MIT Massachusetts Institute of Technology

Evolving for Survival: Learning about Fluid Dynamics through the Urinary System of Mammals

David Hu
Associate Professor of Mechanical Engineering and Biology
Georgia Institute of Technology
Atlanta, Georgia

Patricia Yang
Graduate Student
Georgia Institute of Technology
Atlanta, Georgia


In this lesson, students will learn about fluid dynamics by studying how the mammalian urinary system evolved for survival. This lesson uses simple hands-on experiments to visualize the urinary system and to help students practice the following skills:

  1. Thinking visually and physically to understand how gravity is acting on the fluid in pipes;
  2. Manipulating values of physical quantities with different units;
  3. Comparing the calculation of fluid flow in real life biological systems.

Students will gain experience using Torricelli’s Law, building their own water bottle experiments, and applying their observation to urinary systems inside mammalian bodies. No calculus will be used in this lesson, but familiarity with speed and flow rates is suggested. The lesson will take about an hour to complete. Students should prepare plastic bottles, plastic pipes, hot glue, a timer, and a calculator and, if possible, close access to water for the activities. The activities involve draining out water, so students might get wet. Between lessons, it is recommended that students discuss the experiment and calculations with their neighbors.

Dr. David Hu is a mechanical engineer who studies the movement of animals. He is particularly interested in how animals use specialized surfaces such as water-repellent fur and snakeskin. David has studied how snakes slither, how insects walk on water, and how small insects fly in the rain. Videos and images used in our work may be found on his website

Patrica Yang is a graduate student who studies fluid transportation in the body. She has been studying the fluid mechanics of urination, defecation and digestion. Her dream is to continue work with animals as a professional scientist. 

Here you can download the full paper of this research, published in the Proceedings of the National Academy of Science and entitled “Duration of Urination Does Not Change with Body Size.”

This is a Wikipedia site explaining Torricelli’s Law.

This site, sponsored by the Wolfram Demonstration Project, presents a demonstration of Torricelli’s Law.

This is a video produced by the Science Friday team of Professor David Hu discussing his research on mammalian urinary systems.

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