The kidney: evolutionary marvel

Kidney stones appear at all ages. They are common in older people, but also in the middle aged. They are seen in infants and toddlers, but also in teens and young adults. About 80% of them are calcium stones, 10% struvite stones (from urinary tract infections), and 10% crystallised uric acid, but uric acid ‘seeds’ also promote the formation of calcium stones. That this is so naturally implies that chronic kidney dysfunction must also be common.

Pain associated with a kidney stone can be sharp or dull, mostly depending on the size of the stone either partially blocking or passing through a calix in the kidney or the ureter from the kidney to the bladder, and usually expresses itself as pain in the back or side (easily mistaken for muscular strain), in the abdominal area (easily mistaken for indigestion) or in the groin above which sits the bladder. That such a pain should appear and persist when there are no reasons to suspect either muscle soreness or indigestion indicates that the problem may well be with one or both of the kidneys.

We take almost everything for granted. That we should have air that is not toxic to breathe, water that is not polluted to drink, food that is not contaminated to eat. That we should have a comfortable and warm place to live and work, hot water to shower and bathe whenever we wish, running water wherever we find ourselves. That there should be living plants, insects and animals; soils in which can be planted seeds that will grow; rivers, lakes, seas and oceans in which fish can live, thrive and multiply; mountains, forests and plains in which trees, bushes and grass, beasts, birds and bugs, and every living thing can also not just survive, but thrive. We take these for granted, maybe all the time, and if not, probably most of the time. It is, unfortunately, more than obvious that we should not.

That we take almost everything for granted is even more remarkable when we consider this bodymind (that we customarily and mistakenly call ours), with its countless numbers of specialised cells and tissues, its amazingly intricate organs and systems, and its multitude of facets and functions. What happens when we breathe in, and then when we breathe out? What happens when we drink a glass of water or when instead we drink a glass of juice? What happens when we drink a glass of Coke or a glass of wine? What happens when we eat something: when we eat a an apple or a cucumber, a carrot or a celery stick, a potato or an avocado; when we eat an almond or a walnut, pumpkin or sunflower seeds; when we eat meat or fish, eggs or cheese, olive oil, fresh butter or coconut oil; and what happens when we eat burgers and fries, doughnuts, cookies, cake and candy? What happens in the stomach, in the pancreas, in the liver, in the gall bladder, in the small intestine and in the colon? What happens during the process of digestion? How does digestion take place? What happens in the kidneys? What happens in the bloodstream? What happens in the brain?

Most of us have no idea. But we should, should we not? We take it all for granted: that everything will just work; everything will take care of itself; the body will take care of us. Although this can happen, sometimes, in general it doesn’t. But it should, shouldn’t it? Why does it escape us so thoroughly that this bodymind—every single cell in it—is entirely made from what we eat, drink and breathe? It is so obvious and yet it eludes us. And so, we must consciously come back to this again and again.

When we begin to explore the physiology of the body to find out how things work, we find that both the complexity with which we can appreciate, and the understanding of the various functions and interactions, arrange themselves in layers from coarse and superficial to more subtle and profound. Inevitably, as appreciation and understanding deepen, it becomes impossible to find all of it anything less than amazing. And although this can be said for many, maybe even for all organs, it is particularly true in this case: the kidney is an evolutionary marvel, a true jewel of physiological evolution in animals.

The kidney is without any doubt one of, if not the most refined organ both in architecture and function. To pack together so many tiny, delicate structures, working both independently and in unison in an array of such intricate, complex and subtle functions and interactions is truly mind boggling and awe inspiring. This fact is totally under-appreciated. And for this very reason, I feel it extremely important to raise the point now and bring it to your attention before moving on, so that it can remain clear throughout your reading of this article. I hope that with an understanding of what the kidneys do, how they function and what they need, this appreciation will become permanent for you, coming up on its own every time you drink a glass of water, and also every time you remember that you should have.

What we need to know

The kidneys are two bean shaped organs typically 11 cm in height, 6 cm across and 3 cm thick, on top of which sit the suprarenal (as in: above-the-kidney) or adrenal glands. They are located deep in the abdomen close to the spine, one on either side, in the area of the lower back, just below the rib cage, protected in part by the last couple of ribs but mostly by the tick muscles of the back. The kidney has four main components: a thin layer that covers it like a thick skin called the capsule; a thicker layer just beneath the capsule called the cortex (outer layer), in which are most of the arteries and veins; the inner layer called the medulla (middle layer) constituted by conical structures called the pyramids (there are usually 7 of them in humans) with their wide part or base in the cortex and their tips pointing inwards towards the innermost  part of the kidney; and finally the pelvis (base) with its calyces connecting to the ureter.


