Join Date: Sep 2010
Location: Saskatchewan, Canada
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I'm not entirely sure that holding it in DOES make pee more concentrated, but if it does, I can come up with a mechanism. It's not the resorption thing - that's true for bowels, but the bladder is impermeable.
It is true that "once the urine is in your bladder, it stays there". But, as your bladder gets fuller, the pressure sensors in it start to send signals to send less urine down the way. So your kidneys start concentrating the new urine they are making, causing the concentration of the urine to go up incrementally. I have a hazy memory of learning something to that effect in physiology class. I'm not 100% sure that it's right, though.
Also, it might be worth noting that even if it does concentrate the urine, you might not want to do that. If you are prone to UTIs (which I am), holding it in is never a good idea. And your kidneys have to work a lot harder to make more concentrated urine (that part I definitely remember from physiology class), so you're stressing them out just a little every time you hold it. If you are always really well hydrated and have very dilute urine, don't worry about the occasional "holding it" so you can POAS with confidence, but don't make it an everyday occurence, KWIM?
On a farm with our kiddo (nearly 2), two dogs, two cats, ten goats, two donkeys, nine sheep, a bunch of chickens, and a husband (in the winters). We have another on the way!