Since ammonia is highly soluble in water, my senses keep telling me something like, "Just a little bit of water is enough to flush it, given that a lot of it would dissolve per liter of solvent". But who knew, it's exactly the opposite.

I keep seeing articles saying that since ammonia is very soluble in water and it takes lots of water to eliminate this waste in vertebrates. Uric acid, on the other hand, is very easy to eliminate, since it is not easily dissolved in water.

Why does the water solubility of ammonia would make it much harder to eliminate in birds, reptiles or the human body, compared to urea/uric acid?

Humans also eliminate ammonia, but, apparentely, its high solubility in water, and toxicity, would make it so that is actually more favourable to just convert it to urea inside the liver. On the other hand, its quite easy for bony fish to eliminate ammonia as a nitrogenous waste, since there's plenty of water around to help them flush it.

Once again, i don't get it, if ammonia is highly soluble in water, why does it take an ocean for some vertebrates to eliminate?

Some links: https://en.wikipedia.org/wiki/Metabolic_waste https://courses.lumenlearning.com/boundless-biology/chapter/nitrogenous-wastes/

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    $\begingroup$ Welcome to SE Biology. I would advise you to prepare and edit future questions beforehand to produce a more compact and structured question. This will help you examine your logic and assumptions, and will produce something that is more readable and hence likely elicit an answer. In this case you provide no basis for the assumption in your title that ammonia is more difficult (please avoid non-scientific terms like “tougher”) to eliminate than urea. Where did you read that? Why don’t fish have problems? If you didn’t read it anywhere isn’t it just possible that the problem lies elsewhere? $\endgroup$
    – David
    Jun 25 '21 at 19:17
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    $\begingroup$ I am trying to help you. Some people charge for this sort of educational advice. I am giving it to you for free. Your link says nothing about ammonia being difficult to eliminate — the erroneous assumption of your question. It does contain the statement "Nitrogenous wastes in the body tend to form toxic ammonia, which must be excreted." If you had read that, then the only question I can imagine you could have is one about why transferring ammonia to the bladder would not be enough to prevent its toxic effects on the body. But that is not what you asked. $\endgroup$
    – David
    Jun 25 '21 at 22:33
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    $\begingroup$ What exactly are you suggesting? Because the question i have in my mind is exactly the one that is written. $\endgroup$ Jun 28 '21 at 20:03
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    $\begingroup$ I am suggesting that you accept 1. that the toxicity of ammonia means that land animals have to convert it to a non-toxic compound for excretion, 2. That animals that can afford to carry and loose water convert it to urea which is soluble in water, whereas birds which cannot convert it to the insoluble compound uric acid, which is excreted with their faeces. The valid questions then become 1. Would secreting ammonia to the urine in the bladder solve the problem of toxicity? If so 2 Why does this not happen? A title might be "Why is ammonia converted to urea rather than secreted in the urine?" $\endgroup$
    – David
    Jun 28 '21 at 22:05
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    $\begingroup$ I'm starting to doubt my ability to comunicate in this language because both 1 and 2 were never really in question for me. I knew that more evolved vertebrates, as they started to populate more dry habitats, had to come up with a solution for the toxicity of ammonia, and that would be the urea and uric acid cycles. What i don't know is if the solubility of ammonia worked as an evolutionary pressure to this shift - in response to dry habitat colonization - or was it simply a matter of toxicity. $\endgroup$ Jun 29 '21 at 15:12

The comment of the poster to my request for clarification cites “The formation of ammonia itself requires… large quantities of water to dilute it out of a biological system” I conclude that he assumes that the problem with animals eliminating ammonia as such is one of solubility in water. This is not the case. The solubility of ammonia is such that saturated solutions exceed a concentration of 10 mol/L.

The problem is that ammonia is extremely toxic at low concentrations, so that decreasing its concentration to non-toxic levels would require a large volume of water. The writer of the cited text is, in my reading of it, presenting this as a hypothetical solution, and then rejecting it because the demands of supplying this volume of water would be prohibitive to the organism.

This is not my field, so the best information I have about the toxicity of ammonia comes from an anonymous web page on the University College London web site. It states:

Ammonia is highly toxic. Normally blood ammonium concentration is < 50 µmol /L, and an increase to only 100 µmol /L can lead to disturbance of consciousness. A blood ammonium concentration of 200 µmol /L is associated with coma and convulsions.

The page goes on to state (in red):

200 µmol /L is far too low a concentration of ammonium to affect plasma pH or the normal transport of sodium and potassium ions across nerve cell membranes.

I know almost no physiology, but my inference of the significance of this is that it would be difficult for the kidney to respond to this concentration of ammonia by filtering it out. Hence it needs to be converted to a non-toxic (or much less toxic) compound — urea, in the case of vertebrates.

Further reading suggests that the problem is that there is indeed a renal system involving the proximal tubule for secreting ammonia into the urine by a mechanism coupled to the cation transport and responsive to pH. In fact this system exists to control blood pH by generating ammonia to neutralize blood acid. This would explain the second part of the quotation above — the organism’s mechanism to control blood acid is inconsistent with a secretion mechanism for blood ammonia.

It would be good if a molecular physiologist could comment on or edit this section.

  • $\begingroup$ But why do you think it has nothing to do with soulubilty? I have a Portuguese school textbook that, as this website, states crystal clear that as solubility in water increases, so does the need for water in the process of elimination. It would be great if i could find a peer reviwed article abou this and not just a school textbook. I think your explanation is good enough, but why are you so sure? $\endgroup$ Jun 27 '21 at 12:25
  • $\begingroup$ You can see this idea literally everywhere, but it's never thoroughly explained. Check this out bio.miami.edu/dana/dox/nitrogenouswaste.html. Here the relation between solubility and elimination is stated separately from the toxicity, which makes me think that something is missing. $\endgroup$ Jun 27 '21 at 14:28
  • $\begingroup$ My answer hardly gives the impression that I am “so sure” when I apologise for using an anonymous web page as a source of information and ask for help (answers can be edited) on physiological aspects of the question. As I have a chemical background I knew that ammonia was soluble in water, which is why I felt that solubility was not the problem. As for the Miami U page, that does not impress me. Lists of advantages (like debating arguments) generally contain weak or dubious members to make up the numbers. I imagine a single factor prompted the evolution of ammonia modification. I may be wrong. $\endgroup$
    – David
    Jun 27 '21 at 15:35
  • $\begingroup$ You said, quote, "I conclude that he assumes that the problem with animals eliminating ammonia as such is one of solubility in water. This is not the case.". I felt here you were sure about this. The miami page was just to illustrate that this is a common idea which shows up now and then when reading about nitrogenous wastes. $\endgroup$ Jun 27 '21 at 16:18
  • $\begingroup$ Please don’t take offence. Your written English doesn’t suggest that you are a non-native speaker, but I just wonder whether you are confusing “dilution” with “dissolution” / “dissolving”. Ammonia is so soluble in water that you can dissolve 0.88 g per ml. None of the texts mention solubility. The Miami U page talks about “more water to dilute”. You can only dilute a solution where the solute is already dissolved. This will decrease its concentration. Why would you do that? $\endgroup$
    – David
    Jun 27 '21 at 18:09

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