Trachea - Detailed Anatomy (BAMS Syllabus)

1. Introduction and Definition

2. Extent and Vertebral Levels

• Begins at the lower border of the cricoid cartilage at vertebral level C VI
• Ends at the carina at the level of the sternal angle (angle of Louis), corresponding to vertebral level T IV/V

3. Measurements (Dimensions)

4. Course and Direction

• Begins anteriorly in the neck and slopes posteriorly as it enters the mediastinum
• Lies in the midline, directly anterior to the esophagus
• Shows significant vertical mobility - moves upward during swallowing, and downward/forward on inspiration
• Passes through the superior thoracic aperture (thoracic inlet) to enter the superior mediastinum

5. Relations

Anterior Relations:

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

• Left brachiocephalic vein
• Brachiocephalic (innominate) artery
• Arch of the aorta (crosses on the left)
• Left common carotid artery
• Deep part of the cardiac plexus
• Pretracheal lymph nodes
• Thymus gland (large and fleshy in infants)

Posterior Relations:

• Esophagus (directly posterior, lying anterior to the vertebral column)
• Postero-laterally: Recurrent laryngeal nerves run in the tracheo-esophageal groove

Lateral Relations:

• Lobes of the thyroid gland
• Carotid sheath and its contents (common carotid artery, internal jugular vein, vagus nerve)
• Paratracheal lymph nodes
• Azygos vein crosses on the right side in the superior mediastinum
• Arch of the aorta on the left side in the superior mediastinum

6. Structure of the Tracheal Wall

a) Mucosa

• Epithelium: Pseudostratified ciliated columnar epithelium (respiratory epithelium) with numerous goblet cells
• Rests on a broad basement membrane
• The cilia beat the mucus blanket upwards toward the larynx (mucociliary escalator)
• Lamina propria: Rich in longitudinal elastin fibres; contains numerous seromucous glands producing watery mucus

b) Submucosa

• Slightly denser connective tissue than the lamina propria
• Contains neurovascular structures, tubular glands, and lymphoid patches
• Contains additional seromucous glands

c) Cartilaginous Layer (Hyaline Cartilage Rings)

• 16-20 C-shaped (horseshoe-shaped) rings of hyaline cartilage
• Open ends face posteriorly (toward the esophagus)
• Function: Keep the tracheal lumen patent (prevent collapse, especially during expiration)
• The gap between the free ends is bridged by:
  • Trachealis muscle (smooth muscle with transverse and longitudinal fibres)
  • Fibroelastic tissue (fibroelastic membrane)
• The first tracheal ring is broader and may be joined to the cricoid cartilage by the cricotracheal ligament
• The last ring (before carina) is thick and broad, forming a bridge between the two main bronchi
• With aging, cartilages may calcify or even ossify (develop bone tissue and marrow spaces)

d) Adventitia

• Outermost layer of loose connective tissue
• Blends with the fascia over the esophagus and loose areolar tissue of the mediastinum
• Binds the trachea to adjacent structures

7. Trachealis Muscle

• Smooth muscle located at the posterior (membranous) wall of the trachea
• Has transverse fibres (insert into the perichondrium at free margins of cartilage rings) and longitudinal fibres
• Function:
  • Relaxes during swallowing - allows the esophagus to bulge slightly into the tracheal lumen to accommodate a food bolus
  • Contracts strongly during coughing - narrows the tracheal lumen, increasing velocity of expelled air to dislodge foreign material

8. Bifurcation - The Carina

• The trachea bifurcates at the carina into the right and left main (primary) bronchi
• Level: T IV/V posteriorly (sternal angle/manubriosternal joint anteriorly)
• The carina is a spur/ridge of cartilage seen on bronchoscopy at the bifurcation
• The last tracheal ring has a downward-curving triangular process forming the posterior carina

9. Blood Supply

• Blood supply enters posterolaterally (segmental fashion)
• Branches form a lateral longitudinal anastomosis along tracheal walls
• Anterior branches further anastomose near the midline
• Clinical point: Circumferential dissection around the trachea should be minimized to avoid devascularization

10. Nerve Supply

11. Lymphatic Drainage

• Lymphatics arise in a submucosal plexus and connect with a plexus in the outer fibrous membrane
• Pretracheal nodes → Paratracheal nodes (level VI) → Superior tracheobronchial nodes
• Para-tracheal nodes drain into vessels from the internal thoracic and brachiocephalic lymph nodes
• These form the bronchomediastinal trunks, which drain into:
  • Right lymphatic duct (right side)
  • Thoracic duct (left side)

12. Histological Images

13. Applied Anatomy / Clinical Significance

1. Tracheotomy / Tracheostomy: Surgical opening of the trachea is performed through rings 2-4, below the thyroid isthmus. The inferior thyroid veins and thyroid isthmus must be carefully handled. In infants, the brachiocephalic vein may be higher and at risk.
2. Foreign body aspiration: More commonly enters the right main bronchus due to its more vertical angle, wider lumen, and it being more directly in line with the trachea.
3. Goitre (enlarged thyroid): Can deviate or compress the trachea, causing stridor and dyspnoea.
4. Bronchoscopy: Allows visualization of the tracheal lumen, carina, and main bronchi. The carina appears as a sharp ridge at the bifurcation.
5. Endotracheal intubation: Knowledge of tracheal dimensions and vertebral levels is essential to avoid inadvertent right main bronchus intubation.
6. Recurrent laryngeal nerve injury: During thyroid surgery or tracheal dissection, the RLN running in the tracheoesophageal groove is at risk, potentially causing hoarseness.
7. Cough reflex: Stimulation of tracheal mucosa triggers the cough reflex via vagal afferents, with the trachealis contracting to rapidly narrow the lumen and increase expiratory airflow velocity.

• Gray's Anatomy for Students (neck and thorax sections)
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery (tracheal anatomy chapter)
• Junqueira's Basic Histology, 17e (tracheal histology)
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology (tracheal microanatomy)