PSIO 532 Study Guide - Midterm Guide: Respiratory Tract, Viscosity, Newtonian Fluid
PSL – Respiratory Physiology
Poiseuille’s Law
Poiseuille Law and the Calculation of Airway Resistance
This law describes the relationship between pressure and flow for laminar flow in a tube.
➔ V’ = ∆P/R (Where V’ is flow, P is the pressure and R is resistance); flow is due to the
CHANGE in pressure between the atmosphere and alveoli
Since resistance is the driving pressure divided by flow then we can arrange this equation
to calculate resistance
➔ R = 8l/πr4 (r is the tube radius, is fluid viscosity and l is tube length); resistance is also
going to have an influence in airflow; if you increase resistance, you are going to
decrease airflow; high levels of resistance would also increase the WORK of breathing
→ this can be seen in individuals with chronic obstructive pulmonary disease (COPD);
individuals have trouble breathing because over time, they get very fatigued while trying
to breathe due to the presence of airway resistance
➔ O a da to da asis, the gas isosit doest hage ad either does the airway, so
ad l geerall sta the sae; ispirig Heliu, ould ause the gas isosit to
change and that will impact your airway resistance; but this is not much of an issue
because we just breathe atmospheric air
➔ Therefore, the resistance is primarily going to be influenced by the RADIUS of the
airway! Radius is to the 4th power here, so small changes to the radius can have large
effects on resistance.
Thus, Poiseulles la predicts that resistance to laminar flow (of air or a Newtonian fluid)
is directly proportional to viscosity and tube length and inversely proportional to tube
radius. Therefore, small changes in tube radius can have large effects on resistance.
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