ii) Trachea and Bronchial Tree

Trachea

Structure

The trachea is a hollow tube propped open by a set of incomplete cartilaginous rings. Smooth muscle (the trachealis) extends between the posterior aspects of each cartilage, completing the tube. It begins at the inferior border of the cricoid cartilage, and passes inferiorly through the neck to enter the thorax. At the level of the sternal angle, it divides into the left and right main bronchi.
Although it is quite anterior in the neck, as it descends into the thorax the trachea becomes progressively more posterior, with numerous structures intervening in front.

Microscopic Structure

The trachea is lined with respiratory epithelium, with numerous goblet and ciliated cells. The lamina propria contains mucosal glands and neurovascular structures. The submucosa contains numerous glands. The cartilage or trachealis muscle forms the outer layer.

Relations

In the neck:

  • The isthmus of the thyroid gland is anterior, as well as the strap muscles (sternothyroid etc).
  • The lobe of the thyroid is lateral in the superior part. The common carotid, internal jugular and vagus nerves are lateral inferiorly.
  • The recurrent laryngeal nerves lie in a shallow groove between the trachea and oesophagus, posterolateral.
  • The oesophagus and bodies of the cervical vertebrae are posterior.
  • The larynx is superior.

In the thorax:

  • The oesophagus and thoracic vertebrae are posterior
  • The brachiocephalic veins cross anteriorly and superior. The arch of aorta passes anteriorly, giving off the brachiocephalic artery, left common carotid and left subclavian, before descending on the right side of the trachea. The thymus also lies anterior to these structures.
  • The right pulmonary artery crosses anterior to the carina
  • The inferior vena cava is to the right

Bronchial Tree

Structure

The right and left main bronchi arise from the trachea. The left follows a less acute angle, passing beneath the aortic arch and anterior to the descending thoracic aorta to reach the left hilum, beyond which it branches into left upper and left lower lobe bronchi. The wider right main bronchus descends to the right hilum, giving off the right upper lobe bronchus before passing more inferiorly as the bronchus intermedius. This bronchus terminates as the middle and lower lobe bronchi which then pass to their respective lobes.
Each lobar bronchus divides into 2 - 5 segmental bronchi to the supply the bronchopulmonary segments of the lung. Beyond this point, each bronchus will undergo 20 - 25 further branchings before the alveoli are encountered.
Once reaching a size of < 1 mm, the bronchi will lose their cartilage and become bronchioles. The terminal bronchiole gives off respiratory bronchioles which communicate with numerous alveoli.

Microscopic Appearance

The bronchi are lined with respiratory epithelium and have a similar overall structure to the trachea, with two exceptions:

  • A smooth muscle layer intervenes between the submucosa and the cartilage
  • The cartilagenous rings are smaller and less complete, eventually disappearing when the bronchi become bronchioles.

The bronchioles are lined by a simple cuboidal epithelium, and their wall is much thinner than the bronchi. Their smooth muscle component is relatively larger.

Relations

The right main bronchus passes posterior to right pulmonary artery, inferior vena cava and ascending aorta. The azygos vein passes over its superior surface to enter the inferior vena cava. The oesophagus is posterior.
The left main bronchus passes posterior to the left pulmonary artery, but anterior to the descending thoracic aorta.
The vagus nerve passes anterior and posterior to the main bronchi, but mostly posterior as the pulmonary plexus.


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