http://www.pnas.org/content/111/33/11932.short
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Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved May 14, 2014 (received for review February 6, 2014)
Significance
Animals eject fluids for waste
elimination, communication, and defense from predators. These diverse
systems all rely on the
fundamental principles of fluid mechanics,
which we use to predict urination duration across a wide range of
mammals. In this
study, we report a mathematical model that
clarifies misconceptions in urology and unifies the results from 41
independent
urological and anatomical studies. The
theoretical framework presented may be extended to study fluid ejection
from animals,
a universal phenomenon that has received
little attention.
Abstract
Many urological studies rely on
models of animals, such as rats and pigs, but their relation to the
human urinary system is
poorly understood. Here, we elucidate the
hydrodynamics of urination across five orders of magnitude in body mass.
Using high-speed
videography and flow-rate measurement
obtained at Zoo Atlanta, we discover that all mammals above 3 kg in
weight empty their
bladders over nearly constant duration of
21 ± 13 s. This feat is possible, because larger animals have longer
urethras and
thus, higher gravitational force and
higher flow speed. Smaller mammals are challenged during urination by
high viscous and
capillary forces that limit their urine to
single drops. Our findings reveal that the urethra is a flow-enhancing
device,
enabling the urinary system to be scaled
up by a factor of 3,600 in volume without compromising its function.
This study may
help to diagnose urinary problems in
animals as well as inspire the design of scalable hydrodynamic systems
based on those
in nature.
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