The ELI5 version is if you don’t drink enough water, the blood is slightly thicker and the kidneys don’t get enough blood flow and that causes damage.
The slightly more complex version relates to how the kidneys work. Essentially they dump water (like, a ton of it) into tubes along with waste and salts etc as part of the initial filtration (cells and proteins aren’t filtered out but small things are). Then these tubes go through a complex filtration system to reclaim the water/salts but leave the waste behind, concentrating the fluid. The more concentrated the fluid is to begin with (ie you’re dehydrated) the harder it is for the kidneys to further concentrate it (which they really want to because you’re dehydrated) and that strain, along with the decreased blood flow, causes damage.
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