What the kidneys actually do — and why this happened to your cat
After a diagnosis, thinking usually runs in this order. What do I do → but what actually is this disease → why my cat.
The first question is covered in the day of diagnosis. This article is about the second and third.
1. The kidneys do far more than filter
Cats, like people, have two kidneys. The best-known job is filtering waste into urine. But that is only one of several.
| What the kidney does | What appears when it fails |
|---|---|
| Waste excretion | Uremia — nausea, stomatitis, loss of appetite |
| Water regulation | Can't concentrate urine, so water is continuously lost → dehydration, drinking and urinating more |
| Electrolyte balance sodium, potassium, phosphorus, calcium | Potassium swings, phosphorus accumulates |
| Blood pressure regulation | Hypertension — 20–65% of CKD cats |
| Acid–base balance | Metabolic acidosis — lethargy, potassium abnormalities |
| Producing EPO (red cell hormone) | Anemia — energy and appetite fall together |
| Bone health activating vitamin D | Disordered phosphorus/calcium metabolism, hyperparathyroidism |
2. Why it can't be reversed — the nephron
Inside each kidney are hundreds of thousands of tiny filtering units called nephrons. They are what actually does the work.
So why do some cats live a long time?
Three things work in your favour.
- There are two to begin with. When one declines, the other carries a substantial share
- There is large spare capacity. Kidneys are built to do several times the work actually required, so a great deal can be lost while everything still looks fine
- The remaining nephrons can be made to work better. Reduce dehydration, lower phosphorus, control blood pressure, correct acidosis, and the same number of nephrons operate in a better environment. This is why management is worth doing
Large spare capacity means, conversely, that blood values don't move until a great deal is already gone. "Why didn't I know sooner?" is a common reproach — but this was never a disease you could have known sooner.
3. Why did this happen to her?
Wanting to know the cause is natural, and the thought "was this my fault?" comes with it. The conclusion first — in most cases this was not within an owner's control.
Acute kidney injury (AKI) often has an identifiable cause
- Plants — lilies are the greatest danger. Pollen alone, or the water from the vase, can cause kidney failure
- Medications — human NSAIDs, some antibiotics. Paracetamol/acetaminophen is lethal in small doses
- Chemicals — antifreeze (ethylene glycol) is the classic. It tastes sweet, so cats lick it
- Infection — pyelonephritis
- Urinary obstruction · stones
- Dehydration and hypotension — after anaesthesia or severe vomiting/diarrhoea
Chronic kidney disease (CKD) rarely has a findable cause
The most common cause is ageing, and that is not the kind of thing that can be prevented.
It can also arise secondary to something else.
- Polycystic kidney disease (PKD) — an inherited condition, common in Persian and related breeds, and detectable in advance by PKD1 genetic testing
- Hypercalcemia
- Urinary and kidney stones
- FIP (feline infectious peritonitis)
- Chronic dental disease — persistent oral inflammation places a systemic burden
- Cancer — lymphoma involving the kidney
- Chronic infection and chronic inflammation
About "high-protein food ruins the kidneys"
The confusion arises because high-protein products are frequently also high in phosphorus. So the number to read is not protein — it is phosphorus.
And protein after CKD has arrived is a different discussion entirely — see stage-by-stage diet strategy.
4. Why are there so many CKD cats now?
Paradoxically, because cats now live longer.
Better nutrition, routine vaccination and veterinary care, indoor living and better stress management have all pushed feline lifespan up. Fifteen is now ordinary, and cats past twenty are not rare.
But if the leading cause of CKD is ageing, then the more cats that live to be old, the more cats have CKD. In a sense, CKD becoming common is a consequence of pet care getting better.
5. This was not your fault
If you take one thing from this chapter, let it be this.
- Values don't rise until more than two-thirds of kidney function is gone. There was effectively no way to know earlier
- The most common cause is ageing. It was not preventable
- Cats evolved to hide illness. You didn't fail to notice — she didn't show you
What remains is what can be done from here. That is the rest of this book.
6. Read next
- How to read kidney values on a blood test — the numbers on the report
- Stage and goals — where she stands, and what to aim for
- Feline kidney disease guide — one owner's record of seven years of CKD care