Just like human kidneys, your dog’s kidneys balance certain substances in the blood and filter out the body's wastes as urine. They maintain normal concentrations of salt and water in the body. Kidneys also help control blood pressure, aid in calcium metabolism and sustain phosphorous levels. Additionally, they manufacture a hormone that encourages red-blood cell production. When kidneys don't function properly, toxins build up in the blood and a dog will become ill.