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The kidneys are essential organs that serve the body as a natural filter of the blood and remove wastes which are diverted to the bladder. Furthermore, they function as a regulator, maintaining the acid-base balance and regulating blood pressure by maintaining the salt and water balance). The kidneys also are responsible for the reabsorption of water, glucose and amino acids; they also produce hormones and enzymes. A human being whose kidneys are removed would face certain death, yet the stomachs of ruminant animals contain a mechanism that removes wastes to the stomach so that such an animal could survive even if its kidneys were removed.
On our daf, Rachish bar Papa teaches that although missing kidneys would not render the animal a treifah, if one of the kidneys is diseased, then the animal would be rendered a treifah.