Summarize the anatomy of the trachea.

Anatomy of the Trachea

The trachea is a fibromuscular tube extending from the lower border of the cricoid cartilage (approximately C6 level) to the carina, where it bifurcates into the left and right main bronchi, with a cross-sectional area of approximately 2.5 cm² in adults. 1

Structural Components

Cartilaginous Framework

• The trachea is formed by 16-20 C-shaped (U-shaped) cartilaginous rings that provide structural support anteriorly and laterally 2
• The posterior wall lacks cartilage and is closed by the trachealis muscle, a smooth muscle band that allows for dynamic changes in tracheal diameter 2
• The perichondrium consists of two distinct layers: an outer fibrous layer and an inner transitional layer of immature cartilage cells, both containing type I and type III collagen 3

Wall Layers

• The tracheal wall comprises three main layers from inner to outer: 1
  • Mucous membrane (respiratory epithelium)
  • Smooth muscle layer (positioned between epithelium and cartilage)
  • Cartilaginous or fibrous layer (providing structural support)

Adventitial Layer

• The adventitia divides into two layers: a loose outer layer and a dense inner fibrous layer 3
• Fibrous bundles from the adventitia join the connective tissue of the annular ligaments (connecting adjacent cartilage rings), allowing the trachea to stretch and bend during respiration and neck movement 3

Hierarchical Position

• The trachea represents generation 0 in the dichotomous branching model of the airway tree 1
• It serves as the entrance to the conducting airways, connecting the larynx to the bronchial tree 1

The Carina

• The carina is the ridge at the tracheal bifurcation, most commonly formed by the right main bronchial cartilages (72.41% of cases), with the left bronchial cartilages contributing in 37.93% of cases 4
• The carina is slightly deviated to the left of midline in approximately 4.37% of individuals 4
• This region is highly sensitive to mechanical stimulation and represents one of the most cough-sensitive areas of the respiratory tract 1

Smooth Muscle Distribution

• Airway smooth muscle lines the entire trachea and extends throughout the conducting airways to the terminal bronchioles, but is absent from respiratory bronchioles and alveoli 5
• The smooth muscle layer is positioned in the narrow zone between the epithelium and outer structural layer 5
• Cough-receptor nerve endings reside between the epithelium and smooth muscle layer 5

Clinical Relevance

Vulnerable Regions

• The distal trachea and carina are particularly susceptible to injury from deep endotracheal suctioning, as suction catheters can cause repeated mucosal trauma leading to granulation tissue formation and stenosis 1
• The larynx, tracheal bifurcations, and main bronchi are the most mechanically sensitive regions for eliciting the cough reflex 1

Pathologic Considerations

• Tracheomalacia results from cartilage weakness or loss, causing excessive airway collapse (>50% reduction in lumen diameter) during forced expiration 6
• The two-layer adventitial structure allows for tracheal flexibility but can be disrupted by external compression from masses, vascular anomalies, or surgical trauma 7, 3