Segmental bronchi

At a glance

English

Segmental bronchi

Latin (TA2)

Bronchi segmentales

System

Respiratory

Region

Thorax

In order

From the apical segmental bronchus of right lung (bi) to the posterior basal segmental bronchus of left lung (bx)

About the segmental bronchi

After the trachea divides, each main bronchus splits into lobar bronchi, one for each lobe, and each lobar bronchus splits again into segmental bronchi. These tertiary bronchi are the airways of the bronchopulmonary segments, the wedge-shaped units of lung tissue with their apex at the hilum and their base at the lung surface. Each segment has its own segmental bronchus and segmental branch of the pulmonary artery running at its centre, while the pulmonary veins run in the connective tissue between segments. Because of this a diseased segment can be removed with little disturbance to its neighbours.

The segments and their bronchi are numbered in a standard order that is nearly the same on both sides. In the upper lobe come the apical (I), posterior (II) and anterior (III). Next come IV and V: the lateral and medial segments of the right middle lobe, and the superior and inferior lingular segments of the left upper lobe. In each lower lobe the superior segment (VI) is followed by the basal segments: medial (VII), anterior (VIII), lateral (IX) and posterior (X). The right lung has all ten separately. On the left, the apical and posterior bronchi usually share one trunk, the apicoposterior bronchus (I+II), and the medial and anterior basal bronchi often share a stem, so the left side has fewer independent segmental airways.

All segmental bronchi share the same build. Their walls hold irregular plates of hyaline cartilage rather than rings, a spiral coat of smooth muscle, mucous glands and a ciliated lining. Beyond them come several further generations of subsegmental bronchi and then bronchioles, which have no cartilage. Moving down the series, the vertical and posterior members, such as VI and X, point towards the most dependent parts of the lung.

Knowing the segmental anatomy lets a bronchoscopist name and enter each airway, and lets radiologists and surgeons place disease exactly. Inhaled material follows gravity: in an upright person it tends to reach the basal segments, especially on the right, and in someone lying on the back the superior segments of the lower lobes and the posterior segments of the upper lobes. Physiotherapists use the same map to position patients for postural drainage.

Small variations are common. A segmental bronchus may arise from a neighbour rather than its lobar bronchus, extra subsuperior branches may appear in the lower lobes, and on the right the upper-lobe bronchi sometimes branch in pairs rather than three ways.