Water intoxication danger?

I disagree actually. Whereas a degree in medicine enables one to understand the risks in terms of textbook-based calculations, when someone has a textbook case of a particular condition or illness it's the exception rather than the rule and is unusual enough to be remarked upon. Experience with thousands of patients has led me to the point where i realise that what i learned in the lecture room and even in clinical practice is only the basis of the much fuzzier and more complicated messy reality of people's lives. As a result, whereas my answer is unsatisfactory and gives no figures, unfortunately that's the kind of uncertainty people live with all the time. We are not mass-produced machines made to a particular standard so there will be variation, although there would also be safety margins.

An example of the kind of thing that happens is that when i inflate myself, i'm aware that the maximum capacity of my stomach and intestines is theoretically around fifteen litres, but i know i can get twice as much as that in, partly due to compression and partly due to the fact that the body can adjust to what it has to adapt to. I wouldn't want to risk it but i wouldn't be surprised if you could train your body to resist hyponatraemia to a limited extent. However, i'm also aware that the development of resilience to extremes is sometimes only in one direction, for instance with extremes of temperature it's possible to become more resistant to cold than it is to heat, and i suspect the same is true with hyponatraemia, because in the "wild" people would be more likely to need to resist dehydration than over-hydration. Over-hydration is more likely to be a choice than going without water, so there's less selective pressure to resist it.

Fair enough. I was a bit worried about coming across as supercilious and dismissive in that post, so thanks for taking it in a positive light!

Do you think, though, that there could be some adjustment? I'm also interested in the idea of something like a body fluid replacement which is of similar composition, maybe even with the right proteins, which could be introduced into a potential space within the body fairly innocuously, say subcutaneously.

That would depend on whether you mean saline in the sense of the same ions as are in plasma in the same proportions or something else, other constituents and which part of the body it's in. Assuming a completely isotonic solution in the gastrointestinal tract, yes, i'm sure you could go a long way. If one of the constituents was out of proportion, the problem then becomes something like hyper-/hypokalaemia rather than hyponatraemia, but hypokalaemia strikes with little or no warning, so you may not be aware when you're in the danger zone.

The other thing, and of course i'm now well into the realm of complete fantasy, is the osmotic pressure of protein in the fluid, which only becomes relevant in a body cavity or fluid compartment. A mere solution of saline will not be enough in those circumstances and there would be osmosis between compartments unless there was enough protein or something to pull equally on the fluid. Otherwise you get situations like kwashiorkor or ascites, with fluid content climbing dangerously in those regions. If that happens, i imagine you get additional problems than just electrolyte imbalance, because i would expect three things to happen. One is dilution of blood gases. I would imagine oxygen and carbon dioxide would both diffuse into the fluid and i imagine that would cause hypoxia. Another is acid-base balance, in that the fluid concerned may be quite far from being neutral (e.g. blueberry juice!), so you would probably get acidosis. The final problem is the temperature of the fluid, because without it being at body temperature, the body is going to be struggling either to keep it warm or keep it cool. In my stories, i tend to insert a line about warming the air before it's pumped in, but there are other imponderables with gas because of compression changing the thermal energy in it.

Yes, i've way overthought this!

Sorry, double post.