As for everything that relates to health, understanding how to promote optimal function of a cell, tissue, organ or system requires understanding how it works. It is important to remember that every living cell and organelle does what it does not for our sake, but to maximise its own prospects for survival. When we understand what an organ is trying to do, then we can understand what is needed to make sure that it can do it with ease and efficiency. And when the organ functions with ease and efficiency, it functions optimally. This is the approach to use to maximise our prospects for living a long, healthy and happy life.

So, what is the kidney trying to do?

One: Take out of the blood metabolic wastes and toxins, primarily urea, uric acid and creatinine, all resulting from protein metabolism, while keeping as much as possible of the useful stuff, especially water, minerals and amino acids. Two: Maintain blood electrolyte balance (sodium, chloride and potassium; calcium, magnesium and phosphate), pH (bicarbonate and hydrogen) and osmolarity (concentration of solutes in general). Three: Regulate body fluid content and blood volume and pressure. Sodium is the most important electrolyte and blood pressure regulator, and therefore most closely monitored by the kidney.

What are the main metabolic waste products?

Urea results primarily from the breakdown (oxidation) of amino acids that are not used to build tissue, i.e., protein intake in excess of what can be used at any given time to build and repair cells, (but also from our own tissues). Urea also result from the conversion of ammonia, another byproduct of protein digestion which is so acidic that in high concentration it can cause cell death. The kidney, therefore, tries to eliminate as much as possible of the urea, recycling only what it must depending on the body’s needs, especially to increase water re-absorption when there is dehydration.

Uric acid comes from the breakdown of purines. Some are present in our own cells, and so the natural recycling of the components of dead ones produces uric acid on a more or less continual basis and at a more or less elevated rate depending on how quickly cells are dying (the rate of ageing). Purines are also present in foods we eat and drink: mostly protein-rich foods and alcohol containing drinks like wine and beer. The more purines are present, the more uric acid is produced. All the uric acid needs to be eliminated. When the urine is too concentrated and acidic, however, uric acid cannot be dissolved and thus crystallises.

Creatinine is a breakdown by-product of creatine phosphate, an energy storage molecule used mostly in cells with fluctuating energy needs like those in the muscles and brain. Creatine is made from three amino acids in two steps: the kidney combines the arginine and glycine, and then the liver binds on methionine. Creatine is then transported in the bloodstream to muscles where it is made into creatine phosphate and back to creatine as needed. In the first few seconds of an intense muscular effort or brain activity, creatine phosphate can lend a phosphate group to ADP (adenosine di-phosphate) to form ATP (adenosine tri-phosphate, the energy currency of cells), and help supply the needed energy. Very conveniently, if later there is extra ATP floating around not being used, creatine will take back a phosphate group from the ATP molecule, leaving the latter as ADP, and storing the former for future needs as creatine phosphate once more. Creatine is eventually broken down to creatinine and must be completely eliminated by the kidneys. The need for and use of creatine phosphate depends primarily on muscle mass and level of activity.  Therefore, so does production of creatinine.

How does the kidney do what it does?

By filtering the blood. And the kidneys filter a lot of blood. About 25% of all the blood coming out of the heart flows through them. This is on average 1.2 litres per minute, which amounts to more than 1700 litres per day! Since there are 4-5 litres of blood in the body, it means that every drop goes through the kidneys about 400 times each day! Since the overall flow and pressure of the system must be maintained, only around 20% of the blood flowing through the kidney is filtered (that’s 240 ml/min and 340 l/day). The renal artery supplies the blood, and branches out into smaller arteries that also branch out into smaller arterioles all the way to the filtering unit. Because half of the blood volume is water, this amounts to 850 (1700/2) litres per day flowing through the kidneys. Filtering 20% means that 170 litres of water are filtered each day. Therefore, if one litre of urine is produced and excreted over the course of 24 hours (that’s pretty typical, unfortunately), it means that 169 out of 170 of these litres of water are reabsorbed: a reabsorption efficiency of 99.4% (169/170)! Producing two litres of urine eases this down to an efficiency of merely 98.8% (168/170). Now, that’s what we call high running efficiency.

But what does ‘filtering the blood’ actually mean and how is this done exactly? In each kidney there are about 1 million miniature filters called nephrons; they run from the lower part of the cortex deep into the pyramids. It is in the nephron that the blood is filtered and the urine produced in five main stages, first through Bowman’s capsule (1) and into the proximal convoluted tubule (2), then along the loop of Henle (3) and into the distal convoluted tubule (4), and finally out through the collecting duct (5) and into the ureter to the bladder. The filtrate and the concentrated blood course separately through the nephron only once on a one-way trip through the interstitial medium in which it is embedded in distinct but intertwined vessels. Along this winding course take place the delicate regulation of blood pressure, the filtration, the reabsorption of water and useful substances, the concentration of wastes into the filtrate that will become urine, and the regulation of water content and electrolyte balance. Here’s a description of how it works:

