Anatomy of the Airway and Tracheobronchial Tree

1. Nasal Cavity

• The inferior meatus (between inferior turbinate and floor) is the preferred pathway for nasopharyngeal airways and nasotracheal tubes. Instruments placed elsewhere risk avulsing a turbinate.
• Nasal septal deviation is common in adults; always determine the more patent side before instrumentation.
• The cribriform plate (roof of the nasal cavity) is fragile; fracture creates a nasal-intracranial communication. Nasotracheal tubes are contraindicated with suspected basilar skull fractures.
• The mucosa is richly vascular. Topical vasoconstrictors (e.g., oxymetazoline or cocaine) must be applied before any nasal instrumentation to prevent epistaxis.
• The posterior openings, the choanae, lead into the nasopharynx.

2. Oral Cavity

• Mouth opening requires rotation within the temporomandibular joint (TMJ) followed by anterior sliding (subluxation) of the mandibular condyles. Restricted TMJ mobility (trismus, ankylosis, masseter hypertrophy) limits laryngoscopy blade insertion depth.
• The tongue is anchored by the genioglossus to the mandible. The jaw-thrust maneuver exploits this attachment - pulling the mandible forward pulls the tongue off the posterior pharyngeal wall, relieving obstruction.
• The mylohyoid muscle separates the sublingual space (superior) from the submental space (inferior). Infection or hematoma in these spaces (Ludwig's angina) displaces the tongue posteriorly, causing severe airway obstruction.
• The oral cavity leads into the oropharynx, bounded inferiorly by the tongue and superiorly by the hard and soft palates.

3. Pharynx

4. Larynx - The Critical Gateway

4.1 Cartilaginous Framework

• Thyroid cartilage - the largest cartilage; shield-shaped; forms the anterior "protective housing" of the vocal mechanism. The superior thyroid notch (Adam's apple/laryngeal prominence) is the key anterior neck landmark - palpated to identify midline and locate the cricothyroid membrane. Lies at approximately the level of C4-C5.
• Cricoid cartilage - the only complete cartilaginous ring in the airway. Lies at C6. Forms the inferior limit of the larynx. Anteriorly connected to thyroid cartilage by the cricothyroid membrane. Its signet-ring shape (narrow anteriorly, wider posteriorly) provides circumferential support. It is the narrowest point of the pediatric airway (making uncuffed ETTs appropriate in children); in adults, the glottic opening is the narrowest.
• Epiglottis - a leaf-shaped cartilaginous flap forming the anterior border of the laryngeal inlet. Its anterior surface connects to the hyoid bone via the hyoepiglottic ligament and to the base of tongue by the glossoepiglottic folds, creating the paired valleculae. The tip of the curved Macintosh blade seats in the vallecula; indirect pressure on the hyoepiglottic ligament lifts the epiglottis anteriorly to expose the glottis.

• Arytenoid cartilages - articulate with the posterior surface of the cricoid; serve as the posterior attachments of the vocal cords. Their movement (abduction/adduction) controls glottic aperture.
• Corniculate cartilages - sit atop the arytenoids; visible during laryngoscopy as the posterior cartilaginous prominences at the laryngeal inlet.
• Cuneiform cartilages - embedded in the aryepiglottic folds; create the lateral prominences seen during laryngoscopy.

4.2 Membranes and Ligaments

• Thyrohyoid membrane - connects the thyroid cartilage to the hyoid bone superiorly; the internal branch of the superior laryngeal nerve (SLN) pierces it to provide sensory supply to the supraglottic mucosa.
• Cricothyroid membrane (CTM) - connects the inferior edge of thyroid cartilage to the superior surface of the cricoid cartilage. This is the emergency surgical airway access point.
  • Dimensions: ~8-12 mm wide, 10.4-13.7 mm in height (Barash)
  • Located 1-1.5 fingerbreadths below the laryngeal prominence
  • Composed of yellow elastic tissue lying directly beneath the skin and a fascial layer - minimal subcutaneous fat in most patients, making it relatively accessible
  • Any incision or needle puncture should be made in the inferior third and directed posteriorly to avoid the vocal folds (0.9 cm above the upper border of the membrane) and the cricothyroid arteries (running along the superior border)
  • Ultrasound guidance improves identification, especially in obese patients; only ~one-third of trainees correctly identify the CTM by palpation alone

