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I'm trying to understand what happens with the water while a person is dying from thirst in a deep, narrow mine shaft. Assuming he has dry food for a long time, and some liters of water. Dry air is available, CO2 can get out.

To simplify, let's assume we have box of 1 m³, with a mammal of 1000 cm³ inside. Water and water vapor can not pass the boxes side walls, other substances can. Top and bottom are impermeable. The walls are heated enough to make the water not condense. The animal can live up to 40 °C. Enough food and 3 l water are available.

So, I think the animal will die from thirst after some time. The air will quickly saturate with water, any evaporation stops. It will not starve. But maybe the walls will heat it up too much? The air is saturated with water, so transpiration is not working. When fresh water is no longer available, concentrated urine will collect, until it is too concentrated to be of any use, even if it is used for drinking. Because osmosis into the body does no longer work? So I think there is absolutely no water lost from the body except for urination. And then, the dead body, assumed sterile, will stay perfectly moist. Is that right?

(Not sure I should tag this with experimental physics...)

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  • $\begingroup$ What about sweat, saliva, and mucus? The production of all of those things requires water. In addition, water vapor is exhaled with every breath. $\endgroup$
    – probably_someone
    Commented Apr 26, 2019 at 14:53
  • $\begingroup$ Yes, I missed that sweat can run off the skin without evaporation. Saliva and mucus do not evaporate, and stay in the body. Yes, by aspiration, no water is gained or left. I think it is normally lost there in part. $\endgroup$
    – Volker Siegel
    Commented Apr 26, 2019 at 15:29
  • $\begingroup$ Saliva and mucus (and, in general, the moisture keeping the inside of the nose and mouth and the outside of the eyes from drying out) are exposed to the outside air all the time, though. $\endgroup$
    – probably_someone
    Commented Apr 26, 2019 at 15:40

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let's assume we have box of 1 m³, with a mammal of 1000 cm³ inside. Water and water vapor can not pass the boxes side walls, other substances can. Top and bottom are impermeable.

The mammal will probably suffocate due to lack of usable oxygen and/or inability to get rid of enough CO2 long before it dies of thirst.

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  • $\begingroup$ That is true for the real world case, of course - but I try to understand the water part of it, as in the abstract description, where dry air can freely exchange.. I should make $\endgroup$
    – Volker Siegel
    Commented Apr 26, 2019 at 15:23

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