Dehydration makes cats feel bad. They lose energy and may simply want to rest rather than take part in the activities that they used to love.
Healthy kidneys are also responsible for filtering waste products out of the bloodstream and putting them into urine to be eliminated from the body. Renal disease compromises this important kidney function, which results in increased blood levels of metabolic waste products like blood urea nitrogen and creatinine. This also makes cats feel sick.
Finally, in cases of chronic renal disease, the kidneys no longer produce enough of the hormone (erythropoietin) that stimulates the bone marrow to produce red blood cells. The result is anemia and worsening lethargy.