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Airway resistance quantifies the pressure difference needed to drive a specified volumetric flow rate through the conducting airways. For a steady linear relation with flow $Q$ positive toward the alveoli,The units are pressure divided by volume per time, for example $\mathrm{cm}{\mathrm{H}}_2\mathrm{O}\mathrm{s}{\mathrm{L}}^{-1}$. A stipulated pressure drop of 1 cm water at $Q=0.5\mathrm{L}{\mathrm{s}}^{-1}$ gives $R_{\mathrm{aw}}=2$ in those units. The pressure references matter: mouth-to-alveolar pressure concerns the flow path, whereas transpulmonary pressure also describes elastic loading of the lung.

An instructive idealization is the Hagen-Poiseuille equation. For a rigid cylindrical tube carrying steady laminar flow of a fluid with dynamic viscosity $\mu$,Reducing its radius to 80% while holding length and dynamic viscosity fixed multiplies resistance by $1/0.8^4\approx 2.44$. Real airways form a branching, deformable network and need not carry fully developed laminar flow. The tube equation explains a sensitivity, not a universal law for every lung measurement.

Network structure also matters. For $m$ identical linear branches in parallel, pressure drop is shared and their volumetric flow rates add, giving $R_{\mathrm{eq}}=R/m$. Many narrow distal branches can therefore have a very different combined resistance from one isolated narrow tube. Radius alone is insufficient without the arrangement and number of pathways.

Secant versus differential airway resistance distinguishes two values when the relation is nonlinear. For an original model $\Delta P=aQ+bQ|Q|$, the finite ratio is $\Delta P/Q=a+b|Q|$, while its local slope is $d\Delta P/dQ=a+2b|Q|$ away from zero. Report the operating flow and which definition is used.

A 1959 primary study combined mouth pressure with an intra-esophageal balloon and a differential pressure transducer to investigate resistance and compliance.^[ref] The mouth-to-esophageal difference is not a pure airway pressure drop: its elastic component must be separated. Simultaneous pressure, volume and flow information is what makes this mechanical distinction experimentally accessible.