Stage one: Bowman’s Capsule    The blood coming into the nephron first enters a little spheroidal structure 0.3 mm in diameter (Bowman’s capsule) where about 20% of it is mechanically filtered to separate the fluid part called the plasma from the solids. It is ‘mechanical’ in the sense that it is pressure driven and based on particle size: smaller stuff like water, minerals, glucose and amino acids, together with the metabolic waste like urea and uric acid pass through, whereas large stuff like blood cells, proteins and fats do not. This is similar to how a water filter works: the water goes through the porous but densely packed carbon or ceramic block that stops most of the large particles like chemicals and metals, but allows the water to pass. And just as the filtering efficiency of a given filter depends on the pressure of the water supply, the filtering through the glomerulus in Bowman’s capsule depends intimately on the pressure of the blood supply. If the pressure is too low, the filtering is inefficient. But if the pressure is too high the delicate filtering structures are damaged. The pressure must therefore be just right for the circumstances, (the conditions being obviously very different when we are running and when we are sleeping).

Stage two: The Proximal Convoluted Tubule    The fluid moves from the capsule into the proximal (as in: close-by) tubule. The blood moves from the larger afferent (as in: towards) arteriole where the pressure is monitored before entering Bowman’s capsule, into the smaller efferent (as in: away-from) arteriole after passing through the glomerulus. It is now much thicker and more concentrated. Here, most of the water (about 65%) and almost all sodium are reabsorbed from the filtrate back into the blood, in addition to all of the glucose and amino acids, (none should end up in the urine), and some urea. If the pressure is even slightly lower than it should, the juxtaglomerular (as in: next-to-the-glomerulus) pressure-sensing cells in the afferent and efferent arterioles, secrete renin that flows into the bloodstream, and stimulates the release of angiotensin I from the liver, which is then converted in the lungs to angiotensin II, a powerful vasoconstrictor that promotes the contraction of the blood vessels to raise blood pressure, but also triggers the secretion of aldosterone in the adrenal glands, which in turn stimulates more reabsorption of water and salt in the nephron, also for the purpose of raising blood volume and pressure.

Stage three: The Loop of Henle    Most of the water and salt, and all the organic molecules like glucose and amino acids are reabsorbed from the filtrate back into the blood through a network of tiny blood vessels (capillaries) in the first part of the proximal convoluted tubule, straight after its emerging from Bowman’s capsule. From there, the vessel changes in shape and direction, and becomes what is named the Loop of Henle: a crucial element of the nephron that has a water-permeable descending limb and a water-impermeable ascending limb. As the filtrate travels down, water moves out because of the higher concentration of sodium in the embedding interstitial medium, and is reabsorbed by tiny capillaries back into the blood. The deeper it descends, the higher the sodium concentration grows, the more water comes out of the filtrate, and thus the more concentrated it becomes. As the concentrated filtrate travels back up along the ascending limb of the loop, it is sodium that is now pulled out, but this time by active transport through little pumps instead of by osmosis as for the water in the descending limb. This is necessary to recover as much sodium as possible and maintain the gradient of concentration of the interstitial medium in which the loop of Henle is embedded.

Stage four: The Distal Convoluted Tubule   The next leg of the trip—a very important one indeed—is through the distal (as in: distant) tubule. It is here that pH and electrolyte levels are regulated. It is also here that we find the chemo-sensing macula densa cells tucked in between the afferent and efferent arterioles, next to their pressure-sensing juxtaglomerular cells. Blood pH is regulated by either absorbing bicarbonate and secreting protons to increase acidity, or vice versa, (without a doubt the much more common alternative), by secreting bicarbonate and absorbing protons to make the blood more alkaline.  Sodium can be left to be excreted or it can be reabsorbed and potassium secreted into the bloodstream under the influence of the hormone aldosterone, and calcium can also be excreted or reabsorbed but in this case under the influence of parathyroid hormone or PTH.

Stage five: The Collecting Duct   The distal convoluted tubule is endowed with a system of collecting tubules to which is delivered the filtrate, (now practically urine), and that merge into the main collecting duct that carries the liquid to the ureter into the bladder. On this final stretch in the collecting duct through the interstitial medium of the nephron, a little more water can be squeeze out of the already concentrated urine. This, however, only happens in the presence of the very important hormone vasopressin (also called anti diuretic hormone or ADH), which is secreted when the body is dehydrated.

