Airway resistance is the opposition to flow caused by the forces of friction. It is defined as
the ratio of driving pressure to the rate of air flow. Resistance to flow in the airways depends on whether the flow
is laminar or turbulent, on the dimensions of the airway, and on the viscosity of the gas.
For laminar flow, resistance is quite low. That is, a relatively small driving pressure is needed
to produce a certain flow rate. Resistance during laminar flow may be calculated via a
rearrangement of Poiseuille's Law : 
The most important variable here is the radius, which, by virtue of its elevation to the fourth power, has a tremendous impact on the resistance. Thus, if the diameter of a tube is doubled, resistance will drop by a factor of sixteen.
For turbulent flow, resistance is relatively large. That is, compared with laminar flow, a much larger driving pressure would be required to produce the same flow rate. Because the pressure-flow relationship ceases to be linear during turbulent flow, no neat equation exists to compute its resistance.
While a single small airway provides more resistance than a single large airway, resistance to air flow depends on the number of parallel pathways present. For this reason, the large and particularly the medium-sized airways actually provide greater resistance to flow than do the more numerous small airways.
Airway resistance decreases as lung volume increases because the airways distend as the lungs inflate, and wider airways have lower resistance.
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