Somewhat based on this distrubing thread: http://boards.straightdope.com/sdmb/showthread.php?t=266989
If I have a full bladder and I know I’m not going to be able to drink for a little while, will holding it in allow my body to absorb some of the water already inside my bladder? Or am I better off releasing it and then drinking it?
Someone with a cite can answer that question factually (but I’d bet you would get no benefit either way, as the kidneys have already concentrated the waste as much as they can).
But, your system does absorb water from your colon, and you benefit from that. So don’t poop till you can get some water.
And it costs water to metabolize food, so you only want to eat foods with more than enough water to pay the price. Can someone suggest such foods? --watermelon, …
Pee.
There’s nothing wrong with drinking water. But use your body’s natrual functions and your bod will take care of itself.
This assuming that you are in good health.
The bladder is not designed to absorb fluids in any significant way.
QtM, MD
I know from personal experience that if you don’t void, you can be in a heap of trouble, as the urine will back up into the kidneys eventually. I wasn’t able to void for 3 days and they cathetered about 1500 ml of urine out of my bladder. Luckily, it had not backed up. Even if you don’t drink water, food has plenty of water. So maybe if you don’t eat or drink you’ll be OK until you die from dehydration.
What prevents the urine from flowing back into the kidney and getting reabsorbed there?
Well, urine doesn’t really flow back to the kidneys when the bladder is full. What urine is produce just doesn’t flow into the bladder, eventually saturating the kidneys. The kidneys in humans are one-way devices, they can’t take water from urine back into the blood. So, you have toxins that would be flushed out by the kidneys circulating in the blood and building up. The kidneys would be esp. hit hard in this circumstance, possibly causing irreversible harm if not taken care of soon enough. But eventually other body parts will fail. Ask someone who had a bad Atkins diet experience about kidney failure.
Gravity?
It’s been a while, but I believe the major function of the kidney is to regulate salt concentration in the blood. Through the miracle of counter current…uh, something, excess salt is pushed across a concentration gradient to maintain electrolyte balance. In this process, some water is also transported, ending up in your bladder. It’s essentially a one-way process. As others have noted, the large intestine is responsible for retention/reabsorption/reuptake of water- not your kidneys. This is why uncontrolled diarrhea can lead to fatal dehydration.
Thanks QtM.
Of course I’m asking this in regards to, say, being stuck someplace without access to water Jake.
I recall a story about a ship several hundred years ago that survived (sort of) by drinking urine when they ran out of water. I suppose the body is able to recirculate and re-process some of the water out of the urine until you basically run out of pee?
There is muscle tissue lining the tubes (ureters) that connect the kidneys to the bladder. When that muscle contracts, the force is directed towards the bladder. There is no (active) way to reverse that flow.
What can happen, though, when the bladder cannot empty, is that the pressure rises in the bladder. So, new urine that’s produced by the kidneys can’t make it into the bladder (it can’t overcome the pressure gradient to get there). Then, the ureters themselves dilate to handle it. Eventually, the kidneys dilate.
Functionally, such a backing up is, of course, a bad thing. I still find it remarkable, though, that when the bladder obstruction (or whatever) is relieved, the kidney size and function often return to normal. Thank goodness - otherwise a lot of old guys with swollen prostates would be in real trouble (and, in fact, some still are).
The kidneys can concentrate urine to around 1200 mOsM (blood is at 290 mOsM). So, in theory, if you are trapped in an elevator after an earthquake, your first urine is unlikely to be 1200 mOsM (unless you are already severely dehydrated) and therefore your body can absorb some water from it. I believe some have survived for long periods like this.
The renal pelvicies down to the urethra are lined by a specialized tissue called urothelium. This is a very hardy stratified squamous epithelium with junctions which allow it to stretch. There is no significant water resorption after the renal pelvicies. Although in med school they said that bears can reabsorb a significant amount of water from their bladders.
While it is true that one can reabsorb water from the colon, one cannot reabsorb water from stool already in the rectum.
Maybe I’ve misunderstood how the kidneys work, but I thought they were indeed very good at reabsorbing water from (primary) urine.
I might have this all wrong, but my understanding is that the kidneys first produce primary urine that’s very watery, and then re-absorb water from this using reverse osmosis. This concentrated urine would then be piped away to the bladder. While the kidneys obviously can’t re-process what’s already left them, by understanding was that the amount of water re-absorption depends strongly on the hydration of the bloodstream.
As I said, I’m not an expert, and I’m likelly wrong. - Please point out where, though!
Wow, fascinating thread. Ok, back to trapped without water emergency scenario, it seems like holding it doesn’t help. So it seems to make sense to pee as much as you like, preferably into a bottle so you can drink it. Then, when the urine gets too salty, is it better to administer it as an enema so your large intestines can reabsorb the water, allowing you to excrete the excess salt?