obstruction: Leads to distal atelectasis, air trapping, or infection.
• Bronchial wall thickening: Found in asthma, chronic bronchitis, or cystic fibrosis.
• Infections: Bronchopneumonia may localize to specific segments.
• Neoplasms: Bronchogenic carcinoma may arise from or involve segmental bronchi.

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Key Points and Pearls

• Segmental bronchi are the functional units directing airflow to bronchopulmonary segments.
• Precise localization of pathology on imaging is critical for surgical or interventional planning.
• CT imaging is the modality of choice for segmental bronchial evaluation.
  • If the diameter of the two walls added together is greater than 50% of the lumen it is considered thickened
  • can be overestimated in Expiratory-phase imaging (due to airway collapse).
• Recognizing segmental anatomy aids in understanding diseases like pneumonia, atelectasis, and bronchiectasis.

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Parallels with Human Endeavors

• Segmental bronchi mirror “distribution networks,” such as power grids or water pipelines, where each branch ensures delivery to a specific destination. Like a well-organized system, their failure can disrupt the function of the entire lung.

 

In a Nutshell

Parts

Size Diameters

There are two lungs and they are made up of lobes which are divided into segments and these are discussed in the individual documents dedicated to each of the lungs.

Applied Anatomy: 

Size

Lobar and Segmental Airway Disease

  – Small 

Thickening of the Segmental and Subsegmental Airways

Add the 2 walls and if greater than 50% of the lumen = thickened

Bronchitis

Sarcoidosis

Wegener’s aka Granulomatosis with Polyangiitis

Applied Anatomy  Size – Big 

Diameter enlargement  of the Segmental  Subsegmental and Small Airways

Applied Anatomy – Traction Bronchiolectasis

Position Normal Angles between The Branches

Applied Anatomy – Atelectasis

Angles Between the Airways become More Acutes 

Character –

Mucoid Impaction

Bronchocentric Nodules

Broncholith

Time

The segments are divided into the secondary lobules. The lobules are made up of the small airways including the terminal bronchioles, respiratory bronchioles, alveolar ducts alveolar sacs and the alveoli themselves.

Right Lung Segments 

• Upper lobe
  • superior  segment
  • posterior segment
  • anterior segment
• Middle lobe
  • lateral segment
  • medial segment
• Lower lobe
  • apical segment
  • medial-basal segment
  • anterior-basal segment
  • lateral-basal segment
  • posterior-basal segment

Left Lung Segments 

• Upper lobe
  • apico-posterior segment (merger of “apical” and “posterior”)
  • anterior segment
• Lingula
  • inferior lingular segment
  • superior lingular segment
• Lower  lobe
  • superior segment
  • anteromedial basal segment (merger of “anterior basal” and “medial basal”)
  • posterior basal segment
  • lateral basal segment

RUL bronchus is about 1 cm in diameter, with the apical segment being 4-7mm, and the RML bronchus is about 8mms.  The LUL bronchus resembles its uncle, the right main stem bronchus, since it is also relatively short and fat possibly because it is responsible to give rise to the lingula as well as the left apical segments.  It measures 11mm mm in diameter (but only 9mm in length). The ascending upper division bronchus is approximately 7 mm in diameter.
General diameters of the downstream airways include lobular and segmental bronchi (5-8mm), subsegmental bronchi and bronchiole (1.5-3mm), lobular bronchiole (1mm), terminal bronchiole (.7mm) and acinar bronchiole (.5mm).   (Webb Muller Naidich)The acinus is about 7-15 mms in diameter.

From the lobular bronchus there are 9 to 14
dichotomous branches. Lobular bronchus has 3 to 5 smaller
airways, which are called terminal bronchioles

“A unit consisting of 3 to 5 terminal bronchioles supplied by a
small bronchus with a diameter of 1 mm is called a secondary
pulmonary lobule (Reid’s lobule) (Reid 1958)”.

Applied Anatomy

In Part 1 we learned how radius and flow were related.  When the flow is laminar as it is in the smaller airways Poiseuille’s law applies which states the following

Volume flow rate        =     pressure difference

                                         resistance                

                                    =     P1-P2

                                              R

                                     =    pressure difference X radius⁴

                                               8/pi viscosity X length

Thus it can be seen that a small change in the radius makes a major difference in the volume flow rate.   Number of divisions and length are other  key considerations in flow but they do not carry the same power as radius. Narrowing of the airways at any level significantly affects delivery of air downstream.

