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Main Functions of the Kidneys

How the Kidneys Work

What Happens in CKD

Why CKD Occurs

Why CKD Cannot Normally Be Detected At An Early Stage





Site Overview

Just Diagnosed? What You Need to Know First

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What Happens in CKD

Causes of CKD

How Bad is It?

Is There Any Hope?

Acute Kidney Injury



Phosphorus Control


(High Blood Pressure)



Potassium Imbalances

Pyelonephritis (Kidney Infections) and Urinary Tract Infections NEW

Metabolic Acidosis

Kidney Stones



Nausea, Vomiting, Appetite Loss and Excess Stomach Acid

Maintaining Hydration

The B Vitamins (Including Methylcobalamin)




Ways of Assessing Food Content, Including What is Dry Matter Analysis

How to Use the Food Data Tables

USA Canned Food Data

USA Dry Food Data

USA Cat Food Brands: Helpfulness Ratings

USA Cat Food Brands: Contact Details

USA Food Data Book



Coping with CKD

Tanya's Support Group

Success Stories



Important: Crashing

Alphabetical List of Symptoms and Treatments

Fluid and Urinary  Imbalances (Dehydration, Overhydration and Urinary Issues)

Waste Product Regulation Imbalances (Vomiting, Appetite Loss, Excess Stomach Acid, Gastro-intestinal Problems, Mouth Ulcers Etc.)

Phosphorus and Calcium Imbalances

Miscellaneous Symptoms (Pain, Hiding Etc.)



Early Detection

Blood Chemistry: Kidney Function, Potassium, Other Tests (ALT, Amylase, (Cholesterol, Etc.)

Calcium, Phosphorus, Parathyroid Hormone (PTH) and Secondary Hyperparathyroidism

Complete Blood Count (CBC): Red and White Blood Cells: Anaemia and Infection

Urinalysis (Urine Tests)

Other Tests: Ultrasound, Biopsy, X-rays etc.

Renomegaly (Enlarged Kidneys)

Which Tests to Have and Frequency of Testing

Factors that Affect Test Results

Normal Ranges

International and US Measuring Systems



Which Treatments are Essential

Fluid and Urinary Issues (Fluid Retention, Infections, Incontinence, Proteinuria)

Waste Product Regulation (Mouth Ulcers, GI Bleeding, Antioxidants, Adsorbents, Azodyl, Astro's CRF Oil)

Phosphorus, Calcium and Secondary Hyperparathyroidism (Calcitriol)

Phosphorus Binders

Steroids, Stem Cell Transplants and Kidney Transplants

Antibiotics and Painkillers

Holistic Treatments (Including Slippery Elm Bark)

ESAs (Aranesp, Epogen etc.) for Severe Anaemia

General Health Issues in a CKD Cat: Fleas, Arthritis, Dementia, Vaccinations

Tips on Medicating Your Cat

Obtaining Supplies Cheaply in the UK, USA and Canada

Working with Your Vet and Recordkeeping



Nutritional Requirements of CKD Cats

The B Vitamins (Including Methylcobalamin)

What to Feed (and What to Avoid)

Persuading Your Cat to Eat

2007 Food Recall USA



Oral Fluids

Intravenous Fluids

Subcutaneous Fluids

Tips on Giving Subcutaneous Fluids

How to Give Subcutaneous Fluids with a Giving Set

How to Give Subcutaneous Fluids with a Syringe

Subcutaneous Fluids - Winning Your Vet's Support




Heart Problems



Polycystic Kidney Disease (PKD)


Dental Problems





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The Final Hours

Other People's Losses

Coping with Your Loss




Feline CKD Research, Including Participation Opportunities

CKD Research in Other Species

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Canine Kidney Disease

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Home > What is CKD > What Happens in Chronic Kidney Disease



  • This page explains more about the various functions of the kidneys.

  • It is because the kidneys have so many functions that there are so many possible symptoms of CKD. The symptoms you might see depend upon which functions are affected.

  • This page also explains why CKD is so common in cats, and why CKD cannot normally be detected until two thirds of function has already been lost.