This amazing process takes place in millions of nephrons tightly packed and organised in each of the two kidneys, continuously throughout the day and night, from the moment the kidney starts to work in the not yet born child, to the moment we die, either from kidney failure or something else. And to appreciate just how amazing it really is, consider this back-of-the-envelope calculation: 1 million nephrons are packed into 7 pyramids makes about 150 000 per pyramid. Taking a pyramid to be a cone with a base of 2 cm in diameter gives a surface area for the base of about 3 cm squared (Pi*R^2, and R=1). Dividing 150 000 nephrons by this surface area in which all of them must be packed gives a density of 50 000 nephrons per squared cm. Since there are 100 squared mm in 1 squared cm, this makes a density of 500 nephrons in every square mm over the surface of the base of each pyramid, and remember that they must all squeeze in together even more as they penetrate towards the tip of the pyramid and its collecting calyx. Can you even imagine how small this is, without even considering the incredible complexity with which it all works? Gray’s Anatomy states that the thin part of the Loop of Henle is 30 microns in diameter, whereas its thick part is 60 microns, and it is safe to assume that most tubular parts of the nephron are probably also in this range. This is truly amazing. But appreciating this, we can also appreciate how incredibly fragile each nephron must be. And by the way, once a nephron is dead, it’s dead forever.

Now, blood pressure is intimately related to blood volume (amount of water in it) and blood osmolarity (the concentration of solutes, mostly sodium, and to a lesser extent the other electrolytes as well as glucose). Maintaining these in balance is essential to the functioning of everything in the body. For this reason, there are pressure sensors throughout every blood vessel, and osmolarity sensors in the hypothalamus of the brain, as well as highly sensitive sensors of both kinds in the kidney itself. A drop in volume sensed by the pressure sensors in the blood vessels, or a rise in solute concentration sensed in the hypothalamus, will trigger the release of vasopressin from the pituitary gland. Vasopressin will signal the kidney (the collecting duct) to release more water for reabsorption into the blood stream, in order to counter the drop in blood volume and rise in solute concentration. Vasopressin, just as angiotensin, will make the blood vessels constrict and tighten to maintain the blood pressure constant. It will also stimulate the secretion of glucose from the liver in case fast reaction times become necessary, as well as clotting factors and platelets to make the blood thicker and stickier, and prevent excessive blood losses in case of injury. All of these are part of the standard stress response. Vasopressin will also stimulate the secretion of the stress-induced adrenocorticotropic hormone or ACTH that will act to reinforce all of the above in what will amount to a heightened stress response.

Dehydration—especially chronic dehydration—is probably the greatest source of physical stress in most of us. We, unfortunately, tend to live our lives completely oblivious to this fact, and therefore suffer the consequences a little more acutely with each day that passes.

What we need to do

Although all of this is in many ways awfully complicated, what we need to do to make sure the kidneys function properly is quite simple: drink more water, take more magnesium and less calcium, alkalise the body and its tissues.

More water   This is by far the most important: proper hydration by drinking plenty of water—not fluids in general, just plain water—especially in the morning when the body is most dehydrated, before eating anything, and then before each meal.

Imagine what would happen to a water filter if the incoming water were just slightly cloudy with dissolved clay particles? It would work, but over time, (obviously faster than it would in the absence of clay), it would get clogged up. Now, what if there were more fine clay particles? The filter would get clogged up faster given that its role is to stop and store the particles so that the water coming out can be clean and clear. But in addition to that, because the incoming water would necessarily be thicker and more viscous, the filter would not work as well under the same pressure. To work properly it would need a higher pressure to help push through the more viscous water, but this higher pressure (if it could be adjusted upwards) would inevitably stress the filtration system as a whole and thus shorten its ‘life’. What if, in the extreme, the incoming water were really thick, brown and muddy? It’s pretty simple: no water would make it out of the filter because it would instantly clog up.

This analogy is definitely not exact but it is clear and adequately illustrative. To function well, the kidney needs the right blood pressure, blood flow, blood volume, blood viscosity and osmolarity (concentration). As soon as either pressure, volume or sodium concentration drops, the renin-angiotensin-aldosterone is activated and reinforced by the stress response related to secretion of vasopressin (anti diuretic hormone), all acting to constrict the blood vessels, make the blood more viscous and increase reabsorption of both water and sodium to re-establish a functional equilibrium. Imagine now this thick, viscous, sticky blood going through the exceedingly fine arterioles and capillaries in the nephron, and the difficulty with which wastes would be filtered out and dissolved in the water that should be available but isn’t. Now, picture this happening throughout the 24 hours of the day, week after week and year after year. It’s no wonder kidney problems are so common!

So, at the very least we should drink one litre before breakfast and 500 ml before each of the other two meals, allowing each time 30 minutes for the water to be absorbed into the digestive system and then into the blood before eating. It is better to drink more than this, always on an empty stomach, and to take enough unrefined sea salt to match our water intake. Doing this is already enough to ensure proper kidney function and elimination of the bulk of the metabolic wastes through the urine, preventing in this way the formation of kidney stones.

