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...