Kidney (renal) failure occurs when a dog’s kidneys are no longer able to remove waste and concentrate urine. The bodies of animals produce toxins all day, everyday, and the toxins circulate to the kidneys to be dissolved in water, filtered out and excreted through urination.
A healthy kidney makes highly concentrated urine, meaning a large amount of toxins can be handled and excreted in a relatively small amount of water.
A failing kidney, by contrast, needs more and more water to excrete the same amount of toxins. A dog in kidney failure will drink increasing quantities of water, until eventually he simply can’t drink enough and toxin levels in his bloodstream begin to rise.