More magnesium and less calcium   The formation of calcium stones is more than obviously related to the fact that we are all in general over-calcified, consuming way more calcium than the magnesium needed to keep that calcium dissolved and flowing instead of settling and crystallising in our tissues, blood vessels, joints and kidneys. Therefore, to avoid calcification we must avoid over-consuming calcium (in fact, minimising calcium intake), and we must supplement with magnesium. This will also, over time, dissolve existing calcium stones and deposits in arteries and other tissues throughout the body.

More alkaline and less acidic   The kidney’s main purpose is to excrete acidic wastes by dissolving them in water. But all digestive and metabolic wastes are acidic, and there are many sources and forms of acid wastes that all contribute to increase the overall acid load on the body. In particular, refined sugars and protein. The heavier the load, the more acidic the blood becomes. Since the blood must remain alkaline, the acid can be eliminated, neutralised or stored in tissues. All three lines of defence are used: the kidneys try to eliminate as much as possible, alkaline minerals like calcium, magnesium and potassium are pulled out of the bones to neutralise blood acidity, and excess acid is stored away in tissues. Everything is done to take it out of circulation. The more acid is stored, the more acidic the tissues become. And the more acidic the body is, the less is its alkalising potential and the harder it is for the kidneys to dissolve and eliminate the acid that should be eliminated on a continual basis. There are fundamental physiological arguments that explain how tissue acidosis is at the root of literally every health problem and disease, (I will write about this more specifically on other occasions), but even without any further considerations, the only sensible conclusion is that the less acid-forming foods and drinks we ingest, the healthier the tissues, the kidneys and the body will be.

The most strongly acid-forming foods are refined sugars. Next are meats, eggs and milk products, then flours, grains and starches. The most strongly alkaline-forming (acid-neutralising) foods are raw and green vegetables, especially salads and leafy greens, as well as watery vegetables like cucumbers and celery. The more chlorophyl, the more alkalising. Parsley, basil, cilantro and all grasses are therefore alkalising and cleansing superstars.

Looking beyond single foods we find that certain combinations make the results indigestible and thus promoting of either putrefaction (protein with sugars or starches) or fermentation (simple sugars with most everything else). Both of these lead to the formation of a lot more acid waste in the digestive system a great part of which ends up the bloodstream. Adopting an alkaline diet will very quickly help balance blood pH and promote maximum excretion of acid wastes. Over time, this will allow the body to not only recover proper digestion and elimination on a meal-per-meal and daily basis, but also to eliminate acidic wastes stored in our tissues throughout the body, thus ridding it of aches and pains, the potential for chronic inflammation or infection, as well as for more serious degenerative diseases like arthritis, cancer and multiple sclerosis, for example.

Last words

And finally, to stop taking so many things for granted is simple. We just need to pay attention to the details of our life and allow ourselves to be surprised, intrigued, inspired and amazed by what we encounter. Nothing more. We need to open to how things present themselves and just feel sensations with the actual feeling of the hands and fingers, of the feet and toes, of the belly, the chest, the back and neck. Really feel what is felt: the glass in the hand, the water in the mouth and then flowing in the throat and into the stomach. Actually see what the eyes are seeing: not things but forms and colours, light and dark, space and expansiveness in all directions. Actually hear what is heard in the whole space of hearing. This is how we can stop taking things for granted. Just paying attention to our life with our life. That’s all.

(If you want to read more about water, salt and kidney function you can read How much salt or how much water? For more information about the importance of water in digestion and health read Why we should drink water before meals and Water, ageing and disease. For more on calcification, the importance of minerals in general and magnesium in particular, you can read Minerals and bones, calcium and heart attacks and Why you should start taking magnesium today. For more on the importance of proper hydration in treating chronic inflammation read Treating arthritis I: super-hydration, alkalisation and magnesium.)

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How much salt, how much water, and our amazing kidneys

Salt, the one we put on food, is composed almost exclusively of sodium chloride (NaCl) that very easily dissolves in water into positively charged sodium (Na+) and negatively charged chloride (Cl-) ions. And there is something very special and unique about these ions: in our blood, Na+ and Cl- are present in the highest concentrations and maintained in the narrowest of ranges. This is very revealing, and means, quite plainly, that sodium and chloride are the most important  extracellular electrolytes. This is a simple fact. Now, forget everything you’ve heard, been told, or read about salt being bad for you, and consider this:

Our blood is made of red blood cells (45%) and white blood cells and platelets (0.7%) floating in blood plasma (54.3%). Blood plasma shuttles nutrients to cells around the body and transports wastes out. It consists of 92% water, 8% specialised mostly transporter proteins, and trace amounts of solutes (things dissolved or floating in it). And although circulating in trace amounts, the solutes—especially sodium—are vital. The concentration of solutes in blood plasma is around 300 mmol/l (don’t worry about the units for now). In the highest concentration of all is sodium at 140 mmol/l. In the second highest concentration of all is chloride at 100 mmol/l. The sum of these is 240 mmol/l. So, from these numbers alone, we see that blood plasma is more or less just salty water.

