[Bull, Tina]

Bull, Tina  asked the Naked Scientists:
   
This is a question about human biology, something I grew curious about when rock climbing with my adult children.

Will the human body ever absorb fluids from the bladder, if a person drinks quite a bit, then spends time in the hot sun? I found myself needing a bathroom, yet also feeling the effects of too much sun and heat. Once the bladder fills, is there only one way out? This may sound like a goofy question, but we all found ourselves debating this question. Thank you!

Tina Bull

What do you think?

[rosy]

The water in the bladder leaves the blood in the kidneys and then moves down into the bladder which is basically just where it's stored until it can be got rid of... I'm fairly sure there's no way for water in the bladder to get back into the bloodstream.

[chris]

Hi Tina

It seems intuitive to think that, on hot days when we're running short of water, the logical thing to do would be to dip into the accumulating pool of fluid in the bladder and recycle some of that.

Unfortunately that doesn't happen, for anatomical and chemical reasons.

Anatomically, the urinary tract (ureters, bladder and urethra) is lined by what are called transitional epithelium. This is also sometimes known, with good reason, by the alternative name of "umbrella cells", which are impermeable, preventing the passage of urine (and its associated solutes) back into the body. There is a good reason for this. If these cells were permeable we'd have a hard time excreting excess water from the body because every time we tried to throw some away it would leak back in!

Chemically, the argument is even more compelling. On occasions when an individual is dehydrated and in need of extra fluid, the kidneys are already minimising urine output and saving water to the maximum extent to which they are capable. This is achieved by using energy to pump salts out of the urine and into the kidney tissue. This sets up an osmotic gradient which "pulls" water out of the urine and back into the bloodstream. However, some water is still lost, alongside salts and other nitrogenous waste that we must excrete. This resulting "insensible" loss is liquid that we cannot avoid throwing away.

Therefore, to make this liquid any more concentrated - i.e. extract further water from it - you would need an organ even better at creating an osmotic gradient than the kidneys you already have! The body doesn't do redundancy terribly well, so we instead use the best kidney structure we can to achieve a happy medium between energy consumption and hydration.

Some animals can scavenge back far more water than we can and hence dramatically reduce their insensible losses - to almost zero in fact. Examples include the small mammals that live in dry environments. But this comes at a high cost - to get the extra water back they need to consume far more energy to pump salts across membranes in their kidneys and so create the osmotic gradients that facilitate the water reabsorption.

The situation found in humans therefore represents a happy medium selected by evolution; it's a toss-up, if you will, between water dependency and water conservation at minimum energy expenditure.

The only exception to the situations I have outlined above is the one where a person might be well hydrated on minute but then find themselves dehydrated the next. On this occasion the bladder might be full of very dilute urine that would be useful if you could recycle it. And indeed you can - by drinking it!

Granted, the taste won't be that appealing, but you would extract useful liquid from it. Once it becomes more concentrated next time around, however, this won't work.

Chris

[CliffordK]

The only exception to the situations I have outlined above is the one where a person might be well hydrated on minute but then find themselves dehydrated the next. On this occasion the bladder might be full of very dilute urine that would be useful if you could recycle it. And indeed you can - by drinking it!

Granted, the taste won't be that appealing, but you would extract useful liquid from it. Once it becomes more concentrated next time around, however, this won't work.

I.E.
If you are shipwrecked.  The first day, you might be able to drink your urine.
But, after a week of water rationing, any urine you have would need to be discarded, or at most, used to feed a still.

[Don_1]

Not that affects us humans, but my tortoises can do this. Indeed, when they go into hibernation, as they will do in a few days time, they go with a full bladder and reabsorb water as required.

[Nizzle]

Are you sure they don't just pee in their shell and their skin reabsorbs the water? :p

[Don_1]

Are you sure they don't just pee in their shell and their skin reabsorbs the water? :p

Why, the very suggestion!!! I'll have you know my Tortoises are not given to bed wetting!

If a Tortoise urinates during hibernation, it must wake and drink. Once this happens, they cannot safely go back into hibernation.