Chronic kidney disease is one of the more common ailments affecting older animals.  In most cases, chronic renal disease is not curable without a kidney transplant, but it is a managable condition.  The kidneys perform various important functions for the body.  They remove metabolic waste by filtering the blood through many tiny filtration systems called glomeruli.  They also expend energy to actively remove certain toxins from the body via an extensive system of tubules.  The glomeruli and tubules of the kidneys work to produce urine and help maintain the proper balance of water and various electrolytes in the body.  The kidneys also serve to help maintain proper blood pressure and even produce the hormone erythropoietin which stimulates the bone marrow to produce new red blood cells.  It is because of the relative inefficiency of these important physiological functions that animals with renal disease begin to feel sick.  As the kidneys fail, toxins begin to build up in the bloodstream and animals tend to become dehydrated.  They lose the ability to concentrate the urine and, even though they drink a lot of water, it is difficult for affected animals to stay hydrated.
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Diet Modification:
Animals with renal disease should be fed a good quality low-protein diet.  The reason for this is that the kidneys are responsible for the removal of the by-products of protein metabolism.  After proteins are broken down the liver produces a chemical called urea (aka blood urea nitrogen or BUN) which is a waste product and needs to be removed from the body.  The kidneys are responsible for doing this as well as removing another protein waste product called creatinine.  As these toxins accumulate, animals begin to feel sick.  By providing a low-protein diet you are removing some of the burden the kidneys have to deal with thereby allowing the levels of protein waste to decline and helping your pet to feel better.  There are various diets available through your veterinarian that are ideal for the pet with renal disease.
Since the normal kidneys are responsible for water balance in the body, animals with chronic renal disease have difficulty taking in enough water.  It is important to provide fresh water to your pet at all times.  However, often animals with kidney disease need fluid suplimentation in addition to free choice fresh water.  Giving fluids under the skin (subcutaneous fluids) is a good way to help keep your pet with kidney disease hydrated.  Your veterinarian will help you decided how often and how much fluids you need to give on a case by case basis.  For those who feel comfortable with giving injections, it is possible for your veterinarian to teach you how to give fluids at home.
Phosphorus Restriction:
Excess dietary phosphorus is normally removed by the kidneys.  Therefore, animals in renal failure eventually develop elevated levels of phosporus in the blood.  This excess phosphorus in the blood can lead to serious physiological problems including tissue mineralization and calcium depletion from the bones (additionally, decreased vitamin D conversion to its active form by the kidneys can lower blood calcium and worsen hyperphosphatemia).
Low-protein diets automatically restrict dietary phosphorus but often times this is not enough to keep blood phosporus levels in check.  Another way to help maintain normal phosporus levels is by adding a phosporus binder to your pet's treatment regimen.  These medication work by binding to phosporus in the food your pet eats so that it is not able to be absorbed by the pet's digestive tract.  It is therefore imperative that phosphate binders be given with meals. 
Gastric Protectants:
The toxins that build up in the blood due to chronic renal failure can result is stomach ulcers.  These ulcers can cause nausea, vomiting, inappetence and bleeding (and potentially blood in the vomit).  Because of this, your veterinarian may prescribe medication that is intended to decrease the secretion of acid in the stomach and therefore lessen these symptoms.  Sometimes the use of an antiemtic is also necessary to control vomiting.  Your veterinarian can decide when any of these medications may be indicated.
Treatment for Anemia:
Because of the lack of erythropoietin production by the failing kidneys, the amount of red blood cells in your pet's blood stream may decrease (called anemia).  In most cases of chronic renal failure, anemia develops slowly allowing the animal's body to compensate for the lack of red blood cells.  Red blood cells carry oxygen to the tissues of the body.  Therefore, anemic animals may appear tired or lethargic and may have a decreased appetite.  Erythropoietin can be provided to anemic animals via injection in order to stimulate the bone marrow to produce adequate levels of red blood cells.  Injections are usually given weekly while monitoring red blood cell levels until the appropriate levels are reached.  Your veterinarian can provide you with more information on this type of hormone therapy.
**Adapted fromDr. Cathy Langston, DACVIM, at Animal Medical Center in New York**
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