Don’t you find this amazing? Don’t you find it amazing that nobody has ever told you this straight out in this way? And isn’t it amazing that we have been and continue to be told to avoid eating salt because it is bad for us: that it causes hypertension that predisposes us to heart disease? It really is completely amazing and ridiculous and also rather sad. But misunderstandings of this kind are unfortunately much more common than they should, as you may remember from What about cholesterol and Six eggs per day for six days: cholesterol?, but also from Minerals and bones, calcium and heart attacks and A diabetic’s meal on Air France. As you will understand for yourself in a few moments, the problem is not too much salt; the problem is not enough water:

Hypertension is not caused by excessive salt consumption. It is caused primarily by chronic dehydration and magnesium deficiency.

Taking a look at the other electrolytes, bicarbonate (HCO3-), the primary pH regulator, is the third most highly concentrated molecule in plasma at 20 mmol/l. Potassium (K+) is the fourth at 4-5 mmol/l, then calcium (Ca 2+) and magnesium (Mg 2+) both at about 1 mmol/l. Therefore, the concentration of sodium in the blood is 7 times higher than that of bicarbonate, 40 times higher than that of potassium, and about 140 times higher than that of calcium and magnesium. And as with everything else in our body’s exquisite physiology, there are very good reasons for this:

Every cell in every tissue and in every organ of our body relies on an electrical potential difference between the fluid inside the cell membrane and the fluid outside of it in order to function: produce energy and transport things in and out. This is particularly important in active “electrical” tissues such as muscles and nerves, including neurones, that simply cannot work—cannot contract and relax in the case of muscle fibres, and cannot fire off electrical pulses in the case of nerve fibres and neurones—without a well-maintained and stable potential across the cellular membrane.

This resting potential across the membrane results from the delicate balance of the equilibrium potential and relative permeability through the cellular membrane of the three most important ions: Na+, K+ and Cl-. The potential is maintained by the sodium-potassium pump: a specialised protein structure in the membrane that ensures the concentration of potassium (K+) stays low outside the cell and high inside the cell, and conversely, the concentration of sodium (Na+) stays high outside the cell and low inside. This is the main reason sodium is so important and why it is so carefully monitored and scrupulously reabsorbed by the kidneys, but there are plenty more.

Obviously, this is not an accident. Nothing about the way our body functions is an accident, and no matter how well a particular physiological function or mechanism is understood or not, we can be confident that it is as perfect and finely tuned as it can be because each and every bodily function is the result of adaptations and refinements over billions of years of evolution. This is not a typo: I really did mean to write billions of years. Because every single cell of which we are made has evolved from all of its predecessors as far back as the very first organic molecules that eventually organised in the very first cell: a group of more or less self-organising organelles that developed a symbiotic relationship with one another just because it benefitted them in some way, and found it safer to cluster together behind a fatty membrane through which they could interact with the outside on their own terms.

The aim of every single self-organising entity, from the simplest virus, bacterium or organelle like the mitochondria (our cellular energy-production furnaces), to highly specialised cells in the brain, in the liver or lining a part of the microscopic nephron tubule of one of the millions of these specialised filtering units in our kidneys, to largest groupings of cells in tissues, organs and systems of organs, has always been and always will be the same: survival. Therefore, to understand living systems objectively we have to understand them from the fundamental perspective of the cell itself, the tissue, the organ and the system of organs itself because every adaptation it undergoes is always aimed at improving its own odds of survival. It is very important to keep this in mind and know that everything that happens in a living system always does so in relation to something else and always for good reason, even when we don’t understand the reason, which in itself is also very important to remember.

I use this opportunity to whole-heartedly recommend Lewis Dartnell’s book Life in the universe. Almost every page for me was a delightful discovery of things I was unaware of and found the book truly illuminating.

Coming back to salt, even though we look mostly at sodium and chloride that are the principal constituents of any kind of salt we put on our food, I very strongly recommend always and exclusively using a real salt: any kind of unrefined sea salt (French, cold water, Atlantic salt is particularly clean and rich in trace minerals), Himalayan salt, Smart Salt or Real Salt (the last two are registered trade marks and very rich in trace minerals). On the contrary, I strongly discourage eating chemically manufactured table salt or even refined sea salt, which are not only stripped of trace minerals found in natural, unrefined salts, but contain varying amounts of chemical additives such as whitening agents, for instance.

Now, without regard for polemical disputes, pseudo-scientific discussions and debates, or otherwise unfounded views and opinions about salt, can we answer the simple question: how much salt should we generally eat? I believe we can, but although it may seem so, it is not that simple a question. So let’s first ask a simpler one:

How do we make a solution with the same concentration of sodium and chloride as our blood plasma?

