Anyway, most of us can visualize the transverse colon, the descending colon, and the rest of the GI tract. Gas is expelled downward. But normal physics would seem to want the gas to gather in the transverse, with the solid contents sinking to the bottom–but most of us flatulate more than we defecate.
So, is the large intestine (ascending, transverse, etc.) often pretty airy and under a little pressure, and that’s what keeps forcing out the flatus? Or is peristalsis nudging gas buildup along?
Is it common for gas to back up into the small intestine, or is the ileocecal valve a pretty good gatekeeper at making sure thinks go only one way?
You can swallow while upside-down, right? And live in microgravity, eating, defecating daily, yes? So I assume it’s the peristaltic action of muscles moving everything solid, liquid and gas in only one direction, with check-valves ensuring no damaging backups occur.
IANAD, nor did I take any courses in anatomy/physiology, but why do you think the colon is subject to “normal physics?” It’s a closed system much of the time, with peristalsis squeezing from one end to the other. And the solid material in there isn’t really solid; it’s pretty “spongy” and gas can seep right through it. It seems “normal” that the gas can be squeezed through the solid and travel more quickly to the only way out, while the solid matter is still lumbering along, trying to catch up.
Gas and fluids in the body follow the pathway of least resistance out. That is, from higher pressure to lower pressure. That pathway mostly has little to do with gravity. Peristalsis provides the pressure differential, or at least the vast majority of it, tho local small regions may be affected somewhat by gravity too.
And most gas is generated in the intestines anyway, by action of bacteria there, and that will not tend to (in humans) travel back thru the duodenum to the stomach, up to the esophagus, and out the mouth. Imagine the burps if it did! Eructated gas generally comes from above the duodenum, and its source is swallowed air or carbonated beverages.
This peristalsis-vs-buoyancy question is a false dichotomy. Buoyancy absolutely applies in the human gut the same way it does everywhere else. It’s just that peristalsis is strong enough to overcome it.
I’m not suggesting that anyone in this thread thinks there’s no buoyancy in the human gut. But a casual reading of the thread could give someone the impression that somehow buoyancy “doesn’t apply” to human GI tracts.
Lots of examples in the body’s digestive/excretory systems of stuff overcoming gravitational effects. Just look at vomiting.
Surely You’re Joking, Mr. Feynman!, Feynman recounts a challenge he faced at MIT in proving that urine is forced out rather than merely drained via gravity. He stood on his head and evacuated. I think a simpler, and less messy, proof is just to aim somewhat up. The top of the stream is clearly higher than the bladder. Esp. in a 17 year-old full of beer.
So pushing around some gases is no big deal, esp. with forced semi-solid material behind them.
In the blood’s circulatory system things like clots (heavier) and air bubbles (lighter) can move to all sorts of places to cause trouble.
I wish to Og I had photocopied the Merck Manual article on flatulence from the ca. 1999 edition before it was lost to revision. Whoever wrote that article was a comedy genius.