  Many urological studies rely upon animal models such as rats and pigs whose
urination physics and correlation to humans are poorly understood. Here we
elucidate the hydrodynamics of urination across five orders of magnitude in
animal mass. Using high-speed videography and flow rate measurement at Zoo
Atlanta, we discover the "Law of Urination," which states animals empty their
bladders over nearly constant duration of average 21 seconds (standard
deviation 13 seconds). This feat is made possible by larger animals having
longer urethras, thus higher gravitational force and flow speed. Smaller
mammals are challenged during urination due to high viscous and surface tension
forces that limit their urine to single drops. Our findings reveal the urethra
constitutes as a flow enhancing device, enabling the urinary system to be
scaled up without compromising its function. This study may help in the
diagnosis of urinary problems in animals and in inspiring the design of
scalable hydrodynamic systems based on those in nature.
