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After the liver processes metabolites to produce urea and other by-products, these travel in the blood to the heart, then they are oxygenated, and some travel through the renal artery to the kidneys.

Urea in water can decompose into ammonia which is toxic, as mentioned in a textbook page no 170 (6), under production of ammonia.

Reactant and product are at equilibrium. Also a gaseous product, CO2, is present which could potentially escape during oxygenation of blood. Hence, according to Le Chatelier's principle, an equilibrium shift towards the product is possible

NH2CONH2 + 2H2O → (NH4)2CO3 ⇌ 2NH3 + H2O + CO2

The chance of urea decomposing into ammonia is high while travelling in the blood (i.e taking long route) and in the bladder, but it still does not happen. Why not?

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  • $\begingroup$ best to ask 2 different questions. Just ask this: Why does urea not decompose into ammonia inside the bladder? 1 phrase, simple, easy to read. urine degradation is also decomposition. $\endgroup$ – com.prehensible Mar 31 at 9:37
  • $\begingroup$ I tried to rewrite it very concisely. It's an interesting question. Apparently uric acid is responsible for half of the blood's anti-oxidant potential, despite the blood having a pH of 7.35. At that pH and temperature, ammonia can become gaseous and toxic. Urine has a pH of 6, so it has excess H+, so the ammonia would mostly become ammonium NH4, which is 100 times less toxic than ammonia. The nitrogen cycle in aquariums and in plants is something that helps to understand ponds, tap water, plants, aquariums, so i am always interested in the nitrogen cycle. $\endgroup$ – com.prehensible Mar 31 at 10:59
  • $\begingroup$ it's worth noting that the white substance from bird and lizard poop is uric acid. Fish produce a lot of ammonia, and they also have bacterial symbiosis in their gills to absorb it, and water based animals can produce ammonia directly because there is an excess of water, whereas most land animals have had to adapt to nitrogen waste with limited water. $\endgroup$ – com.prehensible Mar 31 at 11:06
  • $\begingroup$ Another info is here, from reading about fish, that secrete 90% of nitrogen as ammonia/ammonium. it sais: most of the ammonia in fish blood is ammonium ion. side note: pK of NH3 = 9-10; fish blood = pH 7.4) i can't explain that though. $\endgroup$ – com.prehensible Mar 31 at 11:34
  • $\begingroup$ @com.prehensible Can you write your comments into an answer? $\endgroup$ – Chris Mar 31 at 12:07
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The answer to this question is quite simply this:

The activation energy for the uncatalysed reaction is such that the amount of decomposition of urea in aqueous solution at blood temperature and pH is negligible in the time taken for the transfer of urea to the kidney.

The literature supporting this is very old, so I shall first cite a relatively recent (2004) and (I think) freely available paper by Robert P. Hausinger on urease in which he writes:

The substrate [i.e. urea] is highly resonance stabilized (30 to 40 kcal/mol), thus decreasing the reactivity of its carbonyl carbon so that spontaneous hydrolysis of urea has never been observed. Rather, urea decomposes in solution (with an estimated half-life of 3.6 years at 38°C) by the slow elimination of ammonia to form cyanic acid (17)

Reference [17] is a paper by Zerner in Bio-organic Chemistry from 1991 which requires a library subscription. In effect it quotes the same half-life:

The urea molecule is very stable. Between pH 2 and pH 12, the nonenzymatic decomposition of urea in aqueous media is independent of pH and has a half-life of 3.6 years at 38°C (36-38).

References 36 is a book written in 1923, and references 37 and 38 date from 1942 and 1955, respectively. I have not checked the latter, but can if anyone demands it.

All of which is not surprising, as the purpose of the urea cycle in mammals (etc.) is to eliminate toxic ammonia, and, as @Fizz points out, the enzyme urease is needed by bacteria that utilize the compound.

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[Partial answer]

[OP claim:] Urea in water can decompose into ammonia which is toxic.

Probably not as easily as you think. If Wikipedia is correct:

Urea alone is very stable due to the resonance forms it can adopt.

Some bacteria use urease to catalyze the reaction by 14 orders of magnitude (says Wikipedia).

There are some primary sources from the 1930s on the resonance forms of urea; I've not read them just yet.

From experimental data in solutions (not blood), urea would be close to its maximum stability in blood, pH-wise at least:

The stability analysis shows that urea is more stable at the pH range of 4-8 and the stability of urea decreases by increase in temperature for all pH values. Within the experimental range of temperature and initial urea concentration values, the lowest urea degradation was found with lactate buffer pH 6.0. The urea decomposition rate in solution and pharmaceutical preparations shows the dependence of the initial urea concentrations. At higher initial urea concentrations, the rate of degradation is a decreasing function with time. This suggests that the reverse reaction is a factor in the degradation of concentrated urea solution.

As for the reaction speed in terms that even I can understand:

The urea molecule is very stable. Between pH 2 and pH 12, the nonenzymatic decomposition of urea in aqueous media is independent of pH and has a half-life of 3.6 years at 38°C.

(This review cites data from older papers, so presumably they had less sensitive methods, so they concluded a wider pH range where it's most stable.)

For the chemistry experts, there exists a lot more information on the kinetics of the reaction.

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  • $\begingroup$ In effect you are saying the question is based on a false premise, a point of view I imagine is correct, in which case I would vote to close. However neither you or the poster have given any figures for the rate of the uncatalysed reaction. Could you find some so we can resolve this definitatively please. $\endgroup$ – David Mar 31 at 14:06
  • $\begingroup$ @David: I'm trying. The best I found is not in blood, but in solution. ncbi.nlm.nih.gov/pubmed/25043489 I'll have to access the full paper for the details... $\endgroup$ – Fizz Mar 31 at 14:17
  • $\begingroup$ @David: I found more urea chemistry than I can handle right now: hal.archives-ouvertes.fr/hal-01449373/document $\endgroup$ – Fizz Mar 31 at 14:49
  • $\begingroup$ Blood is likely to be irrelevant. Water will do. I’ll have a look later if you can’t find the info yourself. However the poster should be made to justify his claims or withdraw. $\endgroup$ – David Mar 31 at 15:09

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