Comprehensive Study Notes on Upper and Lower Airway Anatomy, Physiology, and Mechanics

The upper airway terminates at the larynx, which houses the vocal cords and the glottis—the opening to the trachea.

Epiglottis: A thin, flat, leaf-shaped cartilage attached at its lower end to the thyroid cartilage. It slants upward and posteriorly toward the base of the tongue.
Valecula: The anatomical space located between the epiglottis and the base of the tongue. During indirect intubation, the tip of a laryngoscope blade is placed into the valecula space to lift the epiglottis away from the vocal cords.
Epiglottitis: Inflammation of the epiglottis, most commonly caused by bacterial infection with haemophilus influenza, which can lead to upper airway obstruction.
Glottis: The opening between the true vocal cords leading to the trachea. It is the narrowest part of the adult larynx, whereas the cricoid cartilage is the narrowest point in neonates and infants.
Vocal Folds: Consist of two pairs of membranes:
- True Vocal Cords: Inner whitish membranes that move to vocalize, generate high intra-thoracic pressures before a cough, and protect the airway.
- False Vocal Cords: Located on the outer surface of the true vocal cords.

Vocal Cord Dynamics: Vocal cords open during inspiration and close during expiration, swallowing, and coughing.
Glottic and Subglottic Edema: Most commonly caused by prolonged placement of an endotracheal tube. Removing the tube when edema is present can cause complete airway closure.
Laryngospasm: A protective motor reflex mediated by the vagus nerve (10th cranial nerve) causing the vocal cords to close over the tracheal opening. It is triggered by aspiration of non-air substances or excessive physical, chemical, or severe thermal stimuli.
Laryngectomy: Complete surgical removal of the larynx (often due to laryngeal cancer). Patients breathe solely through a neck stoma; manual resuscitation requires placing a pediatric mask directly over the stoma.

The lower airway begins below the larynx at the trachea and divides continuously via dichotomous branching up to $23$ generations.

Trachea: Approximately $11\,\text{cm}$ long, extending from the 6th cervical vertebra (C6) down to approximately the 5th thoracic vertebra (T5).
Carina: The main bifurcation point of the trachea into the left and right mainstem bronchi.
Right Mainstem Bronchus: Wider and straighter than the left, branching at a $30^\circ$ angle from midline. Misplaced endotracheal tubes enter the right mainstem bronchus most frequently.
Left Mainstem Bronchus: Narrower in diameter, branching at a $45^\circ$ to $55^\circ$ angle from midline.
Isothermic Saturation Boundary (ISB): Located a few subdivisions below the carina. At the ISB, inhaled gas reaches body temperature ( $37^\circ\text{C}$ ) and $100\%$ relative humidity.

Bronchioles: Airways smaller than $1\,\text{mm}$ in diameter that lack cartilage in their walls. Airway patency is maintained by the retractile elastic forces of the lung parenchyma.
Parenchyma: Supportive lung tissue containing elastic fibers surrounding the airways. Destruction of parenchyma and elastic fibers (as seen in emphysema) causes airways to collapse on exhalation.
Conducting Zone: Comprises generations $0$ through $17$ , ending at the terminal bronchioles.

The acinus represents the respiratory zone where gas exchange occurs, starting at the 17th generation.

Components: Respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.
Alveolar Count: Term newborns possess approximately $50{,}000{,}000$ alveoli; full development reaches $300{,}000{,}000$ alveoli by age $8$ . Total surface area equals approximately half a tennis court.
Collateral Ventilation:
- Pores of Kohn: Direct openings connecting adjacent alveoli.
- Canals of Lambert: Channels connecting terminal bronchioles directly to alveoli.
Atelectasis: Collapsed alveoli.

Respiratory Mucosa: Pseudostratified ciliated columnar epithelium lines the conducting airways down to the terminal bronchioles.
Mucus Production: Submucosal glands (contribute the greatest volume) and goblet cells secrete mucus in conducting airways. Clara cells produce secretions in terminal and respiratory bronchioles.
Mucociliary Escalator: Cilia beat rapidly within the fluid sol layer to move the sticky gel layer and trapped particles upward toward the pharynx.
Inspirated Mucus: Dehydrated, thick, sticky mucus caused by lack of humidity, seen in cystic fibrosis, asthma, and chronic bronchitis.

Epithelial and Endocrine Cells: Serous cells, Kulchinsky cells (precursors to carcinoids and small cell carcinomas), respiratory airways secretory cells (RASCs), and Clara cells.
Inflammatory and Immune Cells:
- Neutrophils: White blood cells ( $40\%$ – $45\%$ of total) involved in bacterial infections; forms purulent (yellow/green) mucus.
- Eosinophils: White blood cells characteristically elevated in allergic asthma.
- Mast Cells: Contain inflammatory mediators (histamine, leukotrienes, prostaglandins) that trigger bronchospasm and edema upon breakdown.
- Alveolar Macrophages: Phagocytic cells in alveoli that digest microbes, clear foreign debris, and neutralize free radicals.
Alveolar-Capillary (AC) Membrane:
- Type I Alveolar Cells: Flat squamous cells covering $90\%$ to $95\%$ of the alveolar surface, optimized for gas exchange.
- Type II Alveolar Cells: Cuboidal cells containing lamellar bodies that synthesize and secrete surfactant.
- Surfactant: Phospholipid complex that reduces surface tension in the fluid lining of alveoli, preventing alveolar collapse during exhalation.

Question: What conditions cause a loss of pharyngeal muscle tone leading to upper airway collapse?
Response: Unconsciousness, drug overdose, anesthesia or procedural sedation, and deep REM sleep.

Question: What is the primary clinical sign of an upper airway obstruction?
Response: Stridor, a loud, high-pitched sound typically heard on inspiration. External anatomical compression (such as thyroid pressure) can also create upper airway noise during sedation, distinct from internal blockages.

Question: How do responses differ between right mainstem intubation and esophageal intubation?
Response: Right mainstem intubation presents with unilateral breath sounds, reduced chest movement on the left, and elevated manual bagging resistance; it is corrected by pulling the endotracheal tube back slightly. Esophageal intubation requires complete removal of the tube and full re-intubation.