Trachea

The trachea is a tubular structure that forms part of the lower respiratory tract. The trachea is continuous superiorly with the larynx and inferiorly becomes the bronchial tree within the lungs. The trachea consists of a support frame of 16–20 semicircular, or C-shaped, rings made out of hyaline cartilage and reinforced by collagenous connective tissue. The posterior wall of the trachea is free of cartilage. In this area, the paries membranaceus forms a plate out of smooth tracheal muscle and connective tissue and forms the border to the dorsally running esophagus.

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Development

The trachea develops as part of the tracheobronchial tree by budding off of the foregut of the embryonic gut tube.

  • Occurs 4–7 weeks after conception
  • Embryonic gut tube: 
    • Forms from the laterally folded endoderm layer 
    • Surrounded by mesoderm
    • Has 3 sections:
      • Foregut
      • Midgut
      • Hindgut
  • Lung bud (respiratory diverticulum): 
    • Buds off of the ventral side of the foregut around week 4
    • Simultaneously grows out (ventrally) and down (caudally)
    • Includes both endoderm and surrounding splanchnopleuric mesoderm
  • Tracheoesophageal groove (or ridge):
    • As the lung bud grows out and down, the tracheoesophageal groove appears as lateral indentations between the new lung bud and the foregut.
    • The grooves/ridges move in medially, pinching off the lung bud, and forming the tracheoesophageal septum.
    • The tracheoesophageal septum creates 2 separate tubes:
      • Esophagus (posteriorly, from the original foregut)
      • Trachea (anteriorly, from the lung bud) 
  • Primary bronchial buds: The trachea bifurcates into the right and left bronchial buds.
    • The primary bronchial buds later continue developing into the bronchial tree and lungs through the pseudoglandular, canalicular, saccular, and alveolar stages.

Gross Anatomy

General characteristics

  • Location:
    • Located in the lower neck and thorax, within the superior mediastinum
    • Continuous superiorly with the larynx at the level of C6
    • Ends inferiorly at the level of T5 and continues as the primary bronchi and the rest of the bronchial tree in the lungs
  • Shape and structure:
    • The trachea is a D-shaped tubular structure in the lower respiratory tract.
    • Approximately 10–11 cm (3.9–4.3 in.) long, with an inner diameter of 1.5–2 cm (0.59–0.79 in.)
    • Consists of 16–20 semicircular, or C-shaped, rings made out of hyaline cartilage reinforced by collagenous connective tissue
    • Has a flat, cartilage-free, membranous posterior wall that contains smooth muscle called the trachealis muscle (known as paries membranaceus)
Cross sectional structure Trachea

The cross-sectional structure of the trachea:
Note the D-shaped, tubular structure and the close proximity to the esophagus.

Image by Lecturio. License: CC BY-NC-SA 4.0
  • Functions:
    • Transports air in and out of the lungs during ventilation
    • Warms and moistens the inhaled air in order to protect the respiratory tract
    • Aids in the act of coughing through the trachealis muscle
  • Portions:
    • Cervical portion: 
      • Direct continuation of the larynx below the cricoid cartilage
      • Begins at approximately the level of C6
      • Ends at the level of the jugular notch of the sternum
    • Thoracic portion: 
      • Direct continuation of the cervical trachea
      • Begins at the upper border of the superior mediastinum (jugular notch of the sternum)
      • Ends at the bifurcation of the trachea into the main bronchi, at the level of T4–T7 (most commonly T5)
Main structure and portions of Trachea

The main structure and portions of the trachea

Image by Lecturio. License: CC BY-NC-SA 4.0

Spatial relations

  • Posterior:
    • Esophagus
    • Descending aorta
    • Vertebral column
  • Anterior:
    • Skin and cervical fascia
    • Jugular venous arch
    • Ascending aorta
    • Brachiocephalic trunk
    • Superior vena cava
    • Thyroid gland and vessels
    • Thymus gland or remnants
  • Lateral:
    • Carotid arteries
    • Inferior thyroid arteries
    • Recurrent laryngeal nerves
    • Vagus nerves
    • Lungs and pleura

Microscopic Anatomy

The tracheal wall consists of 4 layers:

  1. Mucosa: tall, columnar, pseudostratified with cilia and mucin-producing goblet cells
  2. Submucosa: 
    • Connective tissue provides support and elastic recoil
    • Contains elastin fibers, mucus glands, smooth muscle, vessels, nerves, and lymphatics
  3. Musculo-cartilaginous layer: contains the hyaline cartilage of the C-shaped rings and the intervening smooth muscle
    • Trachealis muscle:
      • Can alter the dimensions of the trachea
      • Regulates the amount of air coming into or out of the lungs
  4. Adventitia: composed of fibroelastic, loose connective tissue
Various layers of tissue Trachea and Larynx

The various layers of tissue that comprise the trachea

Image: “Layers of tissue: trachea and larynx” by Phil Schatz. License: CC BY 4.0, edited by Lecturio.

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Neurovaculature

Blood supply

  • Arterial supply:
    • The trachea is mainly supplied by branches of the inferior thyroid arteries, branches of the thyrocervical trunk.
    • Irrigation is supported by ascending branches of the bronchial arteries. 
    • These 2 sets of arteries anastomose along the structure of the trachea.
  • Venous drainage: 
    • Analogous veins drain into the inferior thyroid venous plexus.
    • The inferior thyroid veins drain into the brachiocephalic veins.

Innervation

  • The trachea receives innervation from the pulmonary plexus. 
    • Parasympathetic supply: recurrent laryngeal nerves, branches of the vagus nerve
    • Sympathetic supply: sympathetic trunks
  • Within the walls of the tracheobronchial tree, efferent preganglionic axons from the vagus nerves synapse on small ganglia → regulation of local reflexes and airway control

Clinical Relevance

The following conditions of various origins can affect the trachea:

  • Infectious:
    • Laryngitis is an inflammation of the larynx that can occur after an infection of the nasal cavity and pharyngeal region due to viruses or bacteria, which can get into the trachea. Also, overuse of the voice in a dry air environment can lead to symptoms of laryngitis: reddened pharyngeal mucosa, burning sore throat, and hoarseness or loss of voice. 
  • Congenital:
    • Tracheoesophageal fistula: an abnormal connection between the esophagus and the trachea: Tracheoesophageal fistula is a common congenital abnormality, but when it occurs late in life, it is usually a sequela of a surgical procedure.
    • Tracheomalacia: an abnormal collapsing of the walls of the trachea: Tracheomalacia is due to a cartilage disorder or can be caused by an external source (vascular rings, masses, etc). Symptoms are similar to those of airway obstruction and include stridor and wheezing.

References

  1. Moore, K. L., et al. Clinically oriented anatomy. Lippincott Williams & Wilkins, 2017.
  2. Drake, R., et al. Gray’s anatomy for students E-Book. Elsevier Health Sciences, 2014.
  3. Standring, S. Gray’s anatomy: The anatomical basis of clinical practice, 41st ed. Edinburgh: Churchill Livingstone/Elsevier, 2016.

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