There are pressure changes, but fluids do not respond much to the pressure changes due to the density of the fluids - not very compressible, unlike air. That's why you will see a partly emptied water bottle scrunch up and deform - the air, not the water.
An easy way to think of air pressure changes is to think of sea level, where on a normal 59 F / 15 C temperature at sea level: 14.696 psi, or one atmosphere or one
bar of pressure. Diving into sea water, that changes very quickly - every 10 meters / 33 feet we
double that and at 33 feet we experience two atmospheres of pressure (29.4 psi).
Visually, take a balloon filled with water down there and - no major changes in size due to compression / expansion; take a balloon with air down there, it's about half size now.
Go on a commercial flight - the cabin pressure is maintained to a maximum elevation of 8,000 feet MSL / 2,400 m even if you are flying at 39,000 feet, so now we have less than one atmosphere of pressure - 10.9 psi, or .74 bar or .74 atmospheres of pressure, or 74% what it is at sea level.
A balloon filled with water or similar liquid will experience no changes, the balloon with air will expand, by about 25%.
So - a urinary catheter bag filled with liquid will have a very slight expansion, probably not noticeable to the user. If the cath bag has liquid and a lot of air, prepare for significant expansion of the air in the bag due to the lower surrounding pressure, which could make things uncomfortable to say the least as it might push air up the catheter. On descent, the cath bag with significant air would compress / get smaller, not such a huge deal but possibly still an inconvenience because then it would provide some vacuum to the catheter.
That's not really the long version - I'd have to get more technical!
