The physics of pee

So here’s my question: why is it that, for the most part, I don’t feel like I have to pee as long as I’m sitting down, but as soon as I stand up – even if I’ve just gone to the bathroom – whammo! pee city? This is leaving aside the issue of how Hoss can aim his or her kicks directly into the center of my bladder on a repeated basis while apparently having little to no actual limb control. But the sitting/standing thing I don’t get. It’s fricking annoying! I’ll be all like, yeah, I don’t have to pee, so I can just sit here until my meeting/lunch date/appointment, and then when I stand up and am, like, PEEEEEE! I’m already running late and don’t have time to run to the bathroom. Stupid bladders.