What it shows
The kidney filters blood: large particles stay in, and water, salts and urea leave as urine. The layered bottle shows filtering by particle size.
The science
In the kidney, blood is forced under pressure through the tiny filters of the nephron called glomeruli. Small molecules like water, glucose, salt and urea pass out, while blood cells and large proteins stay. Useful substances are then reabsorbed. The bottle model only shows the first step, filtration by size, because cotton and sand hold back large particles but let water through.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 1 litre plastic bottle cut in half
- A piece of cotton cloth
- Washed gravel, sand and cotton wool
- Muddy water with a pinch of salt and a few paper bits
- Two beakers or glasses
- A drawing of a nephron on chart paper
Safety
For this project: An adult must cut the bottle. Do not drink the filtered water.
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.
An adult cuts the bottle across and turns the top upside down as a funnel in the bottom half.
Put a cloth at the neck to stop sand falling through.
Add cotton wool, then a layer of fine sand, then a layer of gravel, each about 3 cm deep.
Mix the muddy water with a pinch of salt and some paper bits.
Pour it slowly into the funnel and collect the filtrate in a glass.
Compare the first sample and the filtered one for colour and bits.
Taste is not to be tested; instead dip a clean strip of paper in both and compare the stain.
On the chart paper draw the nephron and match each layer to a part, keeping in mind that real reabsorption is not shown.
What you should see
The filtered water is clearer and free of bits while dissolved salt still passes, which matches how a kidney removes large particles but not dissolved wastes at first.
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 functional unit of the kidney?
The nephron.
2What stays in the blood during filtration?
Blood cells and large proteins.
3What does your model not show?
Reabsorption of useful substances like glucose and water.
4Name one waste removed by the kidney.
Urea.
Take it further
Add a second filter and test whether salt can be taken out; explain why it cannot.
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.
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