system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what's in your bladder when it's empty of urine? Is it every empty?

So I was taking a pee today and thought about this. It can't just be a void, and  don't think it should be air?
gpt
1st of all, your bladder never completely empties of urine, there is always some left. But assuming you could empty it completely, the space inside would be the same as many "virtual" spaces in your body (such as the pleural, or pericardial  spaces). It would be the same as a flattened balloon with all the air squeezed out. The inside surfaces would all be compressed and touching each other with no space (except for a little moisture) between them.