How do our lungs work?

• Ask an adult to cut the plastic bottle. Cut off the bottle's bottom so that when a balloon hangs inside the bottle from the spout there is about 1/3 to 3/4 of an inch of empty space below the balloon.
• Place the cut bottle down on the wide opening. Lower a balloon into the bottle until only part of the balloon's neck sticks out. Fold the neck of the balloon over the top of the bottle. The balloon represents a lung.
• Turn the bottle over (keeping the balloon inside) so the bottle top rests on the table. In the next steps you will create and add the diaphragm to your model.
• Make a knot in the neck of the second balloon. At the opposite side of this balloon cut off about a third of the balloon so you are left with a wide opening.
• Stretch the wide opening of the cut balloon over the wide opening of the bottle. Pull the edges of the balloon far enough up the bottle so the balloon surface is gently stretched. Make sure that the knot is on the outside and located near the middle of the bottle opening.
• Like an inflated balloon our lungs are full of air. We have two lungs, which are enclosed in the ribcage and protected by 24 ribs. When you breathe in, air flows into your lungs. When you breathe out, air flows out of your lungs. The balloon inside the bottle is like one of your lungs. The bottle is like your ribcage.

• Hold the bottle so you can see the balloon inside (representing the lung). Gently pull down on the knot. What happens to the balloon inside the bottle?
• Let the knot come back to its neutral position and then gently push it in. What happens to the balloon inside the bottle now?
• Repeat these steps a few times. Does this resemble breathing? Why?
• Which part resembles breathing in and which part resembles breathing out?
• If your model is working well, air will rush into the balloon when you pull the knot outward and flow out when you push the knot inward. Why do you think this happens?
• When we breathe in a relaxed way our diaphragm—the muscle that separates the chest cavity from the abdominal cavity—moves to expand and contract the chest cavity. How is that similar to what you do with your model?
• Push and pull the knot a few more times. Using the model can you find which movement of the diaphragm creates inhalation and which creates exhalation?
• Feel your ribs and breathe in deeply then exhale. Can you feel your ribcage expand and fall back?
• The center of our diaphragm moves more when we take deep breaths: up to four inches! In the model you made, the ribcage (the plastic bottle) is fixed, but you can move the "diaphragm" more by pulling the knot farther and pushing it in more. Try it out. How does that change the volume of air that flows in and out of the lung balloon?
• Extra: Add a windpipe to your model. To do this take the balloon out of the bottle and slip its neck over a straw; secure the balloon to the straw with tape. Hang the balloon—and a short section of the straw—in the bottle's neck and use clay to hold it in place. Make sure the clay makes an airtight seal around the straw and the bottle neck. No change is needed to the second balloon that closes off the bottom of the bottle. Can you see which part models the windpipe?
• Extra: A cough is the body forcefully expelling air to get rid of something that caused irritation. During a cough you breathe in relatively deeply but instead of air flowing out while the chest cavity contracts, your throat closes, and air builds up in the lungs. When the throat opens the chest contracts even more and air flows out in a forceful way. Can you mimic a cough with your model?
• Extra: Find a way to create a model that includes a windpipe that splits into two bronchi, each with a lung attached. The model with a windpipe and one lung is a good start. How can you add a second lung? Can you find a reason why having two lungs is beneficial for us?

When you pulled the knot back, the space inside the bottle increased and your balloon probably filled up with air. In the same way, when the diaphragm in our body pulls back, the chest cavity increases and air flows into our lungs, and we inhale.

When you pushed the knot in, the space inside the bottle decreased, and the balloon probably deflated. In the same way, when the diaphragm relaxes the chest cavity decreases, and air is pushed out of the lungs, and we exhale.  

When you pulled and pushed the knot further the balloon inflated and deflated more. This mirrors what happens when a bigger volume of air is displaced when we breathe more deeply.

This dynamic works because of air pressure, a measure of how hard air presses against objects. Air pressure increases when you decrease the amount of space the air has—and decreases when you give air more space. Close a flimsy empty plastic bottle and try to compress it. It is difficult! The air inside pushes back. Open the bottle and try to compress the bottle again. It is much easier. The air presses back with a much reduced force. Unless something blocks the movement, air will move from areas of high pressure to areas where the pressure is lower, and this is what happens when air rushes in or out of the lungs. When the chest cavity expands there is more space around your lungs. In this condition the lungs can expand, making it a low-pressure area, and air rushes in to balance out the difference in pressure. Then to breathe out the chest cavity and lungs shrink. This raises the air pressure in your lungs, and the air rushes back out.

This experiment is from Scientific American’s “Bring Science Home” series and features 3M scientists and partners.  Reproduced with permission of Scientific American, “How Do We Breathe” by Science Buddies and Sabine De Branbandere, ©2019.   For more activities please go to: Bring Science Home.