How does holding your pee make it more concentrated?

[Laggie]

I see this all the time and it makes no sense to me... Wouldn't your bladder just be fuller? I don't see how the urine would be more concentrated, wouldn't that be accomplished by drinking less fluids?

I'm no biology expert but I thought that once the urine is in your bladder it stays there?

I know FMU is more concentrated but I think that is because your kidneys work more slowly when you are sleeping and because you haven't had anything to drink for 8 hours.

[OllieMama]

Well, they're involving 2 different body systems entirely, but I know that the longer you hold in a bm, the more water gets reabsorbed into your body, making it harder/dryer. So maybe it's a similar situation in the bladder? Osmosis causing water in areas with a higher concentration of it to move across a membrane to an area with a lower water concentration? Then it leaves behind a higher concentration of whatever couldn't cross the membrane... hormones, toxins, waste, etc. I'd guess it wouldn't work so well if you're super well-hydrated though.

But these are my spontaneously generated theories, fwiw.

[Hykue]

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?

[Maurine]

I don't really know...but Laggie I think you're right! Simply holding it won't make it more concentrated...but rather limiting your water intake along with holding it.

[familyaffair]

I've been wondering the same thing.

[karmab]

laggie you are completely right.

the only reason those instructions sometimes say to hold it for 4 hours- and this is a pet peeve of mine- it does nothing to concentrate your pee- the thing is, that if you can't hold it for 4 hours, meaning your bladder is reaching the point of capacity where it is really uncomfortable, then your pee will be dilute, but only because that means you've been drinking that much. the bladder is only a recepticle, it will not reabsorb water like intestines do, and your glomerular filtration rate- how your kidneys process fluid- slows down when you sleep. you hit it on the head.

[Laggie]

I've never seen instructions that say that, but I see people saying it on here all the time and it's always made me confused

Glad I'm not the only one!

[karmab]

yes, some of the hpt companies put that in their instructions, and they shouldnt. women are out there hurting themselves trying not to pee for 4 hours with bladders full to the brim!