Poiseuille’s law (Chap. 2) shows that resistance depends on the length and diameter of the vessel and the viscosity of blood. However, even for a single blood vessel, it is difficult to derive the relation between pressure and flow on the basis of Poiseuille’s law. The diameter of the vessel needs to be accurately known because of the fourth power law. Furthermore, the vessel should be uniform, and, especially for small vessels, the anomalous viscous properties of blood make it impossible to use a single number for viscosity. Accurate calculation of resistance on the basis of Poiseuille’s law is therefore virtually impossible. However, resistance can be calculated from the ratio of the pressure gradient and flow constituting a practical experimental approach. Thus although Poiseuille’s law makes it possible to arrive at several important conclusions regarding vascular function, in practice we use resistance, calculated using Ohm’s law.