What it shows
Breathing happens because the diaphragm moves down and the chest space grows, which lowers the pressure inside so air rushes in. Pushing the diaphragm up squeezes the lungs and air leaves.
The science
The balloons are the lungs, the straw tube is the windpipe and the stretched rubber sheet is the diaphragm. Pulling the sheet down increases the volume of the sealed bottle, so the air pressure inside falls below the pressure outside and air flows into the balloons. Pushing the sheet up reduces the volume, raises the pressure and the balloons deflate. The ribs do a similar job in real breathing.
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 clear 1 litre plastic bottle
- Two small balloons for lungs
- One larger balloon or a piece of a balloon for the diaphragm
- A Y-shaped joint made from two straws and tape
- Modelling clay or tape to seal
- Rubber band
Safety
For this project: An adult must cut the bottle with a sharp knife. Do not blow into the balloons with your mouth if you feel dizzy, and never share a balloon.
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 off the bottom of the bottle so that only the neck and body remain.
Tape two straws together into a Y and tie one small balloon to each short end.
Push the long end of the Y through a hole in the cap or through the neck and seal the gap well with clay.
Let the two balloons hang inside the bottle.
Cut the large balloon, stretch it across the open bottom and fix it tightly with a rubber band, leaving a small knot to pull.
Pull the knot down gently and watch the small balloons swell.
Push the knot up and watch them shrink.
Label the windpipe, lungs, diaphragm and chest cavity with small paper flags.
What you should see
The two small balloons inflate when the sheet is pulled down and deflate when it is pushed up, mimicking inhalation and exhalation.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does the large balloon sheet represent?
The diaphragm, the muscle under the lungs.
2Why do the lungs fill when the diaphragm moves down?
The chest volume rises, the pressure drops and outside air flows in.
3Where does this model fail?
Real lungs have millions of tiny air sacs, and ribs also move.
4Why must the bottle be sealed?
Without a seal, air would enter around the sides and the pressure change would not work.
Take it further
Measure the chest of a friend before and after a deep breath and compare the change to the model.
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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