The failure of air delivery is one of the major causes of fatality during an anaphylactic reaction or a severe asthma attack.  Return of airway patency is the first consideration in resuscitation. In fact, in any resuscitation, the “A” of the “ABC” of resuscitation is to ensure patency of the airway first.
Maintenance of airway diameter is provided by cartilage in the upper components of the airways.  In the trachea a C-shaped cartilaginous ring surrounds the anterior and lateral walls while the posterior wall consists of a membrane allowing for some pliability during the phases of respiration.  The trachea lengthens and dilates during inspiration while it shortens and narrows during expiration.

Some disease states compromise the patency of the airways.  Tracheomalacia is a softening of the cartilage of the trachea and during inspiration the trachea will be unable to maintain its diameter.  This will result in a relative collapse during inspiration and hence diminished air flow.

More downstream, support of airway patency transforms from cartilagenous support to muscular support. Asthma, inflammation, or infection can affect the diameter by inducing muscle spasm while the presence of edema of the wall would also result in narrowing of the lumen.  In most of these diseases the airways of both lungs are diffusely involved and respiratory decompensation can easily result.  Collapse of a lobe, segment, or sub segment of a lung on the other hand may not affect respiratory function at all when the remaining lung is normal or near normal. Localized collapse may occur with foreign body inhalation, mucus or purulent impaction, and tumor growth. Patients who have undergone a pneumonectomy normal pulmonary function usually exists, unless there is disease in the other lung.

Segmental and (Often) Subsegmental Disease

 

Segmental and Subsegmental Disease
Extending into the Small Airways

In the normal patient the pulmonary arterial branches and the bronchi are about the same size until they reach the hallowed halls of the pulmonary lobule. 

 

The reason we are able to see the air filled bronchi within the air filled lung is because they have wall that is made up of the soft tissue allowing for an interface.  As the bronchi get smaller their walls will get proportionately thinner until they are too thin to resolve at which time they will blend into the parenchyma.  They become invisible as discrete structures on high resolution CT when they reach 2mm in size, which corresponds to the bronchioles that are about 2cms from the lung periphery.  The arterioles on the other hand maintain their soft tissue character and their interface with air of the lung is maintained allowing detection even in subpleural regions of the lung periphery.

Bronchiectasis is a condition where disease within the walls of the airways results in weakening resulting in enlargement and hence ectasia.  In this condition therefore the bronchioles will be larger than their arteriole counterpart and they will be seen within 1cms distance of the pleura.

Other Important Dimensions in the Airways

Number of Divisions

The right lung has 10 bronchopulmonary segments while the left has 8. There are approximately 23 airway divisions from the mainstem bronchi to the level of the alveoli.

The number of branches from hilum to periphery is variable with the shortest path to a terminal bronchiole being about 7 divisions and total length of 7 to 8 cms, and the longest pathway having about 25 branch divisions with total length of more than 22 cm. In the upstream portions of the airways the tree does not divide as frequently as they do downstream.  The lungs are large organs and the aim of the airways is to deliver the air to the alveoli.  Distance needs to be covered from the hilum where alveoli are relatively sparse, to the periphery where the alveoli are abundant.  Prior to entering the secondary lobule the bronchovascular bundle branches at approximately 1cm intervals.  Once they enter the confines of the lobule branching becomes fast furious and a new branch originates every 1-3mms.  This frequency of division in the hallowed halls of gas exchange makes intuitive sense as surface area becomes the dominant focus.

 

Thickness

The walls of the bronchi change in character and in thickness as the airways progress downstream.  The following are the measured thicknesses of the downstream airways;

 

lobular and segmental bronchi	1.5mm
subsegmental bronchi and bronchiole	.2-.3mm
lobular bronchiole	.15mm
terminal bronchiole	.1mm
acinar bronchiole	.05mm

Area

Cross-sectional area of the airways

 

Trachea approximately 2.5 cm²

 

Alveoli approximately 11,800 cm²   

Time

Gas exchange takes 0.25 seconds or 1/3 of the total transit time of a red cell under resting conditions. The entire blood volume of the body passes through the lungs each minute in the resting state, that is 5 liters per minute.

 

Applied Anatomy

Knowledge of the relationship of the trachea and esophagus is extremely important for those who are intubating patients or placing nasogastric tubes.  The trachea and esophagus lie very close to each other, and so it is not uncommon for the intubations to be misplaced.

Upper lobe disease such as TB will cause fibrosis and shrinkage of the lung structures with consequent traction effect and elevation of the hilum so that it is pulled upward.

Lymphangitis Carcinomatosis

Elevated Right Main Stem Bronchus and Encasement

 

Amyloid Involvement

 

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