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It occurred to me (while urinating) that this would seem to be selected against because water is a scarce resource. Why are we constantly losing water we don't need to through urination? What is it about the chemistry of urine and the waste products eliminated that make urination necessary as opposed to eliminating them through defecation and recovering the water on the way out?

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Actually this process (in the kidney) helps saving a lot of water. The primary urine is concentrated about 200x before it is excreted into the urinal bladder. – Chris Apr 17 '14 at 15:43
I would hardly call water scarce in the natural world… – canadianer Oct 22 '14 at 6:53
up vote 13 down vote accepted

It is probably true that toilets and other resting-ish area are always a great place to think about biology, I agree $\ddot \smile$.

Why do we urinate?

In short, urine contains the waste from our blood while defecation is just the stuff that we haven't digested. Kidneys are the organs responsible for draining wastes (mostly nitrogen-containing, or nitrogenous, wastes) from our blood.

Trade-off: energy cost vs water loss

You're correct that the loss of water through urination is a considerable cost for an organism (especially those living in dry environments). But the amount of water used to excrete nitrogenous wastes is negatively correlated with the energy it costs to perform this excretion. In other words, there is a trade-off between water and energy loss during nitrogen excretion. Also the question of toxicity is important.

Three ways to excrete nitrogenous wastes

Animals basically have three choices to excrete nitrogenous wastes:

  • Uric acid (excreted by uricotelic organisms)

    • Solid (crystal) with low water solubility
    • Low toxicity
    • Little water is needed
    • Lots of energy is needed
  • Ammonia (excreted by aminotelic organisms)

    • Highly soluble in water
    • High toxicity
    • Lots of water is needed to dilute it because of the toxicity
    • Not much energy is needed
  • Urea (excreted by ureotelic organisms)

    • Solid but highly soluble in water
    • "medium" amount of water is needed
    • "medium" toxicity
    • "medium" amount energy is needed

What method is most beneficial for a given individual?

The amount of energy and the availability of water are very important aspects to understand whether a species is ureotelic, aminotelic or uricotelic. Other developmental constraint must considered too. For example, birds are uricotelic (uric acid) probably because during the development of the embryo the wastes cannot be excreted outside the egg and therefore, excreting uric acid allows to greatly decrease the toxicity of the embryo's environment. In short we must consider

  • Water availability
  • energy loss
  • developmental constraints

Note: This domain is not at all my field of study and I have no reference, typically for what concerns the toxicity, water amount and energy amount. One should not take my words for granted!

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This is an interesting answer. But it doesn't explain why nitrogen can't be excreted in solid or gaseous, rather than aqueous, form. – ThePopMachine Apr 17 '14 at 18:30
@ThePopMachine Well, Amonia is a gaz, Urea is liquid and Uric acid is solid (it makes cristals if I'm not mistaken). So, yes it can be excreted through solid form but it cost lots of energy (ATP). But you're right that my answer does not explain why it is more costly. But I know nothing about physiology and I can't give this answer $\ddot \smile$ I'll let someone else to give info about that if you're waiting for them. – Remi.b Apr 18 '14 at 11:22
@ThePopMachine You might want to create another post and to ask a more specific question about the physiology of Nitrogen excretion and why different solutions (Ammonia, Uric acid and Urea) are differentially costly in terms of energy. Consider this post as it had answered the evolutionary part of the question. – Remi.b Apr 18 '14 at 11:26
Yes, okay this has been here a while, so I will accept. But this answer can be improved further by talking about nitrogen chemistry. Any takers? – ThePopMachine Sep 24 '14 at 14:28
Are you asking why is amonia gazeous while uric acid is solid? If so, you may want to make another post on Chemistry.beta. I personally know very little about chemistry and won't be able to help you further with that. – Remi.b Sep 24 '14 at 15:04

As I have already mentioned in my other post, the most important role of urea synthesis by humans is blood pH regulation and urine concentration, so it is not just about excreting a waste product. I don't think human body is very special in this case, so I think most of the urea excreting mammals use urea for the same purposes.

The urea is created from NH4+ and HCO3- in the liver (mostly) and the kidney because of blood pH regulation purposes. It neutralizes the HCO3- created by the lungs from CO2 and OH -.

The urea cycle (also known as the Ornithine cycle) is a cycle of biochemical reactions occurring in many animals that produces urea ((NH2)2CO) from ammonia (NH3). This cycle was the first metabolic cycle discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the TCA cycle. In mammals, the urea cycle takes place primarily in the liver, and to a lesser extent in the kidney.

In chemical terms, urea synthesis is an irreversible, energy driven neutralization of the strong base HCO3 by the weak acid NH4+, and the average daily excretion of 30 g of urea is equivalent to the disposal of about 1 mol of HCO3 per day. Thus, a major function of hepatic urea synthesis is to effect this neutralization, without which the body would otherwise be confronted by a major load of alkali.

Urea is excreted by the kidney, and is normally present in plasma and body fluids at a concentration of 3.0–6.5 mmol/L.

The kidney reabsorbs urea in order to concentrate the urine

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