Answer:
Their diameters
Explanation:
An equation for the resistance to flow of a fluid through a pipe is
where
η = the viscosity of the fluid
l = the length of the pipe
r = the radius of the pipe
The fourth-power term shows that even small changes in the radius (or diameter) of the passageways will cause large changes in airway resistance.
When the diameter of the airway gets as small as 1 mm, the resistance to air flow is quite large.