I want to design a creature capable of surviving long periods without drinking water, but I am not sure how that creature could store water.
These are my ideas but I am looking for something more efficient or possible:
- Increase amount of water in blood: Each litre of blood has almost half litre of water. Increasing the amount of water in blood could be used to store it but thinking in that I notice a little problem: increasing the amount of water in blood would also raise the pressure of blood and I don't know if that could be bad. Also I have read somewhere  that if you increase the amount of water in blood, red blood cells start absorbing it autocratically (it's a problem of their plasma membrane) until they "explode" and die - a huge problem.
- Store water inside fat: Dromedaries store almost 50 kg of fat in their hump; fat isn't water but fat has water. I don't know exactly the process but when they produce energy with the fat, each kilogram of fat combined with oxygen could produce like waste a litre of water used to survive. This idea seems quite good but I don't want to put a hump in them and also a litre of fat doesn't have exactly a litre of water so it is quite "inefficient".
- Special organ: My last idea is have an inner organ like a bag full of liquid water to store it. Water enter to the organ through the blood system and when it's needed, it's send to the intestine, where it can be re-absorbed. I find the idea excellent but I don't know any specie which does that so I don't know if it's possible. Also I was thinking to store water in an organic material made of water (or at least hydrogen) and more dense than water, so it can store more water in the same volume, but I don't know any compound like that.
I don't like the idea of use humps because:
- A hump isn't "cute", I don't want to put an hump in their back. Also it's quite heavy and it's a hunter creature.
- A kilogram of fat produces a litre of water, but a litre of fat isn't a kilogram of fat (I can't find the value), so it's less efficient than storing the water itself. Maybe you could say that it has the plus of giving energy and you are right but my specie is fine with that, it doesn't need more energy backup, I am looking for water backup.