Bronchi and Their Divisions

The trachea bifurcates at the level of T5 or second costal cartilage or sternal angle into the left and right bronchus.

The Right Bronchus

It is wider, shorter, and more vertical than the left bronchus. It is more in line with the trachea. As a result, any foreign body will have more possibility of going into the right main bronchus as it is the same line. The right main bronchus is about 5 cm in length, and its average angle with the trachea is 25 degrees. It has a diameter of about 17mm plus/minus 4mm in men and about 15 +/- 4mm in women. The right pulmonary artery is anterior to the right main bronchus, and the azygos vein arches over it.

Left Main Bronchus

It is about 5.5 cm long and narrower than the right main bronchus. Its diameter is 2-3 mm less than the right bronchus. It forms 45 degrees with the trachea. It is anterior to the esophagus, thoracic duct, and descending aorta. The left pulmonary artery lies at first anterior and then superior to it.

Also read: Reinke's Edema

Secondary and Tertiary Bronchi

Right Upper Lobe Bronchus

Arises from the right lateral aspect of the parent bronchus, about 12-20mm from the carina. Runs superolaterally to enter the hilum of the lung . Measures 1 cm in length. Divides into three segmental bronchi

• Apical
• Anterior
• Posterior

Apical further divides into apical anterior and apical posterior.

Right Middle Lobe

It arises about 2.5 cm beyond the origin of the right upper lobe bronchus from the anterior aspect of the bronchus. It is directed forward, downward, and laterally. Divides into two segments: median and lateral.

Right Lower Lobe Bronchus

Continuation of the stems of the middle lobe. Four divisions: apical, anterior basal, lateral basal, posterior basal. The medial basal segmental bronchus has a higher point of origin than the other basal bronchi. It runs inferomedially parallel to the right border of the heart. The lower lobe bronchus then divides into an anterior basal segmental bronchus, which descends anteriorly, and a trunk, which divides into lateral and posterior basal segmental bronchi.

Also read: Upper Airway Obstruction: Causes, Management

Left Upper Lobe Bronchus

It arises from the anterolateral aspect of the main bronchus, about 5.5 cm from the carina. Divides into two bronchi to supply the superior lobe of the left lung. The cranial division ascends for about 10 mm before dividing. It divides into

• Anterior
• Apicoposterior

The caudal division descends anterolaterally to supply the anteroinferior part of the superior lobe of the left lung, which is the lingular area. Lingular bronchus divides into:

• Superior lingular
• Inferior lingular

Left Lower Lobe Bronchus

The apical segmental bronchus takes its origin posteriorly from the left lower lobe bronchus, about 1 cm below the upper lobe orifice. The inferior lobe bronchus continues for a further 1-2 cm and then divides into an anteromedial and a posterolateral stem. Anteromedial

• Anterior basal
• Medial basal

Posterolateral

• Posterior basal
• Lateral basal

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Variations

Variations in bronchial divisions can occur, leading to different anatomical configurations. Some notable variations include: Absence of Medial Basal Segmental Bronchus on the Left

Side

• Due to a common origin with the anterior basal segment.
• In 10% of lungs, it arises independently from the left lower lobe bronchus.

Subapical Segmental Bronchus (Left Lower Lobe): In 30% of lungs, a subapical segmental bronchus arises from the posterior surface of the left lower lobe bronchus.

Tracheal Bronchus

It arises from the right lateral aspect of the trachea just above the carina. Clinical importance: May cause confusion during lung resection for carcinoma, making identification of lung sections challenging.

Bronchial Cartilage

Composition: Similar to tracheal cartilage.

Elasticity Variation: Cartilages are more rigid or hyaline near the trachea. Elastin content increases towards the terminal bronchi.

Also read: Thyroid Cartilage: Structure and Functions

Bronchioles

They arise from the tertiary bronchi. These further divide into tertiary bronchioles and the

respiratory bronchioles. The terminal bronchioles refer to the end of the conducting zone, while the respiratory bronchioles refer to the beginning of the respiratory zone, where the exchange of gases takes place. The diameter of these bronchioles is less than 1 mm, as bronchioles become narrow as you go down. These do not have hyaline cartilage and mucus glands.

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