A working lung model takes about 15 minutes to build from a plastic bottle, a few straws, and some balloons. When you pull down on the balloon stretched across the bottom, the “lungs” inside inflate, showing exactly how your diaphragm drives every breath you take. It’s one of the most effective hands-on science projects for understanding the respiratory system, and everything you need costs a few dollars at most.
What You Need
Start with a clean, empty 2-liter plastic bottle. You’ll also need:
- 3 balloons: two small ones for the lungs and one larger one for the diaphragm
- 2 bendy straws (standard size) for the bronchi
- 1 wider straw or plastic tube for the trachea (windpipe)
- Sticky tape: strong packing tape or electrical tape works best
- Modeling clay or poster putty to create an airtight seal
- Scissors
Each part of the model maps to a real body structure. The bottle is your chest cavity (rib cage). The wider straw is the trachea. The two bendy straws are the bronchi, the tubes that branch into each lung. The small balloons inside the bottle are the lungs themselves. And the large balloon stretched across the bottom is the diaphragm, the sheet of muscle that sits below your lungs and powers breathing.
Step-by-Step Assembly
Prepare the Bottle
Remove and discard the bottle cap. Then carefully cut off the entire bottom of the bottle with scissors. If you’re working with younger kids, an adult should handle this step. The cut edge of a plastic bottle can be sharp and splintery, so trim away any jagged spots by snipping off an extra millimeter or two around the rim with scissors until the edge is smooth.
Build the Airway
Take your two bendy straws and cut about a third off the straight end of each one, keeping the bendy section intact. Then cut the wider straw (or tube) in half. You only need one half.
Insert the straight-cut ends of both bendy straws into one end of the wider straw so they form a Y shape. Wrap tape around the joint to hold everything together. Press gently as you tape to avoid crushing the straws. Air needs to flow freely through all three openings.
Attach the Lungs
Carefully push the open end of each bendy straw into the mouth of a small balloon. Wrap tape tightly around each connection so no air can escape. These two balloons now hang from the Y-shaped airway like a pair of lungs dangling from the bronchi.
Seal the Airway Into the Bottle
Lower the balloon “lungs” into the bottle through the top, feeding the wider straw up through the neck so it sticks out the top. Now pack modeling clay or poster putty around the straw where it passes through the bottle’s neck. This seal must be completely airtight. If air can sneak in around the straw, the model won’t work. Press the clay firmly into every gap, but don’t squeeze the straw shut.
Add the Diaphragm
Take the large balloon and tie a knot in its neck. Then cut about 1 centimeter off the opposite (wide) end of the balloon. Stretch this open end over the cut bottom of the bottle so the surface is tight like a drum, with the knotted end hanging down in the center. If the balloon doesn’t grip the bottle well on its own, secure it with tape or an elastic band. The knot gives you a handle to pull on.
How to Use the Model
Hold the bottle steady in one hand. With the other hand, pull the knotted balloon (the diaphragm) downward. Watch the two small balloons inside the bottle inflate. Now push the diaphragm balloon back up, and the small balloons deflate.
That’s breathing. When your real diaphragm contracts, it pulls downward and flattens out, expanding the space inside your chest cavity. This expansion drops the air pressure inside your lungs below the air pressure outside your body, and air rushes in through your nose and mouth to equalize the difference. When the diaphragm relaxes and moves back up, the chest cavity shrinks, pressure inside rises, and air is pushed back out.
The bottle model demonstrates this beautifully. Pulling the diaphragm balloon down increases the volume inside the sealed bottle. Since the bottle is airtight except for the straw at the top, the pressure inside drops and outside air flows through the straw into the small balloons, inflating them. Pushing the diaphragm back up reverses the process.
The Science Behind It
This relationship between volume and pressure is called Boyle’s law: when the volume of a sealed container increases, the pressure inside decreases, and vice versa. It’s the same principle that makes a syringe work. When you pull back the plunger, you increase the volume inside the barrel, which lowers the pressure and draws liquid in.
Your lungs rely on this principle for every single breath. During inhalation, the pressure inside the space surrounding the lungs drops to about negative 8 centimeters of water pressure relative to the atmosphere. That small difference is enough to pull roughly half a liter of air into your lungs with each normal breath. During exhalation, the diaphragm relaxes, volume decreases, pressure rises, and air flows back out.
This is also why deep-sea divers need to be careful. As a diver descends and water pressure increases around the chest, lung volume compresses according to the same law. Ascending too quickly lets the air expand faster than the body can adjust, which can cause serious injury.
If Your Model Isn’t Working
The most common problem is air leaking in somewhere other than the straw. If the small balloons barely move when you pull the diaphragm, check three spots:
- The clay seal at the bottle neck: This is the usual culprit. Press more clay around the straw and make sure there are zero gaps. Even a tiny opening will equalize the pressure and prevent the balloons from inflating.
- The balloon-to-straw connections: If tape isn’t creating a tight enough seal where the small balloons attach to the bendy straws, try wrapping additional layers or using a small rubber band underneath the tape.
- The diaphragm balloon: It needs to be stretched taut across the bottom of the bottle with no wrinkles or gaps at the edges. If it’s loose, air will seep in around the rim. Tape it down firmly or use a thicker elastic band to hold it in place.
Another issue: if the straws are pinched or crushed from over-taping, air can’t flow through the system even when the pressure changes are working. Hold the straw up to the light and look through it to make sure the passage is clear at every joint.
Single-Lung vs. Dual-Lung Versions
You can simplify the project by skipping the Y-connector entirely. Just use a single straw pushed through the bottle cap or neck, with one balloon attached to the bottom end inside the bottle. This version is quicker to build and works on the same principle, making it a good choice for younger kids.
The dual-lung version with two bendy straws forming a Y is more anatomically accurate and makes a stronger demonstration piece for a science class. It shows how the trachea branches into two bronchi, each leading to a separate lung, and both lungs inflate and deflate together from a single diaphragm movement. If you can find a small plastic Y-connector (from aquarium tubing, for example), it makes a sturdier joint than taping two straws into a larger one.
What This Teaches
Beyond the mechanics of breathing, the model shows something important about how bodies are organized. Your respiratory system doesn’t work in isolation. The diaphragm is a muscle controlled by your nervous system. The air that fills your lungs delivers oxygen to blood pumped by your circulatory system. The energy to contract the diaphragm comes from nutrients processed by your digestive system. One simple bottle model opens the door to understanding how all these systems depend on each other.
For students in middle school, this project aligns with science standards focused on how the body’s subsystems interact. Building the model, explaining what each part represents, and then using it to demonstrate inhalation and exhalation covers structure, function, and evidence-based reasoning all in one activity.