Secondary Bronchi

The secondary bronchi, also known as lobar bronchi, are the second-order branches of the bronchial tree. They arise from the primary (main) bronchi and serve as the airways to individual lobes of the lungs. Each secondary bronchus carries air to a specific lobe and further divides into tertiary (segmental) bronchi. Their number corresponds to the number of lobes in each lung—three in the right lung and two in the left.

Location

The lobar bronchi are located within the lungs, beginning at the hilum where the primary bronchus enters. They are entirely intrapulmonary and form the first visible division within the lung parenchyma. They follow the branching pattern of the pulmonary arteries and contribute to the bronchopulmonary segment framework.

Right Lung – 3 Lobar Bronchi

• Superior (upper) lobar bronchus: Supplies the superior lobe
• Middle lobar bronchus: Supplies the middle lobe
• Inferior (lower) lobar bronchus: Supplies the inferior lobe

The right secondary bronchi arise from the wider and more vertical right primary bronchus and are generally shorter and more direct than those on the left side.

Left Lung – 2 Lobar Bronchi

• Superior (upper) lobar bronchus: Supplies the superior lobe, including the lingula
• Inferior (lower) lobar bronchus: Supplies the inferior lobe

The left lobar bronchi arise from the longer and more horizontal left primary bronchus, passing deep to the arch of the aorta.

Branching Hierarchy

The bronchial tree follows a systematic branching pattern:

1. Trachea
2. Primary (main) bronchi – right and left
3. Secondary (lobar) bronchi – 3 in the right lung, 2 in the left lung
4. Tertiary (segmental) bronchi – supply bronchopulmonary segments

Each lobar bronchus continues as a segmental bronchus, supplying a specific anatomical subunit of the lobe it enters.

Bronchopulmonary Segment Association

Right Lung

• Superior lobar bronchus: Apical, posterior, anterior segments
• Middle lobar bronchus: Lateral and medial segments
• Inferior lobar bronchus: Superior, medial basal, anterior basal, lateral basal, posterior basal segments

Left Lung

• Superior lobar bronchus: Apicoposterior, anterior, superior lingular, inferior lingular segments
• Inferior lobar bronchus: Superior, anteromedial basal, lateral basal, posterior basal segments

Structure

The lobar bronchi retain many of the histological features of the main bronchi:

• Epithelium: Pseudostratified ciliated columnar epithelium with goblet cells
• Cartilage: Irregular cartilage plates (not complete rings)
• Muscle: Circular smooth muscle layer beneath the mucosa
• Submucosa: Contains mucous glands and connective tissue

As bronchi continue to divide into smaller branches, cartilage becomes more fragmented and eventually disappears in the bronchioles.

Function

• Conduct air from the main bronchi to each lobe of the lung
• Ensure precise delivery of air to specific bronchopulmonary segments
• Initiate local mucociliary clearance and immune defense via mucosal lining

Relations and Surrounding Structures

The lobar bronchi travel alongside branches of the pulmonary arteries. This close relationship helps segment the lung into bronchopulmonary segments, where each segment is supplied by a lobar bronchus and its arterial counterpart.

Vascular Supply

• Arterial: Bronchial arteries (branches of the thoracic aorta or posterior intercostal arteries)
• Venous: Bronchial veins drain into the azygos vein (right) and hemiazygos or accessory hemiazygos (left)

Innervation

• Parasympathetic: Vagus nerve (CN X) – induces bronchoconstriction and mucus secretion
• Sympathetic: Thoracic sympathetic trunk – induces bronchodilation and reduces secretion
• Sensory: Vagal afferents and visceral afferents – involved in reflex responses

Lymphatic Drainage

• Drains into pulmonary and bronchopulmonary (hilar) lymph nodes
• Then progresses to tracheobronchial and paratracheal lymph nodes

Clinical Relevance

• Lobar pneumonia: Inflammation and infection confined to a specific lobe via its lobar bronchus
• Segmentectomy and lobectomy: Surgical resection of individual segments or lobes relies on precise understanding of lobar bronchial anatomy
• Bronchoscopy: Direct visualization of lobar bronchi is essential for diagnosis, biopsy, or foreign body removal