eli5: Why does the serious urge to pee not correlate with the amount that comes out

Question

So sometimes when the urge to pee is really bad it’s bordering on painful, like after a long sleep or car drive, not that much comes out. But then other times when I’m just chilling at home and go for a pee with no urgency I pee like Seabiscuit. What gives? What causes you to need to pee so badly it hurts cuz it doesn’t seem like it’s a volume thing?

In: 1493

Answer 1

Also, why does it become nearly impossible to hold it while I’m unzipping sometimes. Like after a long car ride – fine walking to the toilet, almost blow it while I’m getting my equipment out. Why?!?