When flow in a straight cylindrical tube is relatively low, fluid particles move smoothly in concentric layers (box Figure, left). This type of flow is called laminar flow. The relation between the pressure gradient and flow is linear and described by Poiseuille’s law. As flow increases, the smooth parallel fluid motion becomes wavy, leading to vortices propagated downstream, subsequently the number of vortices increases and finally fluid motion becomes irregular . This irregular and seemingly random fluid particle motion is called turbulence. Turbulent flow is energetically more costly than laminar flow, because part of the mechanical energy used to maintain flow (i.e., pressure gradient) is lost in the erratic motion between the fluid particles. The resistance to flow is thus higher, which is reflected by the change in slope in the relation between pressure drop and flow (box Figure, right).