Trachea and principal bronchi
The trachea is the continuation of the larynx and commences in the neck below the cricoid cartilage at the level of C6 vertebra, 5 cm above the jugular notch. Entering the thoracic inlet in the mid-line, it passes downwards and backwards behind the manubrium to bifurcate into the two principal or main bronchi on a level just below the lower border of the manubrium. The cricoid cartilage and sternal angle provide easy surface markings for the upper and lower ends of the trachea.
The thoracic part runs through the superior mediastinum in front of the esophagus to the upper part of the posterior mediastinum. In front of this part are the manubrium with sternohyoid and sternothyroid attached on each side, the inferior thyroid and left brachiocephalic veins and the remains of the thymus. The brachiocephalic and left common carotid arteries diverge on either side. The right side of the trachea is separated from the lung by the pleura which plasters the vagus nerve against the side of the tracheal wall, and the arch of the azygous vein hooks forwards over the right bronchus. The right brachiocephalic vein and superior vena cava are anterolateral to the trachea. On the left the left common carotid and subclavian arteries prevent the pleura from coming into contact with the trachea as it does on the right, and the arch of the aorta hooks backwards over the left bronchus, with the left recurrent laryngeal nerve passing upwards in the groove between trachea and esophagus.
The pulmonary trunk branches into the right and left pulmonary arteries rather to the left of the tracheal bifurcation, in front of the left bronchus. Here lies the deep part of the cardiac plexus. The right pulmonary artery crosses just below the tracheal bifurcation, and between the two is part of the tracheobronchial group of lymph nodes.
Blood supply of trachea:
Branches from the inferior thyroid and bronchial arteries form anastomotic networks in the tracheal wall. Veins drain to the inferior thyroid plexus.
Lymph drainage of trachea:
Lymphatic channels pass to the posteroinferior group of deep cervical nodes and to paratracheal nodes.
The mucous membrane is supplied by afferent (including pain) fibres from the vagi and recur¬rent laryngeal nerves, which also provide parasympathetic fibres of uncertain function. Sympathetic fibres from upper ganglia of the sympathetic trunks supply the smooth muscle and blood vessels.
The structure of the trachea conforms to functional needs.
- The wall of the trachea is elastic because it must stretch. The trachea is stretched into elongation during swallowing. Elevation of the larynx elevates the upper end; the bifurcation does not move. Elastic recoil of the trachea restores its original length, pulling the larynx down to its rest position. Normally there is no call on sternothyroid to depress the larynx, and swallowing is unimpaired by loss of this muscle. Per contra, pulling down on the bifurcation by sudden descent of the diaphragm, pericardium and aortic arch produces the clinical sign of ‘tracheal tug’.
- The softness of the elastic wall must be strutted open by bars of hyaline cartilage to prevent collapse during inspiration.
- The curved bars (‘rings’) of hyaline cartilage are incomplete so that the diameter may be controlled by the trachealis muscle. Cine-radiography of a cough shows 30% increase in transverse diameter produced by compressed air in the trachea while the vocal cords are shut, but 10% narrowing of the resting diameter at the instant the cords open. Like the choke barrel of a shotgun this greatly increases the explosive force of the blast of compressed air.
- The mucous membrane shares with the other respir¬atory mucous membranes the property of trapping particulate matter in a surface film of mucus. The soiled mucus is beaten upwards to the larynx by the cilia of the surface epithelium. From the larynx it is expelled by coughing (clearing the throat). Serous glands in the mucous membrane humidify the air.
Principal (main) bronchi arise from the trachea as two terminal branches. The right principal bronchus is wider, shorter and more vertical than the left and is about 1 inch long. Before entering the hilum of the right lung, it gives off the superior lobar branch. This means that on the right side, the superior lobar branch does not arise inside the lung tissue but outside it.
The left principal bronchus is narrower, longer and more horizontal as compared to the right principal bronchus. It is about 2 inches long. It passes to the left below the arch of aorta and in front of the esophagus. After entering the left lung, the principal bronchus divides into superior and inferior lobar bronchi.