The Trachea - Comprehensive Notes

Overview

Gross Anatomy

Dimensions

• Length: 10-12 cm (average adult male: 11.8 cm, range 10-13 cm)
• External diameter: 2.3 cm (AP) x 1.8 cm (transverse) in males; 2.0 cm x 1.4 cm in females
• Paediatric: 3 mm diameter at birth
• Wall thickness: approximately 3 mm
• Shape in cross-section: D-shaped (ovoid)

Extent and Position

• Begins: Lower border of the cricoid cartilage (vertebral level C6)
• Ends: Bifurcation at the carina (level T4/T5 posteriorly, manubrium anteriorly)
• Lies anterior to the esophagus and passes slightly posteriorly from neck to mediastinum
• Mobile: moves upward during swallowing, down and forward on inspiration

Relations

Anterior (cervical)

• Skin, superficial and deep cervical fascia
• Sternohyoid and sternothyroid muscles
• Thyroid isthmus (overlies 2nd-4th tracheal cartilages)
• Inferior thyroid veins

Anterior (mediastinal)

• Left brachiocephalic vein and artery
• Arch of the aorta (crosses left side)
• Left common carotid artery
• Deep cardiac plexus
• Pre-tracheal lymph nodes
• Innominate (brachiocephalic) artery crosses obliquely anterior at ~3rd ring/sternal notch level - important surgical landmark for tracheostomy

Lateral

• Lobes of the thyroid gland
• Carotid sheath and contents
• Para-tracheal lymph nodes

Posterolateral

• Recurrent laryngeal nerves in the tracheo-oesophageal groove (bilaterally)

Posterior

• Esophagus (directly posterior throughout)

Note in infants: The brachiocephalic vein may lie above the suprasternal notch, placing it at risk during tracheal surgery.

Cartilages

• 18-22 C-shaped (incomplete) rings of hyaline cartilage
• ~2 rings per cm; each ring ~4 mm in height
• Maintain patency against the negative pressure of inspiration and prevent collapse during expiration
• The first cartilage is broader and may fuse with the cricoid (or be separated by the cricotracheal ligament)
• The last ring above the carina is thick and broad, forming a bridge between the two main bronchi; its inferior border curves downward to form the carina
• Cartilage rings can become calcified with age (may even undergo ossification with marrow spaces visible on histology)
• Two or more rings may fuse or bifurcate

Posterior Wall - Trachealis Muscle & Fibrous Membrane

• The open posterior ends of the C-shaped rings face the esophagus
• These free ends are bridged by:
  • Trachealis muscle - smooth muscle with transverse and longitudinal fibres
  • A sheet of fibroelastic tissue continuous with the perichondrium
• The fibrous membrane encircles the rings, is composed of collagen and elastin in crossing orientations - permits expansion and aids elastic recoil
• Trachealis functions:
  • Relaxes during swallowing - allows esophagus to bulge into the tracheal space
  • Contracts forcefully in cough - narrows the lumen to increase expiratory velocity and dislodge secretions/debris

Histology

Wall Layers (from lumen outward)

Tracheal Wall Histology (H&E)

Trachea and Esophagus Cross-Section

Epithelium (Respiratory / Pseudostratified Ciliated Columnar)

1. Ciliated columnar cells (most numerous)
   • Extend full thickness of epithelium
   • Each cell has ~250 cilia
   • Basal bodies form a dark line just below the cilia on H&E
   • Beat in coordinated waves (mucociliary escalator) - sweeps mucus toward the pharynx
2. Goblet (mucous) cells
   • Contain mucinogen granules
   • Surface has blunt microvilli (stippled on SEM)
   • Secrete mucus that traps inhaled particles
3. Basal cells
   • Confined to the basal layer near the basement membrane
   • Stem cell function - regenerate other cell types
4. Brush cells - sparse; sensory function
5. Small granule cells (DNES/neuroendocrine) - sparse

Glands

• Seromucous glands in the lamina propria and submucosa (mainly mucous with some serous/mixed)
• Secretion is largely autonomically controlled
• Produce a mucin-rich protective barrier that traps debris and microorganisms

Blood Supply

• Blood supply enters lateral walls in a distributed segmental fashion
• Cervical trachea: supplied by inferior thyroid artery (via 3 tracheoesophageal branches, which may pass anterior or posterior to the recurrent laryngeal nerves)
• Segmental arteries branch superiorly and inferiorly over several rings, forming longitudinal anastomoses along the lateral walls
• Anterior branches further anastomose near the midline
• Posterior branches supply the membranous wall (also receives supply from oesophageal arteries)
• Venous drainage: inferior thyroid plexus
• Thoracic trachea / carina / bronchial tree: supplied by bronchial arteries (usually 3: 2 left = direct aortic branches; 1 right = from 3rd right posterior intercostal artery)
• Bronchial veins drain into the azygos vein (right) and accessory hemiazygos (left)

Surgical note: The blood supply enters posterolaterally. During tracheostomy, the inferior thyroidal artery/vein and anterior jugular veins may lie anteriorly between skin and trachea.

Lymphatic Drainage

• Lymphatics arise from a submucosal plexus and connect with a plexus in the outer fibrous membrane
• Drain into pretracheal nodes then para-tracheal nodes (Level VI)
• Para-tracheal nodes drain into internal thoracic and brachiocephalic lymph nodes
• These form bronchomediastinal trunks draining into:
  • Right lymphatic duct (right side)
  • Thoracic duct (left side)

Nerve Supply

• Autonomic:
  • Parasympathetic (vagus) - bronchoconstriction, increased secretion
  • Sympathetic - bronchodilation, decreased secretion
• The recurrent laryngeal nerves run in the tracheo-oesophageal groove posterolaterally and are at risk during tracheal and thyroid surgery

Carina and Bifurcation

• Trachea bifurcates at the carina at T4-T5 (posteriorly) / manubrium level (anteriorly)
• Right main bronchus: angle of 25-30° with the carina; length ~5 cm; diameter ~17 mm (males)
• Left main bronchus: angle of 45° with the carina; longer and more horizontal
• Clinical implication: aspirated foreign bodies preferentially enter the right main bronchus due to its wider angle and more vertical course

Physiology Summary

Airflow resistance rule: A 50% reduction in inner diameter increases resistance 16-fold (turbulent flow: 32-fold). At 5-6 mm lumen, only ~30% expiratory flow rate is maintained.

Key Clinical Points

• Tracheostomy landmarks: Thyroid isthmus over 2nd-4th rings; innominate artery crosses ~3rd ring
• Tracheal stenosis: Post-intubation (cuff pressure >20 cmH2O exceeds capillary perfusion pressure); post-tracheostomy (A-shaped scar); "sand-glass" stenosis from cuff injury
• Recurrent laryngeal nerve runs in the tracheo-oesophageal groove - at risk in thyroid/tracheal surgery
• Foreign body aspiration - right bronchus more commonly affected
• Age changes: Cartilage rings undergo progressive calcification and may ossify (marrow spaces visible)