Introduction to the Urinary System: Anatomy and Function
The kidneys are responsible for filtering many toxic substances from the
blood, especially the by-products of protein digestion and other
nitrogen-containing compounds. They are also responsible for regulating
precisely the levels of minerals and electrolytes (salt-like compounds) in
the blood stream. Another very important function that is unrelated to its
role in urinary function, is the production of the hormone erythropoeitin,
which stimulates the bone marrow to produce red blood cells. Red blood
cells carry oxygen from the lungs to the rest of the body.
There are two kidneys in the body, both located under the back-bone, close
to where the last rib meets the spine. Through a number of complex
processes, the kidneys filter the blood, remove certain toxins, and retain
certain minerals and electrolytes. The fluid that remains is urine, which
is collected in a central area of the kidney and funneled into a small tube
called the ureter. The ureters (one from each kidney) run from the kidney
to the bladder, which is located just in front of the pubic bone of the
pelvis, and collects the urine until the quantity is sufficient for the dog
to feel the urge to urinate. The urine is retained inside the bladder by a
circular muscle called a sphincter, which relaxes at the same time the
bladder contracts, allowing urine to exit the bladder into the urethra,
which is the passageway to the outside.
Urinary problems can occur at any point along the urinary tract. Infections
are very common in the bladder, especially in female dogs, whose urethra is
wider and shorter than the males. Males, however, can and do get bladder
infections. In intact (non-neutered) male dogs, these infections can be
more difficult to treat than those of neutered males because it is easier
for bacteria to stay hidden from antibiotics in the prostate, which shrinks
substantially following neutering. Recurrent lower urinary tract infections
(i.e. of the bladder and/or prostate) are the number one cause of kidney
infections, because, with time, the infection may travel up the urethra to
infect the kidneys. Although there are several other important causes for
kidney function to be reduced, damage from ascending urinary tract
infections is a significant cause of age-related kidney insufficiency or
failure. Dogs and cats can lose up to 2/3 of their functional kidney
capacity without losing any of the ability of their kidneys to do their
job! After 2/3, most dogs/cats will start drinking more water to help
compensate for reduced function. It is only when 3/4 of the capacity is
gone, that the kidneys can no longer compensate completely and toxins and
abnormal levels of minerals and electrolytes begin to build up in the blood
stream.
Dietary Assistance for Dogs with Failing Kidneys:
Dietary changes for dogs with renal (kidney) insufficiency or failure may
be helpful, but it is important to understand that they are not going to
cure or even treat the disease itself. Diet modification merely gives the
kidneys an easier job to do. It does little to help the function itself.
Because by-products of protein digestion are the main toxins
that need to be excreted by the kidneys, an obvious assumption might be
that all one needs to do is to cut out the protein and the kidneys
wouldn't have any more hard work to do. Of course, things are never that
simple. One certainly would not want to give an excessive amount of
protein, but how much is too much? Dogs do have a minimum daily protein
requirement, the amount of which depends on the quality of the protein
being fed and on the health status of the animal. Proteins are graded on
their "biologic value." High biologic value means that the protein is
highly digestible, easily absorbed by the intestinal tract, and that its
amino acid components (the building blocks of proteins) include all the
essential types of amino acids in their optimal proportions. Egg protein
and milk protein (casein) are the dietary proteins with the highest
biologic value. Lean meat protein from muscle or organ meat, not meat
by-products, are a close second. Normal, healthy dogs, with average activity
levels, need approx. 3 - 3 1/2 gm of high-biological-value protein, "as fed,"
per lb. body weight per day. This translates into approximately 0.6 - 0.7 gm
of dried cooked egg/lb/day. There is significant evidence, however, that
the daily protein requirements actually increase slightly for dogs in
chronic renal failure. Therefore, severely restricting the protein for such
a dog is likely to result in protein malnutrition, in spite of the fact
that the levels of blood urea nitrogen, or BUN (the primary by-product of
protein metabolism) would be correspondingly lower. Typical prescription
"kidney disease diets" formulated by various commercial pet food companies
are designed to provide high quality protein in moderately restricted
amounts (generally 0.9 gm of dry-weight protein/lb/day), while providing
adequate caloric intake from non-protein ingredients. Further adjustment of
the diet must be based upon the individual animal: (1) blood test evaluations
of kidney function (e.g. BUN levels); (2) general appearance of the patient
(e.g., muscle mass, maintenance of body weight; and (3) clinical assessment
of health status. If the BUN levels are in a "good" range, but the dog
is progressively losing weight, then the protein may have to be
increased. Conversely, if the dog is maintaining adequate nutritional
status, but the BUN is steadily climbing, then reducing the protein further
may be of benefit.
Other dietary issues for dogs in chronic renal failure include (1)
maintaining adequate caloric intake to keep other bodily processes
functioning properly; (2) maintaining adequate levels of water soluble
vitamins (B complex and vit.C); and (3) preventing excessive calcium loss
because of the elevated phosphorus levels that develop secondarily in renal
failure.
Caloric requirements are usually met by the addition of extra carbohydrates
and fat to the diet. It is recommended that dogs receive 35 - 50
Cal./lb/day, on average, and that these amounts be tailored to meet the
needs of the individual patient. The commercial kidney diets usually
fulfill these requirements. To prevent excessive loss of the water soluble
vitamins, supplementation of balanced B complex vitamins and Vitamin C is
usually recommended. For dogs with excessive blood levels of phosphorus, a
phosphorus binder such as Amphojel, Basaljel, or Alternagel, is given with
each meal, in order to bind some of the phosphorus present in the food. The
amounts of these supplements should be determined by the veterinarian,
based on the status of the individual patient.
Kidney failure is a varied and complex disease, affecting many organ
systems and affected by numerous intrinsic and extrinsic factors. Dietary
management is only one factor that can affect the quality of a kidney
patient's life. There are many others, which your veterinarian can discuss
with you. Appetite loss, nausea, and other problems can complicate the
picture and a number of symptomatic treatments can provide significant
relief for a time. Fluid therapy, administered either intravenously or
between the layers of the skin, can make a significant temporary
improvement in the animal's clinical status. Unfortunately, most cases of
chronic renal failure are cases of management, not cure. With that in mind,
the "ideal treatment regimen" becomes secondary to the patient's quality of
life. For example, if the dog has no appetite for the food he is "supposed"
to eat, but wants something he is not supposed to have, the question
arises, "Isn't it better to eat something than nothing?" If an antiulcer
medication will decrease the nausea caused by excessive BUN levels, but the
dog fights it bitterly, is it worth the battle? These are very difficult
questions that all owners of kidney patients face. There are no easy
answers, but the support of your veterinarian, the veterinary staff, and
your circle of dog-friends can help you through a lot of them. Dietary
management is only a portion of the bigger picture.
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Copyright 2000 Dr. Lucy Pinkston. All rights reserved.
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