how bladders work.

i was just thinking about bladders.
how do they work? (i do arts. and have never done biology.)
i imagine the bladder is like a big sponge, containing all the pee. and then when it gets saturated, the pee needs to leak out, so u feel this pressure, and go to the bathroom. then something presses down on the sponge bladder, so that all the pee spurts out.

at this point, with the sponge bladder spurting pee out all over the place, i thought, 'but then it would go all over the place, into the blood stream?' so, the sponge bladder must be in a sack, like a balloon, with only one opening. in fact, the bladder is like a sponge in a balloon, with a giant stamp suspended above, so that it can press down on the balloon with the sponge inside which absorbs all the pee, and all the pee squirts out thru the opening of the balloon, down a tube (like a garden hose, even) and out out out of the body!!

i might have slightly misunderstood how the bladder and the entire human body works, but i'm sure that's the gist of it.