Dear student,

For laminar, non-pulsatile fluid flow through a uniform straight pipe, the flow rate (volume per unit time) is given by Poiseuille's Equation:

Obviously, the use of Poiseuille's Equation on the human circulatory system is highly suspect. While the flow is essentially laminar outside of the capillaries, it is definitely pulsatile throughout the arterial subsystem. In addition, the equation is based on the parabolic velocity gradient discussed in the last section. But since pressure waves in arterial walls propagate more quickly than those in blood, the velocity profile is more uniform than parabolic. Beyond that, Poiseuille's Equation assumes a constant viscosity, whereas the viscosity of blood actually changes with velocity, since blood is not a uniform fluid. In fact, the viscosity is much lower in the capillaries than in the rest of the system, since the red blood cells line up in single file to pass through them. On top of everything else, the blood vessels are not straight, uniform pipes!