Airway resistance and its measurement

Principle
Airflow between the nose/mouth and the alveoli is driven by a pressure difference. The airways in between form a resistance

R=ΔP/Q 

where R is resistance, ΔP is the pressure gradient and Q is airflow

Resistance itself is described by the Poiseuille law

R = 8ηl/(πr4)

Where η is the viscosity of the inspired gas, l and r are the length and radius of the airway

Evidently, contraction of bronchi results in decreased airflow.

Measurement
Airway resistance is measured via pneumotachography, which compares pressure differences at two ends of a tube through which the subject breathes.

Increased airway resistance indicates obstruction of the airways.