The Reynolds number increases with fluid density, flow speed, and geometric obstruction. A high viscosity reduces the Reynolds number, making turbulence less likely.
The Reynolds number is an important quantity for the description of the flow of a real fluid. The Reynolds number describes the ratio of inertial forces and viscous forces in a given area of flow. If the Reynolds number is high, inertial forces may overcome viscous forces and lead to turbulent flow.
Cardiologists use the Reynolds number to try to predict turbulence in the cardiovascular system. In the cardiovascular system, important factors which may increase the Reynolds number include increased flow velocity, an obstruction, or decreased blood viscosity (as with severe anemia).
A localized narrowing of an arterial vessel, for example, could lead to increased flow velocity through that area. A two fold decrease in radius would correspond to a four fold increase in flow speed. Repeat that to yourself. If continuity of volume flux applies, a two fold decrease in radius leads to a four fold increase in flow speed. A much higher flow speed can lead to turbulence in an area. Cardiologists are trained to listen for characteristic instances of turbulence.