fluid dynamics - What determines the dominant pressure-flow relationship for a gas across a flow restriction?

If one measures the pressure drop across any gas flow restriction you can generally fit the relationship to

$$\Delta P = K_2Q^2+K_1Q$$ where $\Delta P$ is the pressure drop and $Q$ is the volumetric flow

and what I've observed is that if the restriction is orifice-like, $K_2 >> K_1$ and if the restriction is somewhat more of a complex, tortuous path, $K_1 >> K_2$ and $K_2$ tends towards zero.

I get that the Bernoulli equation will dominate when velocities are large and so the square relationship component. But what's determining the $K_1$ component behavior? Is this due to viscosity effrects becoming dominant? Does the Pouiselle relationship become dominant?