What are the flow mechanics of nasal aerodynamics?

Updated: Jul 13, 2021
  • Author: Samuel J Lin, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Briefly, the biomechanics of nasal resistance relate to the study of turbulent flow. Resistance is pressure (P) divided by flow (Q). Based on principles from the Poiseuille equation for laminar flow equations, a decrease in radius (for example, of the nasal airway) causes a four-fold decrease in flow (where L is length, r is radius, η is viscosity, and ρ is density). [6]

Q = (Δ P π r4)/(8 η L)

Reynolds number = 2rQ ρ/η

Mathematically, a Reynolds number greater than 2000 is equated with turbulent flow. [1] The presence of laminar or turbulent flow in the nasal passageway is pertinent to the physiology of air exchange (see image below). Inspiratory flow is generally considered as laminar flow. Inspiration lasts approximately 2 seconds and ranges from 12-18 m/s at the nasal valve area during quiet respiration. [1] Comparatively, expiratory flow has more components of turbulent flow. By disrupting the boundary between the laminar airflow and the mucosa, [20] turbulent flow facilitates the exchange of heat and moisture. Turbulent flow occurs when transnasal pressures exceed 40-80 Pa. Expiration lasts approximately 3 seconds. [1] As the turbulent air passes over the inferior and middle turbinates during expiration, there is greater contact with the nasal mucosa for heat recovery. [21] Turbulent flow requires more energy expenditure but results in better mixing, which contributes to nasal function. Turbulent flow can prevent clearance of air, which can cause a sensation of obstruction regardless of nasal passage patency.

(A) Laminar nasal airflow at a low inspiratory flo (A) Laminar nasal airflow at a low inspiratory flow rate. (B) Turbulent nasal airflow at a higher inspiratory flow rate. Illustration by William E. Walsh, CMI; Northwestern University medical student and certified medical illustrator.

Nasal resistance is important for respiratory physiology. Automatic positive end-expiratory pressure (Auto-PEEP) occurs from the work that is involved in overcoming resistance during expiration. In postlaryngectomy patients, alveolar collapse is imminent with the loss of nasal airway resistance and Auto-PEEP. The glottis acts as an internal valve to regulate expiratory airflow, thus allowing alveoli to stay open longer during expiration and allowing continued gas exchange.

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