Anatomical Cutoff Between Upper and Lower Airways
The larynx serves as the anatomical boundary between the upper and lower airways, with the upper airway extending from the nose and mouth down to the larynx, and the lower airway beginning at the trachea and extending through the bronchi to the alveoli. 1
Anatomical Definition
Upper Airway Components
- Nasopharynx, oropharynx, larynx, and vocal cords constitute the upper airway structures 1
- The upper airway terminates at the level of the larynx/vocal cords 1
Lower Airway Components
- Trachea, bronchi, bronchioles, and alveolar spaces comprise the lower airway 1
- The lower airway begins immediately below the larynx at the tracheal opening 1
Clinical Significance of This Distinction
Physiologic Differences
- Extrathoracic (upper) airway obstruction affects inspiratory flow more than expiratory flow because negative intraluminal pressure during inspiration is opposed by atmospheric pressure surrounding these airways 1
- Intrathoracic (lower) airway obstruction affects expiratory flow more than inspiratory flow because positive pleural pressure during expiration compresses the airways 1
Diagnostic Implications
- Flow-volume loops can distinguish upper from lower airway pathology: a flat or truncated inspiratory flow-volume loop suggests upper airway dysfunction rather than lower airway disease 1
- Peak expiratory flow (PEF) is generally decreased in both upper and lower airway obstructions, making it less specific for localization 1
United Airway Disease Concept
Despite this anatomical boundary, the upper and lower airways function as a pathological continuum in many disease states 1, 2:
- Inflammatory processes commonly affect both regions simultaneously, particularly in allergic conditions 2, 3
- Treatment of upper airway inflammation often improves lower airway symptoms and reduces medication requirements 4
- All patients with rhinitis or rhinosinusitis should have their lower respiratory tract evaluated, and vice versa 2
Common Clinical Pitfall
Do not assume isolated upper or lower airway disease without comprehensive evaluation—up to 68% of patients with upper airway symptoms have concomitant lower airway abnormalities 5, and patients presenting with either asthma or rhinitis should be considered to have inflammation throughout the entire respiratory tract 3