As our pet’s get older they are more and more prone to picking up illnesses and problems, and one of the more common issues for an elderly pet to have is kidney disease. This article will explain how the kidneys function, how kidney disease affects this, and what we can do to treat kidney disease.
What do the kidneys do?
The kidneys are very important organs in the body, so much so that they receive up to a quarter of the total blood pumped out by the heart each minute. Their role in the body is to filter out waste products and maintain the balance of fluid and salts in the body, as well as producing a number of hormones that have effects elsewhere in the body, such as controlling blood pressure and red blood cell production. The end product of the kidneys filtering the blood is urine, which is an efficient way for the kidneys to achieve their functions of excretion of waste, and balance of fluid and salt in the body.
How do the kidneys work?
The structure of the kidneys is what allows them to perform their main functions, and individual units that make this happen within the kidney are called nephrons. Dogs have around 400,000 nephrons per kidney, and cats have around 200,000 nephrons per kidney. The nephrons are structured in a way that allows the main functions of the kidney to be performed over three main steps.
Filtration
Filtration occurs at the glomerulus, a mesh of small blood vessels at the start of the nephron where fluid is forced out of the blood. After filtration, the fluid contains very few proteins but lots of salts and sugars, in addition to waste products the body is trying to remove from the system.
Reabsorption
Reabsorption occurs throughout the tubular system of the nephron, where water, glucose (the main type of sugar found in the body), small proteins, and the important salts in the body (mainly sodium and potassium) are taken back into the blood stream so they are not lost in the urine. This leaves waste products and any excess salts and minerals that are not needed by the body behind.
Secretion
Secretion occurs in the final portions of the tubular system, where further waste products and excess salts (particularly potassium if there is too much in the body) can be pumped out to remove larger amounts from the blood stream.
Once these steps have been performed, the urine that has been made flows from the kidneys to the bladder.
A 20kg dog will filter approximately 80 litres of blood over a 24 hour period, and assuming they drink a normal amount of water, 79.5 litres of water will be reabsorbed, resulting in half a litre of urine being produced. The amount of urine produced can vary greatly, this is mainly due to how much water the animal drinks, but can also change with certain conditions that affect how the kidneys work.
What causes kidney disease?
Kidney disease can be caused by a variety of issues, and can occur over a very short time, known as acute kidney injury, or can occur over an extended period of time, known as chronic kidney disease.
Acute kidney injury occurs when an insult to the kidneys results in damage and a reduction in function, and with this animals rapidly become unwell. The more common causes for this are usually ingestion of a toxin, such as ethylene glycol (anti-freeze), certain medications (for example ibuprofen) or Lilies in cats. Acute kidney injury can also occur when there is a urinary tract obstruction, such as with kidney stones or in cats with blocked bladders, or with infections in the kidney.
Chronic kidney disease occurs over an extended period of time, resulting in gradual deteriorations of kidney function and therefore slowly progressive signs of being unwell. Chronic kidney disease can occur as a result of long term changes following an acute kidney injury, after low grade damage to the kidneys over an extended period (exposure to toxins for example), or secondary to other changes in the body such as uncontrolled high blood pressure.
How does kidney disease effect kidney function?
In both types of kidney disease there is damage to the nephrons which results in changes to the way the kidney works. The kidney has quite a large reserve capacity, and so it is not until at least 25% of the nephrons are dysfunctional that we can see the earliest changes to blood samples, and often it is not until 75% of the nephrons are dysfunctional that we will see clinical signs.
The clinical signs of kidney disease are related to a reduction in the volume of blood that can be filtered at any one time, resulting in an increase in waste products in the blood stream, and the inability to keep the balance of some of the salts and minerals (mainly sodium and phosphate).
As a smaller number of nephrons are trying to filter the same volume of blood, the fluid moves much more rapidly through the tubules. This causes water to be absorbed less efficiently, resulting in greater water loss through the urine and overall larger amounts of dilute urine being produced.
The nephrons being damaged can cause knock-on problems as well. One of these is increased protein loss through the glomerulus (which can cause further damage to the nephrons itself), which is detected by more protein being found in the urine. The other main knock-on problem is that kidney disease can cause high blood pressure due to sodium retention and activation of hormone systems. Having a high blood pressure can contribute to further damage to the kidneys as well.
What are the signs of kidney disease?
Acute kidney injury
In acute kidney disease we see different clinical signs than with chronic kidney disease, mainly because the damage is more severe and happens over a much shorter period of time.
Depending on the cause of the injury and how many of the nephrons are damaged, the kidneys may react by producing a large amount of urine, or in very severe damage the complete opposite where they only be able to produce a small amount of urine, or none at all. This will be seen as a sudden extreme thirst and your pet needing to urinate much more than normal, or if there is severe damage then a reluctance to drink and not urinating at all. This acute loss of functional nephrons will also result in rapid accumulation of waste products within the body which leads to the other clinical signs – mainly lethargy, inappetence and abdominal pain, but in some cases also vomiting and diarrhoea.
Chronic kidney disease
In chronic kidney disease, often the first clinical signs noticed are an increase in the amount of water your pet is drinking, and also an increase of the volume of urine they are producing, sometimes to the point that they can’t hold on and are urinating inside. This happens because the kidneys have a reduced capacity to concentrate the urine, more urine is produced, and therefore more water is lost. This results in pets feeling thirsty and drinking more to keep their water balance in check.
Some of the other clinical signs of kidney disease are caused by a build up of urea, the waste product made by digesting proteins in plants and meat. When this cannot be excreted by the kidneys, it builds up in the blood stream and circulates around the body. Urea in high concentrations can cause nausea and subsequently vomiting, can lead to diarrhoea, and in very high amounts can lead to ulceration of the membranes in the mouth and intestinal tract.
What can be done about kidney disease?
Thankfully both types of kidney disease can be treated, however, depending on the severity of damage and when it is first picked up, the outcomes of treatment can vary.
Acute kidney injury can be life threatening, and therefore urgent treatment and stabilisation are essential for giving an animal the best chance of recovery. This treatment often entails intravenous fluids to try and flush through the kidneys to give as many nephrons the best chance of recovering as possible, anti-nausea medications, pain relief, and correction of other abnormalities that occur alongside the injury, for example high blood pressure or elevated blood potassium and phosphate levels. The prognosis of patients with acute kidney injury is very variable, and depends greatly on what the long lasting effects of the damage are, however in some cases the damage to the kidneys will be too severe and they will quickly deteriorate once treatment is stopped.
Chronic kidney disease is not an emergency that needs to be treated urgently, however the sooner treatment is started after diagnosis the better. A variety of checks will be performed when your pet is first diagnosed with chronic kidney disease to help characterise it better, looking for signs of change throughout the rest of the body. These tests will involve measuring your pet’s blood pressure, the presence of excess protein in the urine, and to look for any signs of infection that might be damaging the kidneys further. Once these tests have been performed and addressed, the main focus of treating chronic kidney disease involves reducing the workload for the kidneys and minimising further damage to them. The primary way this is achieved is by ensuring the kidneys have enough water to work effectively by encouraging your pet to drink lots of water and reducing the workload for the kidneys by using a prescription diet that is specially formulated.
The treatment of chronic kidney disease can be very complicated and depends on a variety of factors, so I have written another article to discuss this in more detail. Read more here.