Structure
The trachea begins at the lower border of the cricoid cartilage, at C6, and ends at its bifurcation at the level of the sternal angle, the T4/T5 disc. About half its length is in the neck and half in the thorax.
- Cartilage rings: 15 to 20 incomplete, C-shaped rings of hyaline cartilage keep the airway open, joined by fibroelastic membrane.
- Trachealis: smooth muscle closing the gap at the back of each ring, so the posterior wall is flat and lies against the oesophagus.
- Lining: pseudostratified ciliated columnar epithelium with goblet cells, which moves mucus up towards the larynx.
- Carina: a keel-shaped ridge of cartilage inside the bifurcation, one of the most sensitive areas for the cough reflex.
Main bronchi
The right main bronchus is shorter, wider and more vertical than the left, and gives off the upper lobe bronchus soon after it begins. The left main bronchus is longer and more horizontal, passing under the aortic arch and in front of the oesophagus and descending aorta.
Relations
In the neck the trachea lies in the midline, under the thyroid isthmus and infrahyoid muscles; in the thorax it lies behind the great vessels and in front of the oesophagus.
In the neck
- In front: skin, investing fascia, the jugular venous arch, sternohyoid and sternothyroid, pretracheal fascia and the thyroid isthmus over the second to fourth rings. The inferior thyroid veins run down below the isthmus. In children the left brachiocephalic vein and thymus may reach above the manubrium.
- At the sides: the lobes of the thyroid, the carotid sheaths with the common carotid arteries, and the recurrent laryngeal nerves in the grooves between trachea and oesophagus.
- Behind: the oesophagus.
In the thorax
- In front: the manubrium, thymus remnants, left brachiocephalic vein, aortic arch, brachiocephalic trunk and left common carotid artery.
- Left: the aortic arch, left common carotid and left subclavian arteries, and the left recurrent laryngeal nerve.
- Right: the right vagus, the right brachiocephalic vein, the superior vena cava and the arch of the azygos vein over the right main bronchus.
- Around the bifurcation: the tracheobronchial lymph nodes and the deep cardiac plexus.
Blood supply and innervation
The cervical trachea is supplied by branches of the inferior thyroid arteries, and the thoracic trachea by the bronchial arteries. Veins drain to the inferior thyroid venous plexus and the bronchial and oesophageal veins. Lymph passes to the pretracheal and paratracheal nodes in the neck and the tracheobronchial nodes around the carina.
Parasympathetic fibres from the vagus, largely via the recurrent laryngeal nerves, supply the mucosa, glands and trachealis, causing bronchoconstriction and secretion. Sympathetic fibres from the sympathetic trunks relax trachealis. Sensory fibres in the vagus carry the cough reflex.
Clinical relevance
The trachea is the airway accessed in emergencies and ventilation, so its levels and neighbours matter at the bedside.
- Tracheostomy: placed through the second to fourth rings, below the thyroid isthmus, which is divided or retracted. The first ring is avoided to reduce the risk of subglottic stenosis.
- Emergency front-of-neck access: made through the cricothyroid membrane, above the cricoid, rather than the trachea.
- Tracheo-innominate fistula: a low tracheostomy or its cuff can erode into the brachiocephalic trunk, which crosses in front of the lower trachea.
- Inhaled foreign bodies and deep tubes: most enter the right main bronchus, and an endotracheal tube advanced too far can block the right upper lobe bronchus.
- Tracheal deviation: pushed away by a tension pneumothorax or large goitre, pulled towards lobar collapse.
- Widened carina: seen with enlarged subcarinal nodes or a large left atrium.
On the specimen
This station shows the root of the neck and upper mediastinum dissected around the trachea, with the arteries and nerves on each side.
- Thyroid gland and inferior thyroid vein: the gland straddles the upper trachea; the thin vein runs down from its lower border on the front of the trachea.
- Trachea and bronchi: the ringed tube in the midline, dividing into the right and left bronchi; the right is more vertical.
- Right side arteries: the right common carotid runs up beside the trachea; the right subclavian arches laterally. From the subclavian, the right internal thoracic artery runs downwards, and the costocervical trunk arises behind scalenus anterior and passes back over the pleura.
- Left side arteries: the left common carotid and left subclavian arise separately from the arch.
- Right vagus nerve: descends in front of the right subclavian artery, where its recurrent laryngeal branch hooks under the artery.
- Left vagus nerve: descends between the left common carotid and left subclavian arteries.
- Right phrenic nerve: lies more laterally, on scalenus anterior, then passes in front of the subclavian artery into the thorax.
- Right sympathetic trunk: the deepest nerve, ganglionated, lying on the neck of the first rib and vertebral column.