When I was writing my book the Dictionary of Veterinary Terms, Vet-Speak Deciphered for the Non-Veterinarian, the kidneys caused me no end of grief. Not only are the kidneys themselves complicated, but so is the terminology associated with the diseases that affect them.
So here’s a primer on canine kidneys and all the associated verbiage.
Kidneys: Where are They and What Do They Do?
The kidneys are two separate but essentially identical organs that are located between the abdominal space and the lower back. Technically, they do not reside in the abdomen but within the retroperitoneal space (i.e., the space behind the peritoneum or lining of the abdomen). The left kidney is usually a little farther back than the right.
Each kidney consists of hundreds of thousands of nephrons, the functional unit of the kidney.
Think of a nephron as a microscopic filter attached to a long tube.
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| Image: Encyclopedia Brittanica |
The blood that enters the kidney progresses through progressively smaller vessels until it meets a single nephron. There, “bad stuff” (e.g., toxins that are produced by the body’s metabolism) is filtered out and turned into urine. At the same time, the “good stuff” (e.g., red blood cells and water) is either not filtered out in the first place or is reabsorbed before urine production is complete.
Normal kidneys have many more nephrons than there are needed on a day to day basis.
Why is this important? Because we can’t replace a nephron once it is no longer functional. And LOTS of things damage nephrons (more on this later).
But the kidneys do more than just filter out “bad stuff.”
Kidneys also secrete a hormone that
- stimulates red blood cell production
- plays a role in regulating blood pressure
- conserve water
- help balance electrolyte levels,
- and more.
What is Kidney Disease/Failure/Insufficiency
A disease can be defined as anything that adversely affects the functioning of the body. Therefore, kidney disease is anything that adversely affects kidney function… this could be an infection, an immune disorder, degenerative disease, neoplasia (cancer), trauma, etc.
Saying that a dog has kidney disease is basically the same as saying that it has sick kidneys.
What many people mean when they say “kidney disease” is kidney failure.
Failure means as a loss in the ability to perform normal functions to the point where the body experiences a significant adverse effect. Some veterinarians don’t like the term “failure” (it sounds so untreatable) so they use “insufficiency” instead.
A diagnosis of kidney failure usually means that laboratory values associated with kidney function (blood urea nitrogen (BUN), creatinine, urine specific gravity (USG), etc.) are found to be abnormal.
Another word that can apply to this situation is “azotemia.” An azotemic dog has higher than normal levels of BUN and creatinine in its blood.
“Uremia” basically describes the same scenario but also implies that the patient feels poorly because of its azotemia.
What all this means is that most of the kidneys’ nephrons are no longer functioning.
In fact, the earliest sign of kidney failure seen in typical lab work is a low urine specific gravity. This doesn’t occur until about two-thirds of the nephrons have been lost. Azotemia doesn’t occur until at least three-quarters of the nephrons are gone.
Acute or Chronic?
Kidney failure can be either acute (describing something that just happened) or chronic (describing something that developed over weeks, months, or even longer).
Acute kidney failure is caused by things like
- infections
- toxins such as antifreeze or grapes/raisins, and
- episodes of low blood pressure
However, in some cases, an underlying cause can’t be identified.
If the incident kills off enough nephrons, the patient will die without a kidney transplant. On the other hand, if the patient can be supported through the crisis, nephrons that essentially went “offline” but weren’t fatally damaged may recover allowing the body to return to a satisfactory level of functioning.
Chronic kidney failure comes on slowly and is a progressive disease.
As I mentioned before, LOTS of things can damage nephrons. Acute episodes may knock out a bunch all at one time. However, the wear and tear of daily life also takes a toll and gradually reduces the kidneys’ reserve supply of nephrons. Over time, the kidneys may reach the point where only one-third to one-quarter of nephrons remain. That is when the symptoms and laboratory evidence of kidney failure develop.
The International Renal Interest Society (IRIS) has come up with a staging system that lets veterinarians communicate how severe a dog or cat’s chronic kidney disease is.
IRIS uses the term “disease” because they include animals that are not currently in kidney failure but are at high risk for it in the future.
But this also begs the question, what’s up with the “R” in the “IRIS?”
Here’s one more term for you. “Renal” is the adjective form of “kidney.” I guess “kidneyal” was just too much of a mouthful.
Related articles:
Kidney Failure in Dogs
Screening Your Dog’s Kidney Function