Hands-on Activity: Kidney Filtering

Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
An illustration of the human kidneys.
Students learn the role of the kidney as a filtering system for blood


Students filter different substances through a plastic window screen, different sized hardware cloth and poultry netting. Their models show how the thickness of a filter in the kidney is imperative in determining what is filtered out and what stays in the blood stream.

Engineering Connection

Engineers design many different filters that help people live healthier lives. Engineers design filters to clean water, and they also design special filters called dialysis machines. This equipment, essentially a special filter, cleans blood for people whose kidneys are not working correctly.

Educational Standards

  •   Colorado: Math
  •   Colorado: Science
  •   Common Core State Standards for Mathematics: Math
  •   International Technology and Engineering Educators Association: Technology
  •   Next Generation Science Standards: Science

Learning Objectives

After this activity and accompanying worksheet, students should be able to:
  • Explain the role of the kidney as a filtering system for blood.
  • Describe the by-products of the excretory system.
  • Model the filtering function of a kidney on a larger scale.
  • Give examples of filters designed by engineers, such as dialysis machines.

Materials List

Each group needs:
  • 6-inch square pieces of each of the following:
  1. plastic window screening
  2. hardware cloth (½" mesh)
  3. hardware cloth (¼" mesh)
  4. hardware cloth (1/8" mesh)
  5. poultry netting (1" holes)
  • 1-2 sheets of newspaper (to cover desk)
  • 2 measuring cups or bowls (about 4 cups each)
  • large funnel (large enough to have large pebble flow through the neck)
  • ½ cup sand
  • ½ cup small pebbles in various sizes from 1/8" to >1"
  • ½ cup water
  • Filtering System Journal, 4 copies
  • Filtering Worksheet, 4 copies
To bind the screens for safety:
  • duct tape
Optional materials:
  • (optional) round coffee filter
  • (optional) 2 tbsp. flour (only needed if using a coffee filter)


To which human body system do the kidneys belong? The excretory ─ or urinary system! This is an important body system because it helps us remove any harmful substances from our body. The kidneys actually work as a filtering system for our blood. They take blood in from the bloodstream, remove waste products (such as salt, minerals and any toxins, or bad stuff) and combine them with water. Then the body gets rid of this water and waste combination, known as urine. The kidney, in a healthy adult, can process as much as 45 liters of water a day, releasing only 1-2 liters in the form of urine.
A photograph shows a man in a hospital bed connected to a dialysis machine receiving dialysis.
Figure 1. Biomedical engineers design dialysis machines.
Engineers have designed many amazing filtering systems. They have designed systems to filter bad stuff out of water so that people can have clean water to drink, which helps us stay healthy. They have designed filters to remove harmful particles from the air, so we can have clean air to breathe. They even design filters to prevent unwanted noise from coming through on our phones. Engineers also design special filters for people in the hospital or who have medical problems. Many of the machines that you see in hospitals have been designed by engineers.
Sometimes people have kidneys that do not work well, so engineers have designed a special machine, called a dialysis machine, which filters their blood when their kidneys can not. A dialysis system removes a person's blood through a tube, runs it through filters to remove wastes and extra fluids, and then returns the clean blood to their body. It is a truly amazing system! The dialyzer consists of thousands of small fibers. The blood runs through the fibers, and a cleansing solution runs along the outside of the fibers. The solution acts like a sponge, and it soaks up the extra fluid and waste from the blood. The dirty solution is discarded, and the clean blood gets returned to the body.
Today we are going to test some filter materials and decide which ones work best for removing objects from water. Although these filter models are larger than an actual kidney, they help us investigate how kidneys filter waste from our blood. Also, the model allows us to understand how engineers can design filters to create dialysis machines for people whose kidneys are not working properly.
A schematic drawing of the overall dialysis system, showing the steps of dialysis, including: blood being removed from a person's arm, running through the dialyzer, and the clean blood being returned to their arm.
Figure 2. The process of a dialysis machine.
A drawing of kidney dialysis machine's hollow fiber dialyzer showing the blood inlet and outlet, solution inlet and outlet, and fibers.
Figure 3. The fiber dialyzer of a dialysis machine.


The process of separating substances in solution by diffusion through semipermeable membranes.
Equipment that filters blood when the kidneys cannot.
The process that refers specifically to purifying blood by dialysis.


Before the Activity

  • Cut screening, poultry netting, and hardware cloth to the proper size.
  • Bind the poultry netting and hardware cloth with duct tape to cover sharp edges (see Figure 4).
A photograph showing the activity "filters." From left to right are different-sized screening with large holes to small holes. Also shown is poultry netting, a coffee filter, a container of pebbles and a fork and spoon.
Figure 4. The activity filters.
  • Assemble all necessary materials.
  • Prepare overhead of the Excretory System (attached).

