A. Rigid Cylindrical Tube Assumption. Distensibility is much less in small vessels. B. Steady Flow. Flow in all the large arteries is too pulsatile for the simple Poiseuille equation to be used accurately. C. Laminar Flow.
Laminar flow occurs when viscous forces predominate.
All points in the liquid move parallel to the walls of the tube. Laminar flow almost certainly occurs in all vessels where flow is sufficiently steady to consider applying Poiseuille's Law. Turbulence may occur in the aorta at peak of ejection and in certain pathological conditions such as stenosis or severe anemia. Unless the stenosis is severe, the volumetric flow is not decreased by much, partly because it is the arterioles that provide the greatest resistance in the cardiovascular sytem. If the volumetric flow is not reduced too much, the velocity, V, may increase.