What it shows
The kidney filters blood: small molecules such as water, salt and sugar pass out, while blood cells and large proteins are held back. A layered funnel shows the filtering idea step by step.
The science
Each kidney holds about a million nephrons. In the glomerulus, blood is pushed under pressure through thin capillary walls, and water, salts, glucose and urea pass into the tubule, while blood cells and large proteins stay in the blood. This is ultrafiltration. In the tubule the body then takes back all the glucose and most of the water and useful salts (selective reabsorption), and the rest leaves as urine. The model shows only the filtering, not the reabsorption.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A plastic funnel or the cut top of a bottle
- A tea strainer or kitchen sieve
- A piece of clean muslin or cotton cloth
- A beaker or jar to collect the filtrate
- A mixture: water, a pinch of salt, a spoon of sugar, a few drops of food colour, 20 red beads and a teaspoon of fine soil
- A card for the nephron diagram
- Markers and glue
Safety
For this project: An adult must cut the bottle. Do not drink anything from the model; wash hands after handling soil.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Mix the water, salt, sugar, colour, red beads and soil in a jug to stand for "blood plus wastes".
Place the strainer over the funnel and the funnel over the collecting jar.
Pour the mixture through the strainer and see that the red beads (blood cells) stay in the strainer while the liquid passes through.
Line the funnel with the cloth and pour the muddy liquid through it to remove the fine soil that stands for large proteins.
Look at the clear coloured liquid in the jar, which stands for the filtrate that enters the nephron tubule.
Do not taste anything; instead notice that the colour passed through and compare it with the muddy liquid you poured in.
On a card, draw a nephron with the glomerulus, tubule and collecting duct, and label where filtration and reabsorption happen.
Add a note on the card that real kidneys take back glucose and most water, which this model does not do.
What you should see
The strainer keeps back the beads, the cloth holds the soil, and clear coloured liquid with dissolved matter passes into the jar. The labelled card explains which step each layer shows.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the filtering unit of the kidney?
The nephron.
2What stays behind during filtration in the real kidney?
Blood cells and large proteins.
3What does the model not show?
Reabsorption of glucose, water and salts in the tubule.
4Why is filtration helped by pressure?
Blood pressure in the glomerulus pushes small molecules out through the thin walls.
Take it further
Add a second jar of clean water and use a sponge to model how water and sugar are taken back from the filtrate.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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