The friction loss in uniform, straight sections of pipe, known as "major loss", is caused by the effects of viscosity, the movement of fluid molecules against each other or against the (possibly rough) wall of the pipe. Here, it is greatly affected by whether the flow is laminar (Re < 2000) or turbulent (Re > 4000):
* In laminar flow, losses are proportional to fluid velocity, V; that velocity varies smoothly between the bulk of the fluid and the pipe surface, where it is zero. The roughness of the pipe surface influences neither the fluid flow nor the friction loss. 
* In turbulent flow, losses are proportional to the square of the fluid velocity, V;  here, a layer of chaotic eddies and vortices near the pipe surface, called the viscous sub-layer, forms the transition to the bulk flow. In this domain, the effects of the roughness of the pipe surface must be considered. It is useful to characterize that roughness as the ratio of the roughness height ε to the pipe diameter D, the "relative roughness".  Three sub-domains pertain to turbulent flow:
** In the smooth pipe domain, friction loss is relatively insensitive to roughness. 
** In the rough pipe domain, friction loss is dominated by the relative roughness and is insensitive to Reynolds number.
** In the transition domain, friction loss is sensitive to both. 
* For Reynolds numbers 2000 < Re < 4000, the flow is unstable, varying with time as vortices within the flow form and vanish randomly. This domain of flow is not well modeled, nor are the details well understood.