ARRT Ultrasound Practice Test

In laminar flow through a long straight tube, resistance grows with length because the viscous losses accumulate along the path. Poiseuille’s law links pressure drop, flow rate, and tube parameters: ΔP = (8 μ L Q)/(π r^4). If we define hydrodynamic resistance as R = ΔP/Q, then R = 8 μ L /(π r^4). With viscosity μ and radius r fixed, increasing the length L increases resistance linearly. This means for the same flow rate you need a larger pressure difference, or for the same pressure difference the flow rate decreases as length grows.