With the Students

  1. Show students the Excretory System overhead. Ask students to help you identify the location of the kidneys, bladder and ureters. Have students write down these terms under the Vocabulary section of the Filtering System Journal.
  2. Remind students that the kidneys are a filtering system for the blood. Explain that engineers design dialysis machines for people whose kidneys are not working properly. Have students write a few sentences about how the kidney functions under "I've Learned" on their journal sheets.
  3. Explain that this activity is a model for how the kidney cleans the blood. Students should be clear that this is just a model and that the kidney does not actually filter solids, except for blood cells. The products filtered through the screens in the activity are meant to represent the waste products in the blood, which are excreted in urine. Urine contains glucose, sodium, potassium, bicarbonate, water, acid, blood cells, protein and urea (which is what makes urine yellow).
  4. Students should mix the sand, pebbles, flour (optional) and water in the first measuring cup or bowl, as shown in Figure 4.
A photograph showing a glass measuring cup full of pebbles, water and flour. A person is stirring the mixture. Also shown are a large white funnel and a bag of flour.
Figure 5. Preparing the "waste" to be filtered.
  1. The students should take turns filtering the water mixture through the funnel, poultry netting, different-sized hardware cloth, window screening and coffee filter (optional), from large-filter holes to small-filter holes (see Figure 6). The screening should be held over the second measuring cup/bowl. Students should then pour the mixture from the full measuring cup/bowl onto the screen over the empty container and then back again, using a different screen each time.
A photograph showing the activity "filters" in use. On the left is a picture of a person using a funnel to filter waste. On the right is a picture of a person pouring large pebbles onto a screen with fairly large-sized holes.
Figure 6. Filtering waste.
  1. Students should complete the Observations section of the Filtering System Journal.
  2. Discuss with the students what they have learned and have them work through the Filtering Worksheet. Ask the students how the filter model they created might be used by engineers in creating dialysis machines. Have students write down any other things they have learned in their Filtering System Journal under the "I've Learned" section, and answer any questions the students may have written in the "Questions I Have" section of their journal.

Safety Issues

Be careful when handling the sharp edges of the mesh screening.

Troubleshooting Tips

Use a large enough container to contain the poured water.


Pre-Activity Assessment

Vocabulary: Have students write down the terms kidneys, excretory system, ureters and bladder under the Vocabulary section of the Filtering System Journal. Have them discuss with a neighbor what these terms mean and write short definitions next to the terms.

Activity Embedded Assessment

Worksheet/ Observations: Explain to students that engineers and scientists record their observations while they are working with a model. Using the Filtering System Journal, ask them to write down anything they noticed during the activity in the "Observations" section of their journal, and any questions they might have in the "Questions I Have" section. Review their answers to gauge their mastery of the subject.

Post-Activity Assessment

Discussion: Lead a discussion out loud, asking students what they have learned about filters and engineering. Have students write down the things they have learned in their Filtering System Journal under the "What I've Learned" section. Make sure to answer any questions the students may have written in the "Questions I Have" section of their journal.
Math Extension: Have students work through the Filtering Worksheet. Remind students that there are numbers that engineers would need to know in order to size a dialysis machine properly.

Activity Extensions

Have students research engineering systems that use filters (water treatment systems, waste water treatment systems, etc.) Remind students that many engineered systems actually are designed with knowledge that engineers have learned through science and nature.
Invite a medical technician to speak to the class about dialysis machines.

Activity Scaling

For upper grades, explain more of the details of how a dialysis machine works. Some students may have a friend or relative who is or has been on dialysis. Talk about what qualities are important to consider when designing a dialysis machine (sterility, work as similar to the body's function as possible, comfort level of the patient, ease of use for the patient and nurse/aide, etc.). Check out http://kidney.niddk.nih.gov/kudiseases/pubs/hemodialysis/ for more information on dialysis. Have students complete the challenge section of the math worksheet.
For lower grades, discuss why we use the filters in order of largest to smallest mesh size, and not the other way around. Talk with students about how in mathematics and in many engineering applications ─ (as well as in life!) ─ the order in which we do things is very important. Doing things in the correct order helps us achieve and understand the outcomes of a model. The math worksheet may not be appropriate for students under 3rd grade.


U.S. Department of Health and Human Services, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC), NIH Publication No. 06–4281, March 2006, "Prevent Diabetes Problems: Keep Your Kidneys Healthy," http://diabetes.niddk.nih.gov/dm/pubs/complications_kidneys/

U.S. Department of Health and Human Services, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC), NIH Publication No. 03–4666, September 2003, "Treatment Methods for Kidney Failure: Hemodialysis," http://kidney.niddk.nih.gov/kudiseases/pubs/hemodialysis/


Jessica Todd, Emily Weller, Sara Born, Abigail Watrous, Denali Lander, Malinda Schaefer Zarske, Janet Yowell


© 2006 by Regents of the University of Colorado

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder


The contents of this digital library curriculum were developed under grants from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation (GK-12 grant no. 0338326). However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: February 10, 2016

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