Biology 11–12 · Year 12

Modelling haemodialysis with Visking tubing and why fresh fluid is needed

Module 8: Non-infectious Disease and Disorders (Technologies and Disorders: loss of kidney function, dialysis)

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

In dialysis, small solutes diffuse from blood across a partially permeable membrane into dialysis fluid until concentrations equalise, large molecules stay behind, and only fresh fluid keeps removing waste.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

Silver nitrate stains skin and irritates eyes
Iodine solution irritates eyes
Glass beakers

Controls

Eye protection and gloves when using silver nitrate; staff dispense it; rinse splashes at once
Wipe spills promptly

Note

Silver nitrate and iodine: NSW Department of Education Chemical Safety in Schools package (https://education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/1._Section_1_-_General_information_for_all_staff.pdf) and RiskAssess (https://www.riskassess.com.au/).

What you need

Visking (dialysis) tubing, 20 cm, soaked in water; thread
Model 'blood' 20 mL: 1 percent starch (standing for plasma proteins), 1 percent glucose and 1 percent sodium chloride (standing for urea and salts)
Beaker holding 200 mL of distilled water (the dialysis fluid); a second beaker of 200 mL of 1 percent glucose solution for the comparison run
Conductivity meter or probe; glucose test strips; iodine solution; silver nitrate solution 0.05 mol/L in a dropper bottle; stop clock; spotting tile

How to do it

Fill the tubing with 20 mL of model blood, tie it off, rinse the outside and suspend it in the 200 mL of distilled water; stir gently throughout.
Measure the conductivity of the dialysis fluid at the start and every 5 minutes for 30 minutes.
At 0, 10 and 30 minutes test drops of the dialysis fluid on the spotting tile: glucose with a strip, chloride with a drop of silver nitrate (white cloudiness), starch with iodine.
After 30 minutes move the tubing to a fresh 200 mL of water and repeat the conductivity readings to show that more salt leaves.
Repeat the first run with the 1 percent glucose solution as the dialysis fluid and test whether glucose still leaves the model blood.
Fit the rate constant in the model to your conductivity readings, and compare the model with the four to five hour sessions Kidney Health Australia describes.

What you should see

Glucose and chloride appear in the dialysis fluid within minutes and starch never does; conductivity rises quickly and levels off as concentrations equalise. At equilibrium with 20 mL of blood and 200 mL of fluid, 1/11, or 9.1 percent, of each small solute remains in the blood, and after a second change of fluid 0.83 percent remains (computed). When the fluid already contains 1 percent glucose, glucose no longer leaves the blood, showing that the fluid's composition decides what is removed. The learner knows it worked when starch stays inside, the conductivity curve flattens, and the fresh-fluid run raises conductivity again.

What changes

What you change: time; fresh or used dialysis fluid; glucose in the fluid or not
What you measure: conductivity of the fluid and presence of glucose, chloride and starch
What you keep the same: tubing length and volume of model blood
fluid volume
stirring
temperature

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Dialysis filters blood the way a kidney does; it removes solutes by diffusion into fluid and cannot reabsorb what the body needs.
Everything small leaves the blood during dialysis; nothing leaves if the fluid already holds the same concentration.
One exchange cleans the blood completely; diffusion stops at equilibrium, so fresh fluid must keep flowing.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-15 BIO11/12-6
Biology 11–12 Syllabus (2025), Year 11 focus area Cells to systems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-02
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-04 BI-11WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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