In a healthy dog, kidneys fulfill the role of filter for the body's waste. They absorb toxins from the bloodstream and pass them out of the body as urine. In addition, they regulate the level of calcium in the bloodstream and aid in the metabolism of phosphorous. The "filtering" aspect of a kidney's function is carried out by tiny funnel-shaped tubes called nephrons. They number in the millions in each kidney and are essential parts of the filtration process. However, they are incredibly sensitive to damage. Poison, disease, fever, trauma, bacterial infection and aging all can destroy nephrons very easily, and once destroyed they never grow back. This usually isn't a major concern, though, as kidneys are made to continue working with as little as one quarter of their nephrons intact. If the level falls much below that, however, renal failure will rapidly occur.
Renal failure occurs in two different ways, and the implications for each differ dramatically. The first type is acute renal failure, which happens suddenly and unexpectedly as the result of trauma or infection. This type is usually reversible with emergency medical intervention, but should always be taken seriously. The second type is chronic renal failure, in which the kidneys cease to function a little at a time over a period of months or years. This type is completely irreversible, and the process can only be slowed, never stopped.
Either type of renal failure is diagnosed by a failure to urinate, and a blood sample that indicates a sharp deviation from baseline acid levels in the blood stream. During the early stages of renal failure, blood and urine samples must be taken in order to offer an accurate prognosis. Depending on the severity of the failure, its cause, and the length of time over which it can be determined to have been happening (i.e. - whether the failure is acute or chronic), the course of treatment will be very different.
If acute renal failure has been caused by a bacterial infection, catheterization and emergency administration of antibiotic treatment might be enough to restore function to the kidneys. Generally, the animal is not necessarily any more prone to a second attack than they were to begin with. If the acute renal failure is caused not by infection, but rather by a build-up of toxins or by the ingestion of poison such as antifreeze or rat poison, there is generally no treatment available. Failure of this type is most often fatal.
Chronic renal failure on the other hand is the most common cause of death in older dogs. There is no treatment or cure for the disease, but symptoms can be managed and the dog can be made more comfortable by adjusting his or her level of diet and exercise as your veterinarian prescribes. In addition, ensuring that he or she has constant access to clean fresh water is essential. Otherwise, keeping your animal free from other diseases and stress-free is the best you can do for him or her.
One last consideration: drugs used to treat other diseases such as anticancer drugs can often cause kidney damage as an unintended side effect. Be sure that you're aware of the risks of any medication your dog takes.