4.3 Laryngeal Inlet and Interior

• Anterosuperiorly: epiglottis
• Laterally: aryepiglottic folds (containing cuneiform and corniculate cartilages)
• Posteriorly: arytenoid cartilages and interarytenoid notch

1. Vestibule - from the laryngeal inlet down to the false vocal cords
2. Ventricular folds (false vocal cords) - superior folds, not involved in phonation; they have no active function in normal respiration
3. Ventricles - lateral recesses between the false and true cords
4. True vocal cords (vocal folds) - pearlescent white bands extending from the thyroid cartilage anteriorly (anterior commissure) to the arytenoids posteriorly; they contain the vocalis muscle
5. Glottis - the space between the true vocal cords; the anatomically narrowest point of the adult larynx
6. Subglottis - below the vocal cords to the inferior border of the cricoid

4.4 Laryngeal Innervation - Critical for Airway Blocks

• For awake intubation, the SLN internal branch is blocked with injection at the greater cornu of the hyoid, and the RLN is blocked via transtracheal injection of local anesthetic, or topical spray below the cords.
• Unilateral RLN injury causes hoarseness (paralyzed cord remains adducted at midline/paramedian position); the protective role against aspiration may also be compromised.
• Bilateral RLN injury causes complete airway obstruction by unopposed adduction of both cords; requires emergency airway.
• The cricothyroid muscle (innervated by the external SLN) is the tensor of the vocal cords - it is the only intrinsic laryngeal muscle not affected by RLN injury.

5. Trachea

• 16-20 C-shaped cartilaginous rings, open posteriorly
• The anterior and lateral walls are supported by cartilage; the posterior wall is the trachealis muscle (smooth muscle connecting the ends of the C-rings)
• The cricoid cartilage is the narrowest part: average diameter 17 mm in men, 13 mm in women

• Average tracheal length: 10-13 cm
• Lumen narrows slightly toward the carina
• ETT tip should lie at least 2 cm above the carina to avoid endobronchial intubation but below the vocal cords

• The trachea is a midline structure in the neck, flanked by the thyroid gland
• Anterior surface of tracheal rings is subcutaneous in the neck, accessible for surgical or percutaneous tracheostomy
• The esophagus lies immediately posterior to the trachealis muscle

6. Tracheobronchial Tree

Right vs. Left Mainstem Bronchus

Airway Generations (Weibel Model)

• Generations 0-16 = Conducting zone (no gas exchange): trachea → mainstem bronchi → lobar bronchi → segmental bronchi → bronchioles → terminal bronchioles
• Generations 17-19 = Respiratory bronchioles (first alveoli appear in walls; gas exchange begins)
• Generations 20-22 = Alveolar ducts
• Generation 23 = Alveolar sacs (~17 alveoli each)

• Ciliated columnar epithelium in trachea → cuboidal in bronchioles → flat alveolar epithelium
• Cartilaginous support disappears at the bronchiolar level (generation ~11)
• Smooth muscle remains throughout, providing the substrate for bronchoconstriction

7. Summary Table: Structures and Their Intubation Relevance

8. Infant vs. Adult Airway - Key Differences

• Miller's Anesthesia 10th ed - Chapter 40, Functional Airway Anatomy (pp. 5835-5845)
• Morgan & Mikhail's Clinical Anesthesiology 7th ed - Chapter 23, Tracheobronchial Tree (pp. 911-912)
• Barash Clinical Anesthesia 9th ed - Chapter 28, Review of Airway Anatomy (p. 2295)
• Roberts & Hedges' Clinical Procedures in Emergency - Chapter 4, Airway Anatomy