Poiseuille's Law: Breakdown of the Model

Prior to explaining when the model fails, we begin by stating the assumptions of Poiseuille's law. First, we assume that the fluid is in a steady state. This means that the speed at any point inside of our tube always remains the same. Secondly, we assume that the flow is laminar, which means that the fluid acts like layers of thin cylindrical sheets which travel individually without tearing or crossing. Thirdly, the fluid is viscous so that neighboring sheets of fluid create frictional forces between them.

Whenever an assumption is violated, the validity of the law comes into question. When the flow changes with time, the law is inadequate. Note that since the heart beats periodically, this means the law is not completely valid. However, it is still useful; just not accurate. There is a related law that accounts for the time variability. But even more importantly, when the flow is not laminar, the theory breaks down. This situation is referred to as turbulence. Turbulence will occur if the velocity becomes to great or if the viscosity becomes too small. Such is the case in the major arteries where the blood moves very rapidly.