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Water Can Kill?

Exploring Effects of Osmosis


Susan D. Hester
Department of Molecular and Cellular Biology
University of Arizona


This case study presents three true stories of people who suffered fatal hyponatremia (low sodium concentration in their body fluids) after consuming excessive amounts of water. Students examine the tonicity of the extracellular environment created by this water consumption, the effect this has on cells and how this impacts different tissues/organs in the body. After they understand that the immediate cause of death in these patients was due to brain swelling, students are then cast in the role of physicians who must confirm the diagnosis and administer emergency treatment. They also consider why certain populations (athletes, small children, and desert-dwellers) are at increased risk for hyponatremia. A final exercise asks students to calculate how much brain volume increases due to a specified change in solute concentration and a list of simplifying assumptions. The case was developed for an undergraduate introductory molecular and cellular biology class for majors, but is appropriate for any introductory biology, upper division physiology, or cell biology class in which osmosis is discussed.


  • Apply working definitions of the terms hypotonic, hypertonic, and isotonic.
  • Predict the net movement of water and its effect on cells in a solution that is hypotonic or hypertonic relative to the intracellular environment.
  • Explain how osmosis at the cellular level can affect tissues and organisms, even in the absence of cell lysis.
  • Calculate the change in an animal cell's volume resulting from osmosis if the cell is placed in a hypo- or hypertonic solution.


Osmosis; tonicity; hyponatremia; osmotic; hypotonic; hypertonic; isotonic; calculation; quantitative reasoning

Topical Areas


Educational Level

High school, Undergraduate lower division, Undergraduate upper division



Type / Methods

Analysis (Issues), Directed



Subject Headings

Cell Biology  |   Molecular Biology  |   Physiology  |   Biology (General)  |   Science (General)  |  

Date Posted


Teaching Notes

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Supplemental Materials

This case has a set of PowerPoint slides associated with it that provides a review for students of material relevant to the case.  It consists of a series of animations followed by multiple-choice questions. It is recommended for use as a pre-case/pre-class assignment.

   Hyponatremia Power Point

Answer Key

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Brendan Watts
Murray Bridge High School
Murray Bridge (South Australia)
This is a great way to incorporate the science into a real life application that will both engage and interest the students.