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Have you ever wondered how your lungs move air around?

Safety: This activity involves using scissors to cut into a plastic bottle. Ask an adult for help with this step and cover the resulting sharp edges with duct tape.

You will need

• Scissors
• 600 ml plastic bottle
• Duct tape
• 2 balloons
• 1 straw
• Modelling clay

What to do

1. Use scissors to cut the bottom half off the 600 ml bottle. It can be difficult to get a cut started so ask an adult for help with this step. We recommend piercing the bottle with the sharp tip of the scissors or with a utility knife.
2. Cover the sharp edges of the bottle with duct tape. We cut 2 pieces of duct tape about 8-10 cm long and then ripped them in half lengthwise to get 4 thin strips.
3. Cut the straw in half, insert one of the straw pieces into a balloon and then attach them together using a thin strip of duct tape. This is your balloon lung.
4. Insert the balloon lung into the bottle so that the straw pokes through the opening by about 3-4 centimetres.
5. Use the modelling clay to seal off the top with the straw still poking out.
6. Tie off the end of a second balloon then cut off its top half.
7. Wrap the bottom half of the second balloon around the cut end of the bottle. This can be tricky to do on your own. Ask for help or try cradling the bottle between your knees so you can use both hands to wrap the balloon over the bottom of the bottle.
8. Using thin strips of duct tape, tape the second balloon securely into place.
9. Pull down on the second balloon – what happens to the balloon lung? (If your balloon lung doesn’t inflate, make sure the modelling clay completely seals the bottle’s top opening.)
   Recycle the plastic bottle when you’re done!

What’s happening?

You’ve created a working model of a human lung! But did you know that there are no muscles you can control in your lungs? There’s no little pump in your throat either. As your model shows, to breathe, you primarily rely on a muscle in your torso that isn’t directly attached to your lungs. Read on to find out how this works!

Our lungs perform the essential tasks of breathing in oxygen and breathing out carbon dioxide. Because both oxygen and carbon dioxide are gases, your body must be able to move gases in and out of your lungs.

Gases are made up of high energy molecules that are always moving and bouncing off each other. As a result, gases have no fixed shape, and they will expand to fill their container. Examples of containers include soda bottles, balloons, and lungs. When gas particles hit the inside walls of their container, they collectively create a force called pressure.

Pressure helps us understand gas behaviour. For example, pressure increases when the container gets smaller. This is because there is less space for the gas molecules and so they hit the walls more often. The opposite is true when the container gets larger: there is more space to move so the molecules hit the walls less often. But what happens if the container is open? Due to the random movements of gas molecules, gas will naturally move from areas of high pressure to areas of low pressure. This movement of gas continues until the pressure difference disappears.

Your body moves air into your lungs by creating a pressure difference. The major muscle responsible for this is your diaphragm, modelled by the balloon wrapped around the bottom of your bottle. Pulling on this balloon increases the volume of the soda bottle, which decreases pressure around the balloon. The balloon’s pressure is lower than the air pressure in the atmosphere, which draws air into the balloon. Similarly, when your diaphragm contracts, it increases the volume of your chest. This expands your lungs and creates a pressure difference where air rushes in. Relaxing the balloon or your diaphragm reverses the process to expel air.

Human breathing is bidirectional, meaning air flows in the same way it flows out. This type of breathing is a little inefficient! You might have noticed that your balloon lung didn’t completely deflate. Human lungs also can’t completely get rid of stale air. In contrast, birds have a unidirectional breathing where air moves in one direction. This type of lung is much more efficient and is part of the reason why birds can soar over Mt Everest while the humans below need oxygen tanks!