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Your dog’s kidneys & how they work

Posted on by Kristina Johansen

I’ve written about kidney failure on a few occasions in the past – 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 important to your dog’s overall health?

Though they be small…

Like humans, dogs are born (in the majority of 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 microscope 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, the number of which diminish over time, something which in part can contribute to kidney problems. This is because the nephrons act as a kind of filtration system which allow the body to retain substances it needs and to remove toxins and other unnecessary substances.

…they be mighty

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 properly.

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 kidney’s work to distinguish between substances that the body needs and ones 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 which are by-products of normal cellular process like respiration and digestion. These ‘left-over’ substances can’t be used by the body (either they’re surplus or they’re toxic) and so must be removed by the kidneys. Examples of compounds that are 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 did you know that keeping your dog hydrated is a balancing act between water conservation and water removal. And the kidneys play a vital role in maintaining this delicate balancing act.

When your dog is sufficiently hydrated, their kidney’s go about their job as usual. However, in those instances when your dog is dehydrated (which could be for any number of reasons), their kidneys have to switch gear. 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 unnecessary / excess water as quickly as possible, so as to avoid dilution of the blood. In these instances, your dog’s urine will be very dilute (clear in colour).

Sodium/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:

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 out from the blood, then return what’s needed to the blood and excrete anything that’s surplus to requirements 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, it’s detected by these sensors 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. Well these electrolytes are also involved in blood pressure regulation. By regulating the levels of these electrolytes, your dog’s kidneys are also regulating their blood pressure. This is because these substances are involved in maintaining blood volume which in turn is involved in blood pressure regulation.

Calcium/Phosphorus Balance:

In a similar way to how they maintain a balance between electrolytes, the kidneys also regulate the levels of ions including calcium and phosphorus in your dog’s body. They do this through a series of complex processes involving different hormones, pathways, channels and transporters, which ultimately cumulate in the filtration of these ions out of the blood and their subsequent reabsorption or excretion.

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

Red Blood Cell Production:

It might sound odd, but 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, which is what 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 (ERO). 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 release of the hormone ERO.

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 from the blood. Doing so would be dangerous and could cause damage to your dog’s health.

For this reason, the kidneys have mechanisms in place that ensure they only filter out and expel the tiniest amounts of protein (typically excess levels or those under a certain size). These same mechanisms mean that bigger proteins (which are typically those required to carry out various bodily functions) are not filtered out by the kidneys, but instead are 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

Analysis of a generic home-prepared dog diet

Our consultations 

How much protein should you feed your dog?

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