The main job of the kidneys? Excretion.
Excretion of everything the body doesn’t need and especially soluble waste products – metabolic toxins, digestive toxins, moulds and/or mycotoxins, fungi or yeasts, endotoxins produced from pathogen bacteria in the hindgut, which as we know creates a biome dysbiosis which results in laminitis … You name it, the kidneys excrete it.
The horse’s kidneys are much more complex than the horse’s liver – the liver you can imagine being like a big sack-full of of liver cells and blood vessels – of course it’s more complicated than this but this is a good way to imagine it. The kidneys? Waaaaay more anatomically complex.
The horse has two kidneys which sit just under the last rib and before the pelvic bones on each side. Each kidney produces urine which is transported via a tube to the bladder where it’s stored before being excreted to the urethra. Urine is one of the main excretion routes in the body, alongside the fibre faeces from the GI system; the kidneys deal directly with the water-soluble waste, with fat-soluble waste needing to be transformed to water-soluble waste before heading to the kidneys.
There are only 2 organs that get a constant blood volume supply – the brain, which gets around 15% of the body’s blood volume, and the kidneys. Around 25% of the body’s blood volume is constantly pumped into the kidneys, so between the kidneys and the brain they share 40% of the whole body’s blood volume. The remaining 60% of the body’s blood volume is shifted around the body where it’s needed most, as explained earlier in The Liver section, i.e. the inner organs for digestion or the muscles during exercise. However, the kidneys get a constant 25%, so there’s a lot of blood being pumped through the kidneys at all times.
The smallest functional unit of the liver is cellular – hepatocyte cells, and each hepatocyte cell does all the jobs the liver has to do. If one of these cells dies, a neighbouring cell will multiply, so the liver is a highly regenerative unit. This is not the case for the kidneys.
Each kidney is made up of around a million extremely complex filtering units called nephrons, with each tiny nephron including a filter - the glomerulus – which filters the blood coming in, and a tubule, which removes the waste. Unlike the liver, which can regenerate a cell, if a nephron is damaged it can’t be replaced, so if we lose kidney function, the kidney’s capacity stays at the lower level of function.
The nephron is also responsible for making the urine. As the blood passes into the kidneys and filters through the nephrons, they separate out the water content in the blood which contains the nutrients and toxins.
The nutrients are then separated out to be reabsorbed – the horse’s body worked painstakingly hard to assimilate those sugars, amino acids (protein), vitamins and minerals (especially calcium as there’s usually an excess of calcium in forage and this is what makes the horse’s urine look milky/cloudly – it’s simply excess calcium being excreted) from its feed, so it doesn’t want to excrete them; only if there’s an excess in the body will they be excreted so they’ll then stay behind in the remaining water with the toxins.
This becomes the urine - in the space of just 24-hrs a horse can produce up to 550-litres! But – we all know no horse pees that much urine out, and again this is where the kidneys continue their talents. As the watery toxic waste heads out of the nephron to begin its journey to the bladder, the internal cleverness of the nephron continues to regulate and reabsorb some of the water content in the urine and passes it back into the outgoing blood. This beautifully cleaned up, purified and nutrient-loaded blood then leaves the kidneys and rejoins the blood system ready for its journey round the body again.
Lucky for us this is a process that runs automatically so we don’t really have to think about it. A healthy body will do all its own detoxing, with the liver transforming all the waste products into soluble excretable waste products, with the kidneys excreting the waste, collecting them in the bladder ready for elimination. Only if it all starts to go wrong do we see symptoms.
So back to the lovely job of excretion, and toxins don’t just come from feed and the resulting digestive disruption – they also come from the body’s metabolic processes. When there’s too much protein in the diet, this causes significant extra work for the liver and kidneys, because these proteins have to be metabolically degraded by the liver, which produces urea which is also toxic for the body; urea needs to be excreted via the urine, and quirkily, this is where the word ‘urine’ comes from.
There’s also a small production of uric acid but most of the waste product from protein metabolism is urea, so we should always keep an eye on protein levels in the diet.
Also, during spring/autumn when the horse’s coat changes, this is the time when the whole protein metabolism is renewed, where old proteins are degraded and new proteins are created, so at the time of coat change this is a taxing time for the kidneys to excrete all the excess urea created by this excess of protein metabolism. Hence why in spring and autumn it’s useful to assist the liver and kidney function to support them through each coat change.
The same applies to excess vitamins, minerals and trace elements fed to our horses, so if we feed too much salt or magnesium it’ll be naturally excreted via the urine. Vitamin C is an interesting one – many people add vit.c/ascorbic acid to their horse’s feed, especially during winter. Yet a horse naturally produces its own vitamin C in the liver, and in the active form the body recognises and knows what to do with. Adding extra ascorbic acid into the feedbowl is not only you throwing your hard-earned cash away, as being a water-soluble vitamin it heads directly to the kidneys for excretion, but also, as the saying goes, it’s a really expensive way to produce urine 😉
So much of the body’s balance relies on the kidneys as they regulate just about every physiological function in the metabolism, thanks to their ability to ‘excrete or hold back’. Probably the most vital regulation is the water:salt balance, which affects the critical cellular osmotic pressure, so, here’s the How & Why of the kidney’s critical regulation processes.
