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I just read Andy Weir's "The Martian" (which was very good) and one line made me wonder:

[...] a human body only borrows water.

Is that true? Does all water "that goes in, go out"?

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    $\begingroup$ Water is "created" and "destroyed" in biochemical reactions in the body all the time. The overall water balance, as described below, stays at a relatively steady state, but any one water molecule entering the body may be used as a proton (Hydrogen) donor, to make a hydroxyl (-OH) group, etc. $\endgroup$ – MattDMo Sep 17 '15 at 1:30
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Yes, the human body "converts" water.

Out of the hundreds of chemical reactions going on in the human body, there are reactions which take water as an input and "react it away" into something else. There are also reactions which create water out of other molecules.

Example 1: Hydrolase reactions. They have the form of

A–B + H2O → A–OH + B–H

You get water as input, and something else as output.

Example 2: Peroxidases "defuse" the highly reactive hydrogen peroxide, producing water. For example, look at the L-ascorbate peroxidase, which catalyzes

L-ascorbate + H2O2 → dehydroascorbate + 2 H2O

Beside these reaction, there is also a lot of water which doesn't react and is simply excreted after having served its purpose. So, while some water is "converted", some other part "passes through".

The reactions are balanced in such a way that normally, you excrete the same amount of water as you drink, as explained in Amory's answer. But despite this balance of the amount, you cannot claim that the water just passes the body unchanged. Some molecules of it are used up, and others are created de novo.

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Yes, essentially, but in practice you need to consider the context of the novel.[1]

Your body needs water every day - somewhere around 3 liters is sufficient for baseline activity - in order to maintain balance due to obligatory losses. Here's a figure that sums it up (it has a few errors, see below):

enter image description here

That whole online textbook chapter might be of interest, but that figure has a few errors. If you look, the numbers at the bottom for output aren't summed up correctly, and the value for transpiration is repeated. Doing the math on the correct numbers gives 2500 ml output for the 2500 ml input.

So yes, input and output are roughly equivalent. You and I need new sources of water every day since our urine isn't something we'd like to drink, but Watney has a "water reclaimer" and lives in a sealed environment[2]; his entire world is one small, well-balanced water cycle.


[1]: For the uninitiated, "The Martian" is a novel and soon-to-be-movie about Mark Watney, an astronaut stranded on Mars who must use his ingenuity to survive as long as he can. The context of that quote is the need for hydrogen, which Watney will get by hydrolyzing his urine.

[2]: Well, most of the time anyway.

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  • $\begingroup$ So all the $H_2O$ that goes in, comes out as $H_2O$? $\endgroup$ – Nick Russler Sep 16 '15 at 13:50
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    $\begingroup$ Actually, more comes out than goes in: metabolic water is synthesized by the organism. $\endgroup$ – biozic Sep 16 '15 at 19:28
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    $\begingroup$ @NickRussler no, that's wrong. There are several metabolic reactions which require water as input, for example the esterase activity comes to mind. So, when looking at a specific water molecule, it is very much possible that it is consumed, and then reacts to something else, without being excreted as water. What this answer says is that the amount of water going in and out is balanced, not that all water passes through the body unchanged. $\endgroup$ – rumtscho Sep 17 '15 at 12:07
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    $\begingroup$ @NickRussler As rumtscho said here and in the other answer, it's not the same water molecules, but it's the same amount. If you look at the diagram, you'll see that plenty of the input water is from food that we eat, not just liquid water. $\endgroup$ – Amory Sep 17 '15 at 15:06

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