Kidneys are organs in humans which process waste. There are two of them inside the human body and one is capable of functioning properly even if the other is removed; however, failure of both is fatal and must be treated with dialysis or transplantation from another human.
In 2003, Teal'c dreamed that he was a fireman who gave his kidney to a dying Bra'tac. In reality, Teal'c was actually sharing his symbiote with Bra'tac in order to keep him alive after an ambush. (SG1: "The Changeling")
While on Destiny, Dale Volker suffered kidney failure. Because the Destiny infirmary did not contain any device capable of dialysis, Tamara Johansen, using knowledge from the Destiny database, performed a kidney transplation using a kidney donated by Ronald Greer with help from the consciousness of Amanda Perry. (SGU: "Hope")