The trachea (Fig1 , 2 ) is approximately 4.5 in (11.5 cm) in length and nearly 1 in (2.5 cm) in diameter. It commences at the lower border of the cricoid car tilage (C6) and terminates by bifurcating at the level of the sternal angle of Louis (T4/5) to form the right and left main bronchi. (In the living subject, the level of bifurcation varies slightly with the phase of respiration; in deep inspiration it descends to T6 and in expiration it rises to T4.)

Relations

Lying partly in the neck and partly in the thorax (superior mediastinum), its relations are as follows.

Cervical

 • Anteriorly – the isthmus of the thyroid gland, inferior thyroid veins, sternohyoid and sternothyroid muscles.

• Laterally – the lobes of the thyroid gland and the common carotid artery.

 • Posteriorly – the oesophagus with the recurrent laryngeal nerve lying in the groove between the oesophagus and trachea (Fig. 3).

Fig1. The trachea and its anterior relationships.

Fig2. The trachea and main bronchi viewed from the front.

Fig3. The cervical part of the trachea and its environs in transverse section (through the 6th cervical vertebra) (viewed from below).

Thoracic

• Anteriorly – commencement of the brachiocephalic artery and left carotid artery, both arising from the arch of the aorta, the left brachio cephalic vein and the thymus.

• Posteriorly – oesophagus and left recurrent laryngeal nerve.

 • To the left – arch of the aorta, left common carotid and left subclavian arteries, left recurrent laryngeal nerve and pleura.

• To the right – vagus, azygos vein and pleura (Fig. 4).

Fig4. (a) The thoracic part of the trachea and its environs in transverse section (through the 4th thoracic vertebra) (viewed from below). (b) CT scan (axial view) of the superior mediastinum at a level corresponding to that in (a).

Structure The patency of the trachea is maintained by a series of 15–20 U-shaped cartilages. Posteriorly, where the cartilage is deficient, the trachea is flattened and its wall completed by fibrous tissue and a sheet of smooth muscle (the trachealis). Within, it is lined by a ciliated columnar epithelium with many goblet cells.