Lessons Direct

Dialysis tubing is a soft, bendy tube with very tiny holes that you cannot see. These tiny holes let small things pass through but stop bigger things from getting through. This is similar to how a kidney works in your body. Your kidneys clean your blood by letting small waste particles and extra water leave the blood, while keeping important things like proteins and blood cells inside. Because dialysis tubing works in a similar way, scientists and students can use it to model how kidneys filter blood.

In this experiment, the dialysis tubing will act like a model kidney. A liquid that represents blood will be placed inside the tubing. This liquid will have a mix of small and large particles. When the tubing is placed in water, some of the small particles will move through the tiny holes and into the water around it, just like waste moving out of the blood. The bigger particles will stay inside the tubing. By watching what happens, students can see how kidneys help clean the blood and make urine.

Before we start:

1. Read the Procedure. Are there any steps that you are not sure about? Will it help to draw the steps out?
   
2. What do kidneys do?
   
3. Why is dialysis tubing a good material to use for this model?
   

Materials

Dialysis tubing (pre-soaked)
String or clamps
2 clear cups or beakers
Measuring spoons
Stirring spoon
Water
Yellow food coloring
Sugar
Salt
Cornstarch or flour
Glitter or pepper (Optional)
Marker
Paper towels
Conductivity probe or meter (Optional)
Glucose test strips (Optional)

Procedure

1. Prepare the dialysis tubing. Soak the tubing in water until it is soft. Tie a tight knot (or clamp) on one end to seal it. Check for leaks by adding a small amount of water and gently squeezing; re-tie if needed.
2. Make the “blood” mixture. In a cup, mix about ½ cup water with 1–2 drops yellow food coloring. Stir in 1 teaspoon sugar and 1 teaspoon salt until dissolved. Add 1 tablespoon cornstarch (or flour) and stir well. (Optional: add a pinch of glitter or pepper to represent cells.)
3. Fill the model kidney. Pour the “blood” mixture into the dialysis tubing using a spoon or funnel, leaving a small space at the top so it can be tied. Tie or clamp the open end to seal the tubing. Rinse the outside of the tubing with water to remove any mixture on the outside.
4. Set up filtration. Fill a second clear cup with about 1 cup of clean water. Place the sealed dialysis tubing into this cup so it is fully submerged. Label the cup water as “Urine (outside)” and the liquid inside the tubing as “Blood (inside).”
5. Observe changes over time. Let the setup sit for 15–30 minutes. Every 5 minutes, students record observations of both liquids (color, cloudiness, visible particles). Gently swirl the outside cup once or twice during the waiting period to keep the water moving (do not squeeze the tubing).
6. Collect the filtrate (“urine”). After the waiting period, remove the tubing and place it on a paper towel. Pour the outside water into a labeled cup marked “Urine.” Observe the color and clarity of the “urine” and compare it to the mixture still inside the tubing.
7. Optional tests (if available). Test the “urine” cup for dissolved substances:
   • Glucose strips: Dip a strip into the outside liquid and record results.
   • Conductivity: Measure conductivity of the outside liquid to show salt moved through.
8. Clean up. Dispose of the liquids in the sink with plenty of water. Throw away cornstarch solids in the trash if needed. Rinse cups and wipe down tables.

Data

I. What does the “blood” look like as it passes through the dialysis tubing?


II. What does the water outside the tubing look like over time?


III. What does the “blood” and outside water look like at the end of the experiment?


IV. Where did the materials end up? (Inside the dialysis tubing or outside in the water)?

Food coloring:

Salt:

Sugar:

Cornstarch or Flour:

“Cells” (Glitter or Pepper):


Reflection Questions

1. What materials were placed inside the dialysis tubing to represent “blood”?
2. Which substances were able to pass through the dialysis tubing into the outside water?
3. Which substances stayed inside the dialysis tubing? Why do you think they stayed inside?
4. How did the dialysis tubing act like a real kidney during this experiment?
5. How was the liquid outside the tubing similar to urine? How was it different from real urine?
6. Why is it important for the body to keep substances like proteins and cells in the blood instead of letting them leave as urine?
7. What evidence did you observe that filtration had occurred?
8. If the dialysis tubing had larger holes, how do you think the results would have changed? Explain your reasoning.
9. How could this experiment explain what might happen if someone’s kidneys do not work properly?

Future Directions

1. What changes could you make to your model to make it work more like a real kidney?
   
2. What steps in the methods were unclear or hard to follow?