Chronic renal insufficiency - an overview
"Chronic" means that a disease is long-standing, or had been present for a while. In the case of kidney disease in cats, most of the time it means the disease has taken a while to develop, and will take more time to progress. Chronic diseases are those that we think of as "dragging on" (like arthritis), as opposed to acute diseases which come on quickly and sometimes go away quickly (like the flu).
"Renal" is just the Latin word for the kidney. We use "renal" as an adjective, as in renal insufficiency or renal failure. "Nephro-" is the Greek version, and is usually the form we use when we are connecting it to another medical term. nephrotoxic, for example, means toxic to the kidneys. Now you know a couple of $10 words.
"Insufficiency" in this case means that there is insufficient function to do the job properly.
But what is the job of the kidney? What does it do? We all know that the kidneys make urine. There is much more to it than that. Have you thought about what is in urine? And even more importantly, what is not?
The kidneys have FIVE very important functions, not all of which are directly related to producing urine. The first is to remove toxins from the body. Our bodies produce lots of things that we don't want to keep. These chemicals are breakdown products or byproducts of our everyday metabolism. Every time we use a muscle or have a gland excrete, our cells produce small (or large) amounts of waste. Part of the kidney's job is to get rid of some of these wastes and toxins.
The second big job is to retain things. We don't want to throw the baby out with the bathwater, and so we don't want our kidneys excreting good things like glucose (sugar), useful proteins, electrolytes like potassium and sodium, and salts. We also like our kidneys to be able to retain water when appropriate. If we are walking through a desert without a canteen, we are going to rely on our kidneys to conserve water and not turn it all into urine.
Our kidneys (and those of dogs and cats) have sensors in them that detect changes in blood pressure. Through a specialized signaling system the kidneys help to maintain normal blood pressure. Loss of this regulatory function can result in high blood pressure.
Kidneys are also big producers of a variety of hormones, and are involved in the conversion of vitamin D to its active form. One of the hormones the kidneys produce stimulate the bone marrow to make red blood cells (erythropoietin). Others are involved in blood pressure mechanisms and salt balance.
And finally, and probably most obscurely and biochemically, they help to regulate the acid-base balance in the body. Very important, but not very exciting, sadly.
The kidney has a ton of redundancy. You don't need all of your kidney capacity in order to survive. If you think about it, you'll realize that you don't even need half of your capacity - that's why you could donate a kidney and still be a perfectly normal person with normal renal function, with only half of your initial kidney mass.
Redundancy is an excellent thing. It means that the kidneys can take a lot of abuse over time, and your cat's kidneys can still make good urine and get rid of toxins and do the things they ought to do. As a matter of fact, we don't start to see any changes in the blood or urine tests until 2/3 of the functional ability of the kidney is lost.
Damage and loss of function
The functional unit of the kidney is the nephron. Nephrons are the little microscopic bits of the kidney that do all the filtering and urine-making. There are millions of nephrons in each kidney. When 2/3 of those millions of nephrons are so damaged that they can't work properly, the kidney cannot produce appropriately concentrated urine. On our lab tests we start to see cat urine that is more dilute than we consider normal. The kidneys have lost the ability to retain water, even when the cat is clearly becoming dehydrated.
When 3/4 of the nephrons are damaged, there is evidence that more functions are affected. We start to see evidence that the kidney's filtering abilities are lost; they are not longer able to remove toxins. We see a rise in blood levels of urea (blood urea nitrogen, or BUN) and creatinine. Potassium levels go down as the kidneys fail to retain electrolytes. Protein starts showing up in the urine. Phosphorus levels in the blood rise, as the kidneys are unable to remove it. Most of these cats become anemic over time, as the hormone that stimulates the bone marrow to produce red blood cells is lost. Blood pressure rises.
And that is the picture of feline chronic renal insufficiency. Over time the kidney nephrons are damaged, and the kidney is no longer able to perform its functions. As these functions are lost, the cat will exhibit signs of clinical disease as well as laboratory changes. It should also be well noted that many cats live for many years witha good quality of life with this disease. Diagnosis is not necessarily a death sentence.