hit F1 or \ (the browser version doesn't work on my laptop so I don't know if it has the cheat console). The stuff in the other reply about not being able open the console or change the character's capability of holding stuff is wrong. IIRC if you type in set (character name) into the console it'll autofill some options to change bowel and stomach capacities. You can increase the amount of air/water/food in them by doing basically the same thing.
Just tested it out and here are some example commands:
set Player bowels_capacity 10000 -sets the players capability of holding stuff in their bowels to 10000
set Aria stomach_capacity 10000 -sets Aria's capability of holding stuff in her stomach to 10000
set Amber bowels_water 5000 -sets the amount of water in Amber's bowels to 5000
Just play around with names and the autofill.