Poiseuille's Principle

In addition, Poiseuille's equation tells how much pressure differential is required for a fluid of a given viscosity to flow through a tube with a given radius and length. The image below shows Poiseuille's equation. In the equation, F stands for the rate at which the fluid flows, r is the radius of the tube, delta p is the change in pressure from one end of the tube to the other, the lower-cased n (actually lower-case Greek symbol eta) stands for the viscosity of the fluid, and L stands for the length of the tube.