The Haversian Canal

A classmate, Nicole, has been collecting bottles with those sports tops, you know, instead of just screwing off, they have a bite nipple that you pull up to allow water out? Like a lot of Gatorade bottles. She was collecting them for a science project at a school where she helps out. Today was the big day and she had an example in class. It's great. You cut off the bottom two thirds of the bottle, keeping the top third. Pull the mouth of a party balloon over the nipple, and drop the balloon through the mouth of the bottle top (the pink balloon in the example below). Tape the inverted nipple-top to the mouth of the bottle. Then take a second balloon, cut the neck of it off, and pull it over the bottom of your bottle (the green one below). You can then pull on the bottom one, like it's your diaphragm, and the pink balloon will inflate and deflate visibly inside the bottle. Just like your lungs.

Well, sort of. And it's probably the right way to do it when illustrating the principle for kids because they can visually appreciate that the force must be transmitted through the air, making the fluid, material nature of air more believable. But, hey, I'm a nerd! I can make this more anatomically correct! As a friend said, "Are you an engineer, because, man, if it ain't broke, you can always add a feature..." Anyway. So I took the green balloon off, turned the whole thing upside down, put a drop of hand soap and some water in the bottle, swished it around, dumped the excess water, and put the green balloon back on. (Simulating low-surface-tension pleural fluid.) Then I pulled the lip of the pink balloon over the mouth of the bottle to seal that end, and blew into the pink balloon, while pulling one bit of the green balloon away from the side, to vent the excess air in between the balloons. Releasing the green balloon, the two balloons now have a vacuum in between them, and a little bit of fluid in between the surfaces. Just like real lungs! And the lung is even pink! You'll note there is a fairly substantial concavity to the green balloon while at rest. This is because the forces all have to balance out. The pink balloon is pulling the green on in as much as the green one is pulling the pink one down.

This in turn makes me wonder to what extent the liver is really pushing the diaphragm up, as much as the rest of the guts and the hoop tension of the abdominal muscles keeps the liver up in the concave diaphragm. No, they didn't teach me that in school. So I can't believe it and it must not be true. I must now return to the Borg for retraining.

One minor point is that the bottle didn't have the inherent hoop strength to resist the fairly substantial tension brought to bear by my prestressed version, so reinforced it with a scrape of 12 gauge solid core wire coiled to a resting diameter greater than the diameter of the bottle, then tensed and released inside the base of the bottle. You can actually see the form of the wire through the green balloon on picture left.