It happens gradually over time. A constant trickle coming down from both kidneys. And no, there is no mechanism for getting the water back. If you had a full bladder then went on a hike and got dehydrated you couldn’t obtain any of the water out of the bladder (unless you pee it then drink it again). I often think that this is evidence against intelligent design, because in a more efficient system we would be able to do this.
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