Is it “obviously” the colon, or are you “guessing” it’s the colon? Or are you UNCERTAIN?
It’d be a good adaptation if water could be reclaimed this way, but humans haven’t evolved it.
Our kidneys cannot stop producing urine because the blood that enters them cannot be cut off. The pressure of urine in the kidneys is derived from blood pressure, and blood pressure (2-3 PSI) is much higher than bladder pressure. Therefore, the urine cannot travel back up the ureters.
What if you have two girls but only one cup?
Actually, there’s a deeper reason the adaptation cannot be evolved:
Imagine that the urine begins to travel back up into the kidneys. It traveled up the dense network of tubes that carry out the extraction of water. There, the urine would be made more concentrated. And then what? Where would this re-concentrated urine go, if urine is flowing up the ureters instead of down. The urine inside the filtration tubes would reach such a concentration of urea and wastes that water could no longer be extracted from it, and the kidneys would simply stop extracting any more water.
(Btw, it might also be helpful to add that the bladder wall simply cannot do the job that the complex, high-surface-area renal tubes perform–namely, very selectively deciding which substances are left and which are extracted from the filtrate, not to mention extracting water against very great osmotic pressure. The bladder wall is impenetrable, because a permeable membrane would let everything that the kidneys do be undone.)
For one’s own feces I’m fairly certain. Putting my own feces right back where it came from seems safe, whereas ingesting it seems questionable.
There must be a reason that we say “eat shit and die”.
Anyway, I’m sorry to have started this disgusting conversation.
Sorry, didn’t read carefully enough to see the distinction you were making.
Suppose that the adapation involved sensory data within the nervous system that signalled dehydration, and the bladder had an additional drainage pipe that opened during dehydration and it could shunt sufficiently dilute urine into the colon for reabsorption. Insufficiently dilute urine would prevent the pipe from opening.
Not saying that it could really happen, but if we’re going to imagine an adaption that doesn’t exist, we might as well imagine one that works 