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Steady mass conservation makes $\alpha u=Q$. For uniform area, $u=U=Q/{\alpha }_0$ is constant and the pressure-area law of a collapsible tube gives $p_x=p_e^{\prime }(x)$. The acceleration vanishes, so momentum balance requiresThe external pressure must therefore decrease linearly along a positive-flow tube. Its additive constant is free; the constant transmural pressure is $\tilde{P}({\alpha }_0)$.

The first equation is mass conservation for an incompressible flow whose area may change: local area storage balances the divergence of the volume flux $\alpha u$. The two left-hand momentum terms are local acceleration and convective acceleration, while $-p_x$ is the driving pressure gradient and $-R(\alpha )u$ models viscous drag per unit mass. The pressure-area law of a collapsible tube expresses transverse wall balance using internal minus prescribed external pressure.

For example, choose a reference area ${\mathcal{A}}_{\ast }$, length $L$ and velocity $U_{\ast }$. If dimensional variables are $\mathcal{A},s,t_d,v,P$, takeThe dimensionless external pressure uses the same scale. A dimensional friction rate $r_f(\mathcal{A})$ becomes $R(\alpha )=L{r}_f({\mathcal{A}}_{\ast }\alpha )/U_{\ast }$, and the dimensional wall pressure law is divided by $\rho U_{\ast }^2$. These choices make both inertia and the pressure-gradient coefficient unity; other equivalent reference scales are possible. On the mechanically stable wall branch, increasing area requires increased transmural pressure, whereas an enlarged tube reduces resistance. Thus $\boxed{{\tilde{P}}^{\prime }(\alpha )>0,R^{\prime }(\alpha )<0}$.

The pressure-area law of a collapsible tube describes transverse mechanical equilibrium of a deformable tube wall. On a stable branch ${\tilde{P}}^{\prime }(\alpha )>0$. Linearizing mass and momentum conservation about a stationary uniform tube gives the displayed wave speed. External pressure is prescribed separately and is not part of the elastic wall stiffness.