Bronchopulmonary Segments

Bronchopulmonary segments are the functionally and anatomically distinct subdivisions of the lung lobes. Each segment is aerated by a single segmental (tertiary) bronchus and supplied by its own branch of the pulmonary artery. These segments are separated by thin connective tissue septa, making them discrete units that can be surgically removed without affecting adjacent lung tissue. Understanding bronchopulmonary segmentation is essential for thoracic anatomy, radiology, and segmental lung resection.

Defining Characteristics

• Each segment is pyramid-shaped with the apex facing the lung root and base toward the pleural surface
• Each has its own segmental bronchus and pulmonary artery branch
• Pulmonary veins course between segments (intersegmentally)
• Segments are separated by connective tissue planes

Number of Segments

The number of bronchopulmonary segments varies slightly between the right and left lungs:

• Right lung: 10 segments (3 in upper lobe, 2 in middle lobe, 5 in lower lobe)
• Left lung: 8 to 10 segments (4–5 in upper lobe including the lingula, 4–5 in lower lobe)

Right Lung Segments

Superior (Upper) Lobe

1. Apical
2. Posterior
3. Anterior

Middle Lobe

1. Lateral
2. Medial

Inferior (Lower) Lobe

1. Superior
2. Medial basal
3. Anterior basal
4. Lateral basal
5. Posterior basal

Left Lung Segments

Superior (Upper) Lobe

1. Apicoposterior (fusion of apical and posterior in many cases)
2. Anterior
3. Superior lingular
4. Inferior lingular

Inferior (Lower) Lobe

1. Superior
2. Anteromedial basal (anterior and medial often fused)
3. Lateral basal
4. Posterior basal

Depending on variation, the left lung may have 8 or up to 10 segments if apical/posterior and anterior/medial basal segments are not fused.

Vascular Segmentation

Each segment receives:

• A bronchial branch: A segmental (tertiary) bronchus from the lobar bronchus
• A pulmonary artery branch: Carrying deoxygenated blood from the heart

In contrast, pulmonary veins travel in the connective tissue septa between segments, collecting blood from adjacent areas (intersegmentally).

Shape and Orientation

Bronchopulmonary segments are shaped like irregular pyramids. Their apex points toward the hilum, where the bronchus and artery enter, and the base faces outward toward the lung surface. This configuration supports efficient airflow distribution and facilitates segmental drainage and ventilation.

Functional Importance

• Smallest resectable unit of lung tissue
• Facilitates surgical removal of diseased or infected tissue without disturbing adjacent healthy lung
• Important for radiological interpretation of segmental pathology
• Segmental infections (e.g., pneumonia or tuberculosis) may remain localized

Relations with Pleura and Fissures

While lobes are separated by fissures (oblique and horizontal), segments do not correspond exactly to fissure lines. Some segments cross over fissures, particularly near the hilum. The pleura envelops the entire lung and does not subdivide between segments internally.

Segmental Bronchial Tree Hierarchy

The bronchial tree divides as follows:

1. Trachea
2. Main (primary) bronchi – right and left
3. Lobar (secondary) bronchi – 3 on the right, 2 on the left
4. Segmental (tertiary) bronchi – one for each segment

These segmental bronchi continue to branch into subsegmental bronchi, then into bronchioles and eventually terminate at the alveolar level.

Clinical Relevance

• Segmentectomy: Removal of a single bronchopulmonary segment; useful in localized disease without affecting other segments
• Postural drainage: Knowing the segmental orientation helps in draining secretions in conditions like bronchiectasis or pneumonia
• Segmental collapse (atelectasis): Can be visualized radiologically if a segmental bronchus is obstructed
• Localized infections: Tuberculosis or aspiration pneumonia can often be confined to a specific segment