It is just my educated guess that since a primary function of the kidneys is to filter the blood that unclean kidneys come from unclean blood. This of course leads one to ask - how do we get dirty blood? It's at this point the task of educated guessing gets quite a bit more complicated because there are so many functions and processes integral to the human body that are facilitted by the flow of blood, and I'm talking way beyond the simple processes that begin when a person willingly takes stuff into their body by what they breath, drink and eat, but a person who exercises healthy control over the things they willingly breath, drink and eat probably stands a better chance of enduring all those other things they unknowingly breath, drink, eat and absorb.