Internal friction in a fluid is a very important factor in the working of the human body. The friction, called viscosity, causes the flow to occur in layers which move relative to each other. For example in a pipe the friction with the walls causes the flow to be biggest in the center.
Every liquid has some viscosity,
where Q is the flow in m3/s, r is the radius in m,
Notice that reducing the diameter of an artery by a factor of 2 due to plaque build-up reduces the flow by a factor of 16. If you force the liquid to flow at too high a velocity, you go into turbulent flow and the equation is no longer valid. This turbulent flow causes noise, which can be observed by a physician looking for blockages.