Main Functions of the Kidneys


The kidneys have five main functions:

  1. the regulation of fluid levels in the body

  2. the regulation, including filtering and disposal, of waste products in the body

  3. the regulation of electrolytes (salts in the body's cells which are necessary for survival) and minerals in the body

  4. stimulation of red blood cell production

  5. production of renin, which controls blood pressure

In cats with CKD, as the kidneys become more and more damaged and their ability to function declines, an imbalance can arise in any or all of these areas.  So, for example, a cat whose kidneys struggle with the production of red blood cells will develop anaemia.


How the Kidneys Work


in order to understand what happens in CKD, it helps to have a rough idea of how the kidneys work.



The main work of the kidneys is performed by units called nephrons. The nephrons filter the blood flowing into the kidneys via their glomeruli (see below). Healthy cats have about 170,000 - 190,000 nephrons, far more than they need, which is known as the renal reserve.


Khan Academy has a clear video showing how the nephrons work.


Glomeruli and Glomerular Filtration Rate (GFR)

Each nephron contains a glomerulus, a very fine blood vessel. The glomeruli perform the first stage in the process of filtering waste products to produce urine. GFR measures their filtering ability.


If the glomeruli are damaged, blood proteins, which would normally be recycled into the bloodstream, may leak out into the urine (proteinuria).


The glomerular filtration rate (GFR) is an accurate measure of how effectively the glomeruli and nephrons are working, but unfortunately this is difficult to measure, so it is not checked very often.


Long Beach Animal Hospital clearly explains more about the way the kidneys work.


The Merck Manual explains kidney function (with diagrams).


What Happens in CKD


There are a number of different causes of kidney disease. Management and treatment of chronic kidney disease in cats (2016) Caney S In Practice Oct 2016 pp10-13 states "CKD is the end result of a wide range of primary disorders that cause irreversible damage to nephrons, eventually leading to reduced glomerular filtration rate (GFR)."


Interstitial nephritis is the most commonly seen type of kidney problem in cats, so when a vet says a cat has CKD, s/he often means chronic interstitial nephritis.


Chronic interstitial nephritis is not a disease as such. Renal fibrosis in feline chronic kidney disease: known mediators and mechanisms of injury (2015) Lawson J, Elliott J, Wheeler-Jones C, Syme H & Jepson R The Veterinary Journal 203(1) pp 18-26 says "In most cases the underlying aetiology is unknown, but the most frequently reported pathological diagnosis is renal tubulointerstitial fibrosis. Renal fibrosis, characterised by extensive accumulation of extra-cellular matrix within the interstitium, is thought to be the final common pathway for all kidney diseases and is the pathological lesion best correlated with function in both humans and cats."


Renal diseases in cats (2015) DiBartola SP Presentation to the Idexx Finland Congress says "Chronic interstitial nephritis is a common morphologic diagnosis and may represent the end result of several different renal diseases including chronic glomerulonephritis and chronic pyelonephritis. In most patients, however, the inciting cause of the progressive renal disease cannot be determined."


If you're not a vet, you are probably none the wiser after reading the above. Essentially, whatever the initial cause of the CKD, inflammation and fibrosis (the formation of excess fibrous connective tissue) occur and the end result is a cycle of kidney damage. Cats with chronic interstitial nephritis have small, shrivelled kidneys with scar tissue, and this is what is seen in most CKD cats. Feline chronic kidney disease (2015) Grauer GF Today's Veterinary Practice 5(2) pp36-41 says "the end point of irreversible glomerular or tubular damage is the same: fibrous scar tissue replacement of nephrons."


Renal dysfunction in small animals (2016) Brown SA Merck Veterinary Manual has some information on interstitial nephritis and CKD.


Why CKD Occurs


There are a number of reasons why CKD occurs in cats, see Causes of CKD and immediately above. However, in most cases there is little you could have done to prevent it because one of the main factors in the development of CKD in cats is aging. Studies indicate that around 10% of cats over the age of ten will develop CKD. Older cats are at even greater risk: Chronic Renal Disease in Cats (1989) Current Veterinary Therapy X, Krawiec DR & Gelberg HB, Ed. Kirk RW, WB Saunders Company p. 1170-1173 found that 30% of cats over the age of 15 had the disease.


