How does breath inflation work?

actually, you have a nice idea - good work!

It looks like a mix of InflationFetishGuy/TheGrumpyWriter with Critical Volume, with best parts of each fantasy.

And I think it's all you need - your own idea, genuine, new, all yours, and detailed enough for it's purpose - stories.

Also, let me use this opportunity to praise your stories again!

To OP

Inflation Fetish Guy writes about cavities of fat appearing in the body as a lady matures, but with a very important addition - cavities are filled with gelly-like fat, while your idea is that they are filled with air.

Another idea where cavities are filled with air is - Critical Volume, it's explained in Inflation Olympics. Laurie, Lindsey, Amanda, Sandy and all others have that "bypass system" you write about implanted into them. They can breathe normally with their lungs, and they also can open a tiny valve by curling their tongue and licking under it. Then the air will fill their implants. That's how they prevent embolism - implanted cavities contain air. Athletes can either inhale open air though that's difficult, it is much easier to put a hose of heliumtank into the mouth and breathe in. Then they close a valve and exercise, and after they finish exercising they open the valve to let helium out deflating back to normal.

There are a dozen of other ideas, instant recommends are Carnatic (clean popping) and Nineteenthly (messy popping) - if I'd be interested in inventing inflation, I'd certainly talk to them first ^_^

As for my fantasy of inflating into a balloon, floating like a balloon and either popping like a balloon or deflating back to your normal self - it's just a crazy fantasy ^_^ I've seen Maria inflating with our child like a balloon and I tell for sure that recalling inflation situations in a quiet evening is pleasant, arousing and fascinating - but living them through is none of that. You're not aroused even slightest when you solve an urgent problem

Still, when I was making VSG I couldn't just pretend she goes "HUFFF" and - "POW" she's twice bigger. Okay, when Maria takes a deep breath she does look twice bigger to me but that's not the case ^_^

I don't know anything about game design, so I've done what I know - a sports simulation for athletes, with a little bit of imagination. You'll be surprised how little imagination it needs! Inflating like a balloon doesn't even require bending physics - it's within our laws of nature, the only fantasy is in biology

Since proteins of any complexity are already synthesised in cells, and we already produce our mass every day in glucose byproducts alone - how much fantasy is producing our mass in substances that just happen to be gases at room conditions?

What if characters in imaginary world have such a fantasy gene that commands their bodies to turn into gas and back?

Then a character will be able to inflate - healthy and naturally. Including - by taking deep breaths or putting a hose of a heliumtank in the mouth. It won't matter whether it's a tooth or an eye because each cell will take care of itself and surroundings, there are molecules containing calcium and other metals that are gases at room temperature. Even more - biochemistry allows doing it in a variety of ways ^_^

When such character takes a deep breath or puts a hose into her mouth, she's stuffing herself until she can't take anymore - and then she'll inflate like a balloon! Some parts like epidermis and a few muscles will have to remain uninflated - to keep together, to serve as kernels of deflation later and to store energy for it. Some animals have muscles with amazing super-stretching capability - what if humans could stretch a few of their muscles like that? Then anatomy of an inflating character would be gradually simplified to accomodate more and more helium until a character is nothing but a balloon

Normally muscles are attached to bones, but a balloon has no bones - instead muscles that remain are attached to each other, they keep the surface together and allow distending wider than you're inflated. Real example - a mouth that has a circular muscle allowing us to smile cutely, puff our lips or open wide. Or muscles are attached to skin and cross the balloon like invisibly thin straight lines, they pull a surface together and allow contracting smaller than you're inflated or holding at your limits. Real example - face muscles that let us look embarrassed or happy

When fully inflated, such character will retain her mass in gases (including helium she's inflating with), she'll be at least as wide as she is normally high and will have a skin as thin as a party balloon. I guess, if she feels risky she'll be able to stretch further like a party balloon

What will happen if you thrust a needle into a party balloon? After a thunderous BANG tiny pieces of epidermis will float all around, they are stretchy like rubber and look like scraps/confetti. All gas will come out and the body will die unable to rebuild itself. What will the gas look and smell like? That depends on reactions you choose for your inflation gene, I prefer to think it's transparent with a very delicate fragrance - sweet, musky and slightly narcotic. It's a weaker fragrance than lady's perfume. Burst cloud can look much worse - with beads of liquid, bad odours and coloured smokes depending on reactions you choose to program in DNA. Okay, party balloons don't burst into confetti - some pieces are large while others are barely visible. Like in CattyN's drawings where you see half of abdomen with a navel and one intact boob flying in one piece while another boob disappears in explosion... Ugh, enough of that! ^_^

The reverse process - deflation - will certainly require speeding up metabolism if we don't want our body to be built from raw materials for 40 weeks like in mommy's tummy ^_^ Bears speed up their metabolism by hundreds of times without any ill effects when they recover from winter sleep - what if humans could do that? (some yogins do that ^_^ ) I guess that means deflation burns calories, you'll be hungry like a bear after it ^_^ Maybe you won't notice it's enough and might pack on weight, idk ^_^

If you're interested in numbers, VSG shows some of them, and you can add a line to output more from the simulation - forces, sizes, pressure, stress and strain... It's more or less how I imagine inflation and it's connection to deep breaths in a fictious, imaginary universe ^_^

Such amount of detail is needed for freedom (giving multiple opportunities in gameplay) and balance (reasonable payoff for every effort), for inventing ability scores, items and skills. I think it's too much for stories. At least, when I write stories, I don't unleash all that on poor reader - characters inflate, that's it - the rest is inflation look and feel, plot and characters.

When you read someone's story, you might feel there's a whole world behind it, with depth and history - for instance, ATG, Carnatic, Clovis, Dan, Demonette, Double, HeliumGirl, Luther, Nineteenthly, Replacement, SourflourPrincess, Sven, Zumberge and some other writers make me feel that way. Many writers here have their own answer to how to do you inhale and inflate. And it's awesome you now have yours! It will certainly show in your stories ^_^

Good luck!