Your dog’s kidneys and how they work

I’ve written about kidney failure in dogs many times before—what it is and how it can be treated.

But what I haven’t spoken about is what the kidneys are. What exactly do these bean-shaped organs do, and why is keeping them healthy so crucial to your dog’s overall health?

Small but mighty

Like humans, dogs are born (in most cases) with two kidneys. They’re situated – one on either side – at the back of your dog’s body in the abdominal cavity just under the backbone, close to where the spine and last rib meet.

Shaped like their namesake beans, each kidney is not much bigger than 3-5.3 cm for small dogs, up to 6.6 – 9.3 cm for large dogs, and is comprised of microscopic structures that help it carry out its plethora of functions. The most important of these is the nephron. This tiny structure is itself comprised of a renal corpuscle and a renal tubule, all of which work together to help the kidneys perform their many tasks.

At birth, each kidney contains about 1 million nephrons. This number diminishes over time, something which in part can contribute to kidney problems. This is because the nephrons act as a filtration system, allowing the body to retain substances it needs and remove toxins and other unnecessary substances.

Despite their relatively small size, kidneys carry out a whole range of functions that are vital to keeping your dog healthy, indeed alive. This is epitomised by how ill dogs can become when they stop working correctly.

While many people have some idea of the role of the kidneys, not many know the full extent of what these little organs are capable of.

Toxin removal

The kidney function that you’re likely to have heard of is toxin removal. Using their internal filtration system, the kidneys work to distinguish between substances the body needs and those it doesn’t, and to aid the expulsion of these substances from the body in the form of urine.

Urine is typically comprised of water, metabolic waste and toxins.

Metabolic waste is the broad term given to compounds and substances that are by-products of normal cellular processes, such as respiration and digestion. The body can’t use these ‘left-over’ substances (either they’re surplus or they’re toxic) and so must be removed by the kidneys. Examples of compounds considered metabolic waste include nitrogen, carbon dioxide, and sulphates.

While metabolic waste isn’t necessarily poisonous, toxins (which are a type of metabolic waste) are. These compounds are produced within and by living cells/organisms and can cause a lot of harm if allowed to accumulate within the body or if they are absorbed by bodily tissues. For this reason, the kidneys work to identify these toxins and expunge them from the body. Examples of toxins include peptides and small molecules.

Water conservation

It’s a simple fact that your dog needs water to survive. But keeping your dog hydrated is a balancing act between water conservation and water removal. And the kidneys play a key role in maintaining this delicate balancing act.

When your dog is sufficiently hydrated, their kidneys go about their job as usual. However, when your dog is dehydrated (which could be for any number of reasons), their kidneys have to switch gears. They need to work to retain and conserve as much water as possible while still removing metabolic waste and toxins. They do this by mixing these toxins and waste with as little water as possible, which is why concentrated urine (yellow in colour) could be a sign that your dog is dehydrated.

Conversely, if your dog drinks a lot of water, the kidneys will work to remove the excess water as quickly as possible to avoid dilution of the blood. In these instances, your dog’s urine will be very dilute (clear in colour).

Sodium and potassium balance

Among the various complex compounds that need to be regulated to keep your dog healthy are a group of substances known as electrolytes. Sodium and potassium are among the more well-known electrolytes, and they’re important because when dissolved in blood, they help:

• Regulate muscle and nerve function
• Maintain acid-base levels
• Maintain normal fluid levels in fluid compartments

It’s unsurprising, therefore, that a change in the levels of these electrolytes – and indeed the balance between them – can lead to damage and health problems for your dog. This is where their kidneys come in. In a series of complex steps involving sensors and regulators, these small organs filter water and electrolytes from the blood, then return what’s needed to the blood and excrete any surplus via urine.

In this way, the kidneys help maintain sodium and potassium levels, the correct balance between them, and the levels of other electrolytes.

Blood pressure regulation

One of the lesser-known functions of the kidneys is their involvement in blood pressure regulation. As a reminder, blood pressure is the pressure required for the arteries in your dog’s heart to carry blood from the heart to the rest of the body. And there’s a healthy range within which your dog’s blood pressure should always be.

There are two ways in which the kidneys keep your dog’s blood pressure in check:

Hormonally

Within their complex structure, the kidneys also contain sensors which monitor blood volume as it passes through the kidneys. When these volumes are low, these sensors detect it, which in turn stimulates the release of a hormone called renin. When this happens, it triggers a cascade of events that stimulates the kidneys to retain sodium and water, and blood vessels to constrict, all of which raises blood pressure.

Maintenance of the fluid circulation

We’ve already spoken about how the kidneys help regulate sodium and potassium levels. These electrolytes also play an important role in controlling blood pressure. By managing their levels, your dog’s kidneys help regulate blood pressure too, since sodium and potassium influence blood volume—and blood volume directly affects blood pressure.

Calcium and phosphorus balance

In a similar way to how they balance electrolytes, the kidneys also regulate the levels of ions, such as calcium and phosphorus, in your dog’s body. They do this through a series of complex processes involving various hormones, pathways, channels, and transporters, which ultimately result in these ions being filtered from the blood and then either reabsorbed or excreted.

It’s essential that the kidneys regulate calcium and phosphorus levels accurately, as an imbalance in their levels can lead to various disorders, including skeletal problems, seizures, arrhythmias and respiratory problems.

Red blood cell production

The kidneys also play a big role in the production of red blood cells. These round, flat cells are made in the bone marrow inside your dog’s bones and contain a protein called haemoglobin. It is this protein, inside red blood cells, that allows them to carry fresh oxygen around the body.

So, if these important cells are made in the bone marrow, what do the kidneys have to do with their production? Put simply, the kidneys help regulate when red blood cells should be produced. They do so by producing a hormone called erythropoietin (EPO). When it’s produced and released from the kidneys into the blood, ERO in turn prompts and stimulates the bone marrow to produce new red blood cells.

This means that while the kidneys aren’t directly involved in actually producing red blood cells, they indirectly stimulate their production in the bone marrow through the release of the hormone EPO.

Protein conservation

Proteins are the building blocks of life, and as such, are very important for your dog’s overall health. This means that when filtering your dog’s blood, the kidneys do not want to remove all the proteins. Doing so would be dangerous and could cause damage to your dog’s health.

For this reason, the kidneys have mechanisms that ensure they filter out and expel only the tiniest amounts of protein (typically excess levels or those below a certain size). These same mechanisms mean that larger proteins (which are typically those required to carry out various bodily functions) are not filtered out by the kidneys, but are instead reabsorbed into the blood, where they’re needed.

The take-home message

Your dog’s kidneys are vital to their survival in lots of different ways, some more obvious than others. That’s why it’s important to ensure you feed them a diet that helps to keep their kidneys working at an optimal, healthy level. Doing so will help ensure your dog remains happy and healthy.

Learn more

Diet for Dogs with Kidney Disease

FAQs on Kidney Disease in Dogs

How much protein should you feed your dog?