Secondary (Lobar) Bronchi

1. Overview

The secondary bronchi, also known as the lobar bronchi, are branches of the primary (main) bronchi that deliver air to the individual lobes of the lungs. They form the second division in the bronchial tree, playing a central role in directing airflow to distinct anatomical regions of each lung. Each secondary bronchus enters and supplies a specific lung lobe, making them crucial in both respiration and clinical lung segmentation.

2. Location

The secondary bronchi are located:

• Distal to the primary bronchi, branching off within the hilum of each lung

• Proximal to the tertiary (segmental) bronchi, which arise from the secondary bronchi and supply bronchopulmonary segments

Number and distribution:

• Right lung: 3 secondary bronchi (superior, middle, and inferior lobar bronchi)

• Left lung: 2 secondary bronchi (superior and inferior lobar bronchi)

This distribution reflects the anatomical division of the lungs into lobes: three on the right and two on the left.

3. Structure

The secondary bronchi share the same basic wall architecture as other conducting airways but with adaptations for branching:

• Cartilage: Irregular plates of hyaline cartilage (instead of complete rings), providing structural support

• Epithelium: Lined with pseudostratified ciliated columnar epithelium with goblet cells to trap and move mucus

• Submucosa: Contains smooth muscle, seromucous glands, and connective tissue

• Vascular supply: Receives blood from bronchial arteries (systemic circulation) and drained by bronchial and pulmonary veins

4. Function

The primary function of the secondary bronchi is to:

• Conduct air: Serve as dedicated airways that distribute air from the primary bronchi into specific lobes of the lungs

• Initiate branching: Give rise to tertiary (segmental) bronchi, which further divide and define functional lung units

Additional functions include:

• Air filtration: Via mucociliary clearance mechanisms

• Moistening and warming air: Through mucus production and vascular heat exchange

5. Physiological Role(s)

In respiratory physiology, secondary bronchi contribute to:

• Ventilation-perfusion distribution: Ensure air reaches each lung lobe for optimal gas exchange

• Mucociliary defense: Cilia and mucus trap and transport pathogens and debris toward the oropharynx

• Structural airway maintenance: Cartilage plates prevent collapse during expiration

• Airflow resistance regulation: Smooth muscle can constrict or dilate in response to autonomic input

6. Clinical Significance

Lobar Pneumonia

Bacterial infection can affect a specific lung lobe via a secondary bronchus. Symptoms include fever, productive cough, and localized crackles. Chest X-ray shows consolidation in the affected lobe.

Bronchial Obstruction

Blockage of a lobar bronchus due to:

• Tumor (e.g., bronchogenic carcinoma)

• Mucus plug

• Foreign body

May cause post-obstructive atelectasis, collapse, or infection of the corresponding lobe.

Bronchial Asthma and Hyperreactivity

Although more distal bronchioles are usually involved, lobar bronchi can participate in bronchospasm. This can lead to:

• Increased airway resistance

• Wheezing

• Reduced airflow to the lobe

Treated with bronchodilators and anti-inflammatory medications.

Lung Resection Surgery

Lobectomy (removal of a lung lobe) involves the dissection of the relevant lobar bronchus. Accurate anatomical knowledge is essential for avoiding vascular or airway complications during surgery.

Bronchoscopy Navigation

The secondary bronchi are easily visualized during flexible or rigid bronchoscopy. Identifying lobar openings is critical for:

• Biopsy of suspected lesions

• Bronchial lavage or culture collection

• Stent placement

Endobronchial Tumors

Tumors arising within a lobar bronchus can cause:

• Lobar collapse

• Hemoptysis

• Obstructive pneumonia

Management may involve surgical resection, bronchoscopy, or stenting.