To answer this our approach is simple: use the mean concentrations of sodium and chloride in the blood to calculate how much salt we need to match these such that drinking our salt water solution will neither increase nor decrease their concentration. It might seem a little technical at first, but bear with me, it is in fact quite simple.

This approach is rather well motivated physiologically because the kidneys’ primary function is to maintain blood pressure and concentration of electrolytes—sodium above all others, and each within its typically narrow range of optimal concentration—while excreting metabolic wastes. The kidneys do this by efficiently reabsorbing most of the water and electrolytes from the large volume of blood that goes through them continuously throughout the day and night, getting rid of as much as possible of the metabolic wastes, and carefully adjusting the elimination of ‘excessive’ amounts of water and electrolytes. (You will soon understand why I placed quotation marks around the word excessive.) Let’s start.

You already know that the mean concentration of sodium in the blood is 140 mmol/l. What we haven’t mentioned is that it must be maintained in the range between 135 to 145 mmol/l. You also know that the mean concentration of chloride is 100 mmol/l, and it must be maintained between 95 and 105 mmol/l. The atomic mass of Na is 23, hence one mole (abbreviated mol) is 23 g, and thus one millimole (abbreviated mmol) is 23 mg. The atomic mass of Cl is 35.5, hence one mole is 35.5 g, and therefore one millimole is 35.5 mg. The molecular mass of NaCl is the sum of the atomic masses of Na and Cl, which implies that one mole of NaCl is 58.5 g, and a millimole is 58.5 mg. (A mole is the amount of substance that contains 6×10^23, Avogadro’s number, elementary entities, in this case, atoms. The molar mass is the same as the atomic or molecular mass.)

Multiplying the concentrations in mmol/l by the molar mass in mg/mmol we get the concentration in mg/l. For Na this equals 140 x 23 = 3220 mg/l or 3.22 g/l, and for Cl it is 100 x 35.5 = 3550 mg/l or 3.55 g/l. This is the mean concentration of sodium and chloride there is in our blood. For a small person like me, weighing, say, 56 kg, there are 4 litres of blood that contain a total of 13 g of Na and 14 g of Cl. This is equivalent to about 2 tablespoons of salt.

It is important to note that this is truly quite a lot in comparison to other ions or molecules in our blood. Glucose, for example, which many—probably most people—mistakenly think as the ‘energy of life’, giving it such great importance, is ideally maintained around 80 mg/dl or 0.8 g/l. This is, therefore, also the amount we would need to add to our salt and water solution to make it have, in addition to that of the salt, the same concentration of glucose as that of our blood. And 0.8 g/l for 4 litres of blood makes a total of 3.2 g of glucose in that (my) entire blood supply. This is about 10 times less than the amount of salt!  What does this tell you about their relative importance in our system?

Now, given that Cl (35.5) is heavier than Na (23), NaCl will have a higher mass fraction of Cl: its mass will be 60% chloride (35.5/58.5) and 40% sodium (23/58.5). This just means that 10 g of NaCl or salt has 6 g of Cl and 4 g of Na. So to get 3.22 g of sodium, we need 8 g of sodium chloride, which provides 4.8 g of chloride.

The simple conclusion we draw from this calculation is that dissolving a somewhat heaping teaspoon of salt in one litre of water gives a solution that has the same concentration of sodium as that of our blood (with a little extra chloride).

Does this mean that we should generally drink such a salt and water solution? No, I don’t think so. Are there times when we should? Yes, I believe there are. But say we drink 4 litres per day, 8 g of salt per litre adds up to 32 g of salt just in the water we drink! If we add even half of this amount to our food, we are looking at about 50 g of salt per day! Isn’t this utterly excessive, especially since we are told by the medical authorities to avoid salt as much as possible, with some people today consuming nearly no salt at all? (This article here takes a sobering look at the evidence—actually, the lack thereof—of the claimed benefits of salt reduction.) And more questions arise: What happens when we eat less salt? What happens when we eat more? What happens when we drink less water? What happens when we drink more?

Eating more or less salt. Drinking more or less water.

Remember that the kidneys try very hard to maintain the concentration of solutes in blood plasma—to maintain plasma osmolarity. Also remember that sodium is by far the most important in regulating kidney function, and it is also in the highest concentration. It is nonetheless total osmolarity that the kidneys try to keep constant, and besides sodium, the other important molecule used to monitor and maintain osmolarity by the kidneys is ureathe primary metabolic waste they are trying to eliminate.

As an aside to put things in perspective about the importance of sodium, plasma osmolarity is typically estimated by medical professionals using the sum of twice the concentration of sodium with that of urea and glucose: calculated osmolarity = 2 Na + urea + glucose (all in mmol/l). Since sodium is typically around 140 mmol/l whereas glucose is less than 5 mmol/l and urea about 2.5 mmol/l, it’s obvious that we could just forget about the latter two whose contribution is less than 3% of the total, and look exclusively at sodium concentration (2 Na = 280; glucose + urea = 7.5, so their contribution is 7.5/(280+7.5) = 2.6%).

