There are several important determinants of airway resistance including: In fluid dynamics, the Hagen–Poiseuille equation is a physical law that gives the pressure drop in a fluid flowing through a long cylindrical pipe.
and given the above definition shows:- While the assumptions of the Hagen–Poiseuille equation are not strictly true of the respiratory tract it serves to show that, because of the fourth power, relatively small changes in the radius of the airways causes large changes in airway resistance.
Therefore, resistance is greatest at the bronchi of intermediate size, in between the fourth and eighth bifurcation.
This means that a large increase in pressure difference is required to maintain flow if it becomes turbulent.
In cases of upper airway obstruction the development of turbulent flow is a very important mechanism of increased airway resistance, this can be treated by administering Heliox, a breathing gas which is much less dense than air and consequently more conductive to laminar flow.
Therefore, diseases affecting the respiratory tract can increase airway resistance.
Specific airway conductance is often measured at FRC, in which case: [3]