Prepare the bottle and one balloon before the lesson:
Saw or cut the drink bottle so that the distance from neck to the cut is about the length of a deflated balloon. Sand the cut edge, so that it doesn't pierce a balloon later.
Tie a knot in the neck of one balloon, then cut the tip off the other end (the rounded part).
Distribute materials to each student: a cut bottle, one knotted and cut balloon, one whole balloon.
Have a partner hold the bottle, or stabilize it between the knees, with the bottle mouth pointing upwards.
Push the rounded main part of the intact balloon into the mouth of the bottle, then stretch the neck over the bottle mouth to secure it. The balloon should hang down in the bottle, with a hole into it at the mouth of the bottle.
Turn the bottle the other way up and stabilize, with the cut end of the bottle pointing upwards.
Stretch the open end of the knotted balloon over the cut end of the bottle, so that it is secure with the knot outwards.
Work the lung model:
The balloon hanging in the bottle is a lung (the physics is the same even though there are not two lungs).
The knotted ballon covering the cut end of the bottle is the diaphragm muscle.
Push the knot upwards so the diaphragm goes up into the bottle. This is the position of the diaphragm when it is relaxed. The lung should crumple and have no air in it.
Pull the knot downwards so the diaphragm ballon is pulled away from the bottle. This is the position of the diaphragm when it is contracted (it is actually straight across, not pulled down). The lung should inflate with air, fully or partially,
This model shows that we do not actively suck air into, or push air out of, our lungs (as it feels like). Air flows in and out of the lungs as the diaphragm muscle changes shape and the chest cavity changes size.
When the diaphragm contracts, it moves downwards, which increases the volume of the chest cavity, which decreases the air pressure in the chest cavity. Atmospheric air rushes into the lungs to equalize the pressure.
When the diaphragm relaxes again, it curves upwards, which decreases the volume of the chest cavity, so increasing the air pressure. Air leaves the lungs to equalize pressure.
In addition, when we breathe in, our rib muscles also move our rib cage up and outwards, further increasing the chest cavity, and making even more atmospheric air rush in.
Lung breathing gif:
Clear image of lungs and diaphragm only - https://upload.wikimedia.org/wikipedia/commons/9/9c/Diaphragmatic_breat… (but incorrect in that the same molecules enter and leave the lungs).
Shows diaphragm and ribs moving - https://www.luanamaso.com/wp-content/uploads/2019/09/AdolescentTastyFel…
Also shows heart and intestine - https://giphy.com/gifs/body-systems-organs-lckhIaarcbT20CXRDo
