How do I start, well I have noticed something weird about this our Urinary system I would love scholars to enlighten us on. The first instance is this :
1: Why do we urinate when we step into the bath tub, even when we don't feel pressed in any way.
2: If you are still thinking about the first scenario, think about this on the other hand when u step into a pool or jump into the pool to urinate doesn't come like it does in the bathroom.

Speculation: Is it that our brain releases some signal to the part that makes us urinate when we water flows from the head, maybe that's why its not the same when we are in the pool.....Please Doctors in the house, Biologist, Chemist etc.

It's all about the Brain.. The brain has been conditioned to associate urination with the toilet/bathroom right from childhood. A good scenario is the case of where a scientist conditioned its dog to associate the sound of bell to lunch time and so whenever the scientist rang the bell, the dog started to salivate..
Haven't finished my research on the brain but I think it has to do with the limbic part of the brain which has to do with higher functions of the brain...My little knowledge