A more recent study, Disease surveillance and referral bias in the veterinary medical database (2010) Bartlett PC, Van Buren JW, Neterer M & Zhou C Preventive Veterinary Medicine 94(3-4) pp264271, found that 28% of the cats over the age of 12 who were examined at four US veterinary hospitals had CKD. Therefore, if you have a cat aged 12-15 or over, s/he has a one in three chance of developing CKD.


Younger cats may also develop CKD but it is less likely. If they are very young (less than two years old), they may have a congenital problem. Other possible causes include kidney infections, blockages or exposure to toxins (though cats with these problems may have acute kidney injury).


The Cat Doctor mentions that CKD occurs twice as often in Siamese, Maine Coon, Abyssinian, Russian Blue and Burmese cats.


Why CKD Cannot Normally Be Detected at an Early Stage


I often hear from people who are kicking themselves for not realising sooner that their cat was sick. But it is actually normal for CKD not to be diagnosed until as much as 66% of function has been lost, if not more.


The 170,000 - 190,000 nephrons found in a cat's kidneys are many more nephrons than are needed for normal function; plus nephrons can increase their individual function to some extent when other nephrons die. This is why people can donate a kidney and still manage perfectly well with one kidney. In the case of a kidney transplant, if you remove one kidney from the donor, the donor's glomerular filtration rate (GFR, a measure of kidney function) will immediately fall to half of what it was, but will then gradually improve as the remaining nephrons increase their function to compensate for the loss of one kidney. Eventually the nephrons in the remaining kidney will reach almost the same level of function as two kidneys.


It works in a similar way in a cat with kidney disease, i.e. as damaged nephrons die (they are often described as "scar tissue"), other nephrons take over their work. Eventually, however, all the remaining nephrons will be working fulltime (i.e. there will be no renal reserve left). It is at this point, when around 66-75% of function has gone, that you will probably start to see symptoms in your cat, as the remaining nephrons start finding it harder to cope with the workload.


Feline chronic kidney disease (2015) Grauer GF Today's Veterinary Practice 5(2) pp36-41 states "Progressive diseases that slowly destroy nephrons allow intact nephrons to undergo compensatory hypertrophy, which can delay onset of renal failure. Therefore, when renal failure occurs (< 25% of the original nephrons functional), nephron hypertrophy can no longer maintain adequate renal function."


So please do not feel guilty for not noticing sooner there was probably nothing for you to notice, plus cats are very good at hiding signs of illness. CKD is not normally painful, so this makes it easier for the cat to hide what is happening.


There are a number of possible methods of Early Detection, but some of these are quite specialised, and most people wouldn't know about them; in fact, not all vets are familiar with all of them, although there is a new test (introduced in 2015) called the SDMA test. This is a simple blood test available through IDEXX Laboratories which may help detect CKD sooner, but even then it can only detect CKD when 40% of function has gone.


Don't waste your energy beating yourself up. What you need to focus on is the fact that cats with CKD can often manage quite well on limited kidney function. For some cats, things only become critical when they have lost as much as 90% of function, and there are some cats who cope astonishingly well with even less function. So the goal is not to worry about the function that has already been lost, but to try to retain the remaining function for as long as possible, and keep your cat feeling as well as possible. This site aims to help you with both goals.


Links to More Information


Cornell University College of Veterinary Medicine has a helpful video overview of feline kidney disease. Click on Part V first, Understanding kidney disease.


Kidney disease (2007) Syme H Royal Veterinary College is a 22 minute long podcast which you can download. Scroll down to RVC10 if necessary.


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This page last updated: 30 June 2020


Links on this page last checked: 30 June 2020







I have tried very hard to ensure that the information provided in this website is accurate, but I am NOT a vet, just an ordinary person who has lived through CKD with three cats. This website is for educational purposes only, and is not intended to be used to diagnose or treat any cat. Before trying any of the treatments described herein, you MUST consult a qualified veterinarian and obtain professional advice on the correct regimen for your cat and his or her particular requirements; and you should only use any treatments described here with the full knowledge and approval of your vet. No responsibility can be accepted.


If your cat appears to be in pain or distress, do not waste time on the internet, contact your vet immediately.



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