Water and salt balance
They’re completely connected. Water is essential for all living beings and the important factor is that we shouldn’t have too much or too little water in the body. Thing is, the body can’t pump water around from A to B so the body uses salts for the water to follow because salt is hygroscopic – as in it attracts moisture – so where salt goes, water follows. This connected regulation is all controlled by the kidneys by excretion – or holding back – of salts. So, the more salt the kidneys hold back, the more water remains in the body – if there’s water retention in the body, the kidneys excrete salt so water is then excreted too via the kidneys.
Hence, one of the reasons why the body needs salt, and thus why focusing on hydration is so important.
Now we move to the body’s cells – this is also where this salt:water connection is so important. The body’s cells need a distinct pressure to function, reason being that unlike plant cells which have a solid wall, mammalian cells don’t – our cells have a membrane wall around them, and in order to work they need the right water pressure inside each and every one of them. This is called osmotic pressure, and it’s regulated by – you’ve guessed it – salts and water.
So, if a cell’s intelligence knows that it’s drying out and shrinking, it takes up salts from the extracellular environment (the area outside the cell) and water follows. Equally, it can excrete salt for the water pressure to follow if it’s too full of water. The cell’s natural intelligence regulates its own osmotic balance, but there has to be enough salt and water available which is regulated by the kidneys via its natural holding back or excreting salt and/or water.
We know when this balance breaks down by typical symptoms of unexpected diarrhea, or when a dog vomits for days on end – this usually means that the salt/water regulation is significantly out of balance and the body dehydrates. This is also why diarrhea in young foals is extremely serious because the body is so small, so they dehydrate quickly, so adding more salt/electrolytes to the feed is vital.
When we think about blood pressure we usually think about the heart, which is fair enough as the heart is the pump and the heart pumps the blood into the blood vessel system.
However, the blood pressure is regulated by how wide the blood vessels are – the wider the vessels, the less pressure we have; the narrower they are, the higher the pressure. Same as our yard hose - if we’re washing down our horse with the yard hose and there’s some stubborn mud, we squeeze the end of the hose between our fingers to get a higher pressure by dilating the ‘vessel’.
Hence the heart:kidneys connection – the kidneys need a healthy, regulated pressure to perform the urine excretion; it’s the kidneys that regulate how wide the blood vessels are and therefore the blood pressure of the horse.
As we know, BP is a big issue in humans yet in horses it’s overlooked, probably because it’s assumed that it all works just fine. However, mild colic is one such symptom when the weather changes, especially during a hot humid summer – this is a blood pressure issue, indicating a potential heart and kidney issue.
There’s also a unique, reciprocal association with the endocrine (hormone) system, and specifically with the adrenal gland. The adrenal gland hormones regulate the various tasks of the kidneys, which in turn regulate the hormones. This is especially noticed in the case of Cushing’s horses who will have significant kidney disfunction alongside an overproduction of cortisol hormone.
Addressing kidney function is crucial for our Cushing’s horses to rebalance the relationship between the two, instead of simply relying on Prascend which, like most pharmaceuticals, simply suppresses the symptoms caused by the imbalances in the metabolism.
And as if this isn't critical enough, the kidneys also manage production of Vit. D from the sun's UVA rays, which is essential for bone and muscle health.
Again, these are all processes that run automatically so we don’t really have to think about it. Only if this process is disturbed then we start seeing symptoms – blood works will show changing parameters, but if the kidney function is depleted this can be disastrous for the horse as the kidneys can’t regenerate themselves. Hence it’s always best to spot the signs and act on them immediately, with the skin, hair and hooves being early beacons.
The skin is the emergency kidney of the body; if the kidneys can’t excrete enough, the body uses the skin, despite it not being ideally suitable, but better to have an itchy skin that to die from toxification. Diminished kidney function = diminished immune function.
... and this is a Big One. The primary issue with sweet itch is not a skin issue - it’s a gut issue relating to dysbiosis in the hind gut. The microbiome is massively disturbed, with extensive inflammation in the gut walls which leads to a high histamine level. This triggers a hyper immune reaction making it extremely sensitive to allergic reactions, which means a huge increase in toxins.
The kidneys now have to excrete so many extra toxins that they become overloaded and unable to excrete everything, so the kidneys use the emergency detox pathway – the skin. Cue an excess of toxin excretion via the skin surface, which causes the skin surface to itch like crazy, especially during summer when there’s additional insect bites. So, not only should we cover the skin but also seriously address kidney and liver health.
If it’s obvious that there’s itchy skin, we should also be asking the question, Why are the kidneys not working properly? Working upstream, if the skin has a problem, the kidneys have a problem, which means the liver is overburdened, which is going to affect the biotransformation process of the toxins, so the kidneys can’t excrete them. This takes us further upstream to the gut system, and here’s where it all starts again – a gut imbalance. Too many toxins coming in, with not enough important nutrients, so the liver and kidneys can’t do their job. So, the body uses the skin as an emergency pathway for excretion, and herein lies the classic symptoms for sweet itch - and also mallenders – all typical symptoms when the kidneys aren’t working normally anymore.
Sadly, all this started as a manmade issue because we started feeding our horses differently to how evolution created the horse to adapt perfectly to its environment. Then 50-years ago the advent of intensive farming practices changed everything – overnight us humans thought we knew better than nature, and switched everything around from previously large to limited amounts of dried forage, and unnaturally large amounts of weird concentrates, all absolutely not suitable for the ecology of our horses’ gut systems.
Long term this has led to all these metabolic conditions, and they always start with these early markers we see today in our horses.