The trachea and bronchi

Radiological features of the pleura

Plain films On a chest radiograph the pleura is visible only if tangential to the beam and if fat or air is on each side of it. Thus the pleura is visible in a normal subject at: • Fissures; • Sites where parietal pleura lies on extrapleural fat: — seen just below the second rib and — extending vertically upwards from the costophrenic recess; and • Junction lines (see Figs 4. 6, 4. 9, 4. 40, 4. 41 and 4. 45) (see also section: Mediastinal lines): — Anterior junction line: anterior to the arch of the aorta the two lungs may come in contact w i th one another, separated only by four layers of pleura. This pleura is then seen as the anterior junction line on a PA chest radiograph; — Posterior junction line: if the lungs lie close to one another posteriorly, a posterior junction line is seen on a PA chest radiograph, extending vertically downwards from the apices (approximately T1) for a variable distance. It disappears where the lungs envelop the aortic arch and may reform inferiorly. Where the junction lines are seen well, a mass between the lungs in that area can be excluded.

Computed tomography On axial CT the pleura cannot usually be distinguished from the thoracic wall or mediastinum unless it is thickened (see section on fissures, p. 119). The pulmonary ligaments can occasionally be seen extending below the inferior pulmonary vein caudally and posteriorly to the diaphragm. The right pulmonary ligament lies close to the inferior vena cava (IVC), whereas the left pulmonary ligament lies close to the oesophagus.

THE TRACHEA AND BRONCHI (Figs 4. 10-4. 12)

The trachea

The trachea begins at the lower border of the cricoid cartilage at the level of C6 vertebra. It extends to the carina at the level of the sternal angle (T5 level, T4 on inspiration and T6 on expiration). The trachea is 15 cm long and 2 cm in diameter and is made up of 15-20 incomplete rings of cartilage that are bridged posteriorly by the trachealis muscle. The trachea is lined by ciliated columnar epithelium.

The trachea in children is very pliable. It may be deviated to the right at almost 90° in a normal expiratory f i l m. It only deviates to the left if the aortic arch is on the right side.

Relations of the trachea

Cervical (see Figs 1. 34-1. 36) The anterior relations are as follows: • Anterior: — Isthmus of thyroid anterior to the second, third and fourth rings — Inferior thyroid veins — Strap muscles: sternohyoid and sternothyroid; • Posterior: oesophagus and recurrent laryngeal nerves; and • Lateral: lobes of thyroid gland — Common carotid artery.

Thoracic (see Figs 4. 7, 4. 17, 4. 42 and 4. 43) The thoracic relations are as follows:

• Anterior: — Brachiocephalic and left common carotid arteries — Left brachiocephalic vein • Posterior: oesophagus and left recurrent laryngeal nerve • Left lateral: — Arch of the aorta — Left common carotid and left subclavian arteries; and • Right lateral: right brachiocephalic artery — Right vagus nerve — Arch of the azygos vein — Pleura (in direct contact unlike the other side).

Blood supply of the trachea The upper trachea is supplied by the inferior thyroid artery and the lower part is supplied by branches of the bronchial artery.

Venous drainage is to the inferior thyroid venous plexus.

Main bronchi (see Figs 4. 10-4. 12)

Carina

This is the anteroposterior ridge at the junction of the main bronchi. It lies at T5 vertebral level (T4 on inspiration and T6 on expiration) and at the level of the sternal angle. The carinal angle measures approximately 65° - that is, 20° to the right of the midline and 40° to the left. This angle is slightly larger in children. The carinal angle increases by 10° to 15° in recumbency.

The right main bronchus (eparterial bronchus) The right main bronchus lies at about 25° to the median plane. It is 2. 5 cm long and 1. 5 cm wide. It is thus wider, shorter and more vertical than the left main bronchus. Relations

The relations of the right main bronchus are as follows: • Anterior: — Superior vena cava — Right pulmonary artery; • Posterior: azygos vein; and • Superior: arch of azygos vein.

The bronchus to the upper lobe arises almost immediately after the tracheal bifurcation, entering the hilum of the lung separately and thereafter dividing into anterior, apical and posterior bronchi. The right bronchus continues as the bronchus intermedius, which then divides into middle- and lower-lobe bronchi. The middle-lobe bronchus has medial and lateral divisions. The apical segment bronchus of the lower lobe comes off opposite the bronchus to the middle lobe. The lower-lobe bronchus divides into four basal segment bronchi - posterior, lateral, anterior and medial.

Left main bronchus (hyparterial bronchus) The left main bronchus lies at 40° to the median plane. It is 5 cm long and 1. 2 cm in diameter.

Relations The relations of the left main bronchus are as follows: • Anterior: pulmonary trunk; • Posterior — Oesophagus — Descending aorta; and • Superior: — Aortic arch — Pulmonary artery.

The left main bronchus divides into upper- and lowerlobe bronchi within the lung. The upper-lobe divisions are similar to the right. The posterior and apical segmental bronchi usually have a common apicoposterior bronchus, which then subdivides. The lingular lobe bronchus comes off the upper-lobe bronchus and has superior and inferior divisions. The lower-lobe bronchus has apical, lateral, anterior and posterior basal segments but no medial basal segment.

The anatomy of the bronchial tree is shown diagram¬ matically in Figure 4. 12. Naming basal bronchi laterally to medially is easier if the constant relationship of anterior, lateral and posterior (ALP) is remembered, w i th only the medial bronchus in the right lung changing its relative position.

Blood supply Whereas the lungs receive the entire output of the right heart, their own nutritive supply arises from the bronchial arteries, branches of the thoracic aorta (see Aorta).