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Up to this point, all my life I assumed that stomach and intestines are filled with air. I got this impression from all the anatomic drawings in schoolbooks and encyclopedias that show empty stomach and intestines like a big sacks, with only small amount of digestive fluids in stomach at the bottom of it.

But getting some in-depth knowledges of scuba/free diving, it seems that it was a lie, all along! If it would have been filled with air, divers would need to "equalise" it, like they do with middle ears, sinuses, mask and lungs. Otherwise, they would face pretty serious barotrauma.

So, if it's not filled with air... Is it filled with liquid? I don't think it's likely, in this case we would always feel "fullness" and it would add too much weight.

My current hypothesis is, that stomach and intestines are so stretchy (distendable?), that it just collapses naturally, when empty, and maybe only contains small amount of mucus on it's walls to decrease friction of the walls. Is this correct? I couldn't find this information anywhere on the internet, strangely. Maybe it's so obvious that nobody writes about it?

I can imagine intestines shrinking, but stomach? It's kind of hard to imagine how such a big "bag" can shrink so much to not contain any gas/liquid in it...

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    $\begingroup$ When empty there is almost nothing in them. However one is frequently swallowing sinus mucus and saliva. My doctor told me a couple gallons in 24 hours; but I have no internet source so not an answer. $\endgroup$
    – blacksmith37
    Dec 4, 2021 at 16:54
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    $\begingroup$ "Is it filled with liquid? I don't think it's likely, in this case we would always feel "fullness" and it would add too much weight" How would you know this if it's the normal state of being? If it were this way since infancy or before? You would feel "normal". Which is normal. Also, your urinary bladder shrinks when empty. Why not your stomach? $\endgroup$
    – anongoodnurse
    Dec 4, 2021 at 17:24
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    $\begingroup$ As far as barotrauma goes, how distensible are the sinuses, the middle ear, the lungs and the diving mask? Not very. But the intestines? Very. If you're absolutely full of gas, you'll get gas pains (not the bends) on ascending, but you don't need to equalize something that you can't possibly equalize. Barotrauma of the GI tract is rare; much more commonly, the lungs are involved. I wrote a full answer including Xrays, but got the erroneous message that your Q was deleted, so my answer was discarded. Sorry. $\endgroup$
    – anongoodnurse
    Dec 4, 2021 at 19:26
  • $\begingroup$ @anongoodnurse That's true, I meant filled with liquid to the extent that is shown on all the anatomy pictures in encyclopedias. $\endgroup$
    – ScienceDiscoverer
    Dec 8, 2021 at 7:41
  • $\begingroup$ @anongoodnurse NP, your comment is good enough for my understanding! I think that probably one possibility of barotrauma is when diver swallows some high pressure breathing gas on very deep depth, so that it would greatly extend when coming back to surface quickly (for example in an emergency assent). $\endgroup$
    – ScienceDiscoverer
    Dec 8, 2021 at 7:44

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There is nothing. The tissue compresses together.

The entire digestive tract, including the esophagus, is pressurized by muscles.

Gases do get produced by digestion (and disfunctionally by air swallowing) and those gases must be removed from the body.

A small amount of air and carbon dioxide can naturally exist in the stomach, but if this increases it will rapidly cause disomfort or other symptoms, such as burping.

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    $\begingroup$ A rapid transit time is 24 hours; more normal is 36-40, and even 72 hours is still considered normal. Unless you're NPO for >72 hours or you've undergone a rigorous bowel prep (ugh!), there's almost no time that the entire GI tract is empty. Also, I can't recall ever seeing an upright abdominal film where there wasn't an air-fluid level in the stomach. Air (and saliva) in the stomach is the norm. $\endgroup$
    – anongoodnurse
    Dec 5, 2021 at 17:39
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    $\begingroup$ @anongoodnurse The OP is asking about gases, not fluids. Obviously the digestive tract has fluids. $\endgroup$
    – Imprisoned Rhesus
    Dec 5, 2021 at 21:18
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    $\begingroup$ It's not obvious from your answer. "There is nothing." That's not quite the extent of it. $\endgroup$
    – anongoodnurse
    Dec 6, 2021 at 1:40
  • $\begingroup$ If by "nothing" he meant air, then I'm fine with it! $\endgroup$
    – ScienceDiscoverer
    Dec 8, 2021 at 7:39

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