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Research Article

Duration of urination does not change with body size

Patricia J. Yang, Jonathan Pham, Jerome Choo, and David L. Hu
  1. Schools of aMechanical Engineering and
  2. bBiology, Georgia Institute of Technology, Atlanta, GA 30332

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PNAS August 19, 2014 111 (33) 11932-11937; first published June 26, 2014; https://doi.org/10.1073/pnas.1402289111
Patricia J. Yang
Schools of aMechanical Engineering and
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Jonathan Pham
Schools of aMechanical Engineering and
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Jerome Choo
Schools of aMechanical Engineering and
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David L. Hu
Schools of aMechanical Engineering and
bBiology, Georgia Institute of Technology, Atlanta, GA 30332
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  • For correspondence: hu@me.gatech.edu
  1. Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved May 14, 2014 (received for review February 6, 2014)

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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.

  • urology
  • allometry
  • scaling
  • Bernoulli's principle

Footnotes

  • ↵1To whom correspondence should be addressed. Email: hu{at}me.gatech.edu.
  • Author contributions: P.J.Y. and D.L.H. designed research; J.P. and J.C. performed research; P.J.Y. and D.L.H. analyzed data; and P.J.Y. and D.L.H. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1402289111/-/DCSupplemental.

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Urination time does not change with body size
Patricia J. Yang, Jonathan Pham, Jerome Choo, David L. Hu
Proceedings of the National Academy of Sciences Aug 2014, 111 (33) 11932-11937; DOI: 10.1073/pnas.1402289111

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Urination time does not change with body size
Patricia J. Yang, Jonathan Pham, Jerome Choo, David L. Hu
Proceedings of the National Academy of Sciences Aug 2014, 111 (33) 11932-11937; DOI: 10.1073/pnas.1402289111
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Proceedings of the National Academy of Sciences: 111 (33)
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