Eating anything at all, but especially salt or salty foods, raises plasma osmolarity. In response—to maintain constant osmolarity—the kidneys very efficiently reabsorb water and concentrate the urine. Drinking water dilutes the blood and therefore lowers its osmolarity. In response, the kidneys very scrupulously reabsorb solutes and eliminate water, hence diluting the urine.

If we eat nothing and just drink plain water, beyond the body’s minimum water needs, every glass will dilute the blood further and thus cause the kidneys to try to retain more of the sodium while eliminating more of the water. We are drinking quite a lot, but as the day progresses, we are growing more thirsty. We drink more but go to the bathroom more frequently, our urine grows more diluted, and by the end of the day we find ourselves visibly dehydrated, with chapped lips and dry skin. This seems paradoxical in that while drinking water, we are getting increasingly dehydrated. But it is not paradoxical. It is simply the consequence of the kidneys doing their work in trying to maintain constant blood plasma concentrations of sodium (and solutes). For those of you who have fasted on plain water for at least one day, you mostly likely know exactly what I’m talking about. For those who have not, you should try it and experience this first hand for yourselves. Avoiding dehydration in this case is simple: eat salt to match water intake.

If, on the other hand, we do not drink, then the blood gets more and more concentrated, the concentration of sodium and other ions, urea, and everything else for that matter, rises with time, and the kidneys keep trying, harder and harder with time, to maintain the osmolarity constant by retaining as much as they possibly can of the water that is present in the blood. You might think: why not just eliminate some of the solutes to lower their excessively high concentration? But eliminating solutes can only be done through the urine, which means getting rid of water that, in this state of increasing dehydration, is far too precious, and the kidneys therefore try to retain as much of it as possible, hence concentrating the urine as much and for as long as possible to make full use of the scarce amount of water that is available for performing their functions. But here is a crucial point to understand and remember:

In order to reabsorb water, the kidneys rely on a high concentration of solutes—hyperosmolarity—in the interstitial medium through which passes the tubule carrying the filtrate that will eventually be excreted as urine. This is how water can be reabsorbed from the filtrate: the higher the difference in concentration, the more efficient the reabsorption. If there is plenty of excess salt—sodium and chloride ions—then these solutes is what the kidneys prefers to use to drive up and maintain the hyperosmolarity of the interstitial medium, and urea can be excreted freely. If, however, there is a scarcity of sodium and chloride ions, then the kidneys will do everything to reabsorb as much of the precious ions that are in circulation to maintain adequate concentrations of these in the bloodstream, and at the slightest sign of water shortage and dehydration—to ensure the hyperosmolarity of the interstitial medium for maximum water reabsorption—the kidneys will begin to recycle urea, excreting progressively less of it as dehydration increases.

Most of you will have experienced a long day walking around, maybe while on a trip visiting a city, during which you did not drink for several hours. You might have also noticed that you probably didn’t go to the bathroom either, which you may have found unusual compared to the frequency with which you usually go pee when you’re at home or at work. You will have noticed that your mouth was drier and drier as the hours passed, but also that you felt more and more tired, heavy-footed and without energy.  Eventually it struck you just how thirsty you were, or you were finally able to find water to drink, and drank to your heart’s content. As you drank, you might have felt a surge of energy within as little as a minute or two or even immediately following the first few sips. Soon after, you finally did go to the bathroom, and noticed how incredibly dark and strong smelling your urine was. Now you understand what was happening in your kidneys, why you didn’t go pee for these long hours, why your urine was so dark and smelled so strong. However,  the reason why you felt your energy dwindle as the hours passed, and then return when you drank is still unclear.

Water in the blood regulates its volume. And volume in a closed system determines internal pressure. Our circulatory system is a closed system in the sense that there are no holes where blood either goes in or comes out. Yet at the same time it is not a closed system because water enters and leaves the system: it enters the bloodstream through the wall of the intestines, and leaves it through the kidneys and out into the urine. All physiological functions depend intimately on blood pressure: whether it is shooting up through the roof as we face a huge brown bear towering over us and growling at the top of its lungs, and priming us in this extremely stressful fight-or-flight situation for some kind of high-energy reaction in response, or whether it is as low as it can be during our most soothing and restful sleep deep into the night, when the body is repairing and rebuilding itself. And what is the primary regulator of blood pressure? The kidneys.

I will address the details of how the kidneys function and regulate pressure and osmolarity in another post. For now, what is relevant to understand why your energy faded as the hours passed or, more precisely, as the body got progressively more dehydrated, is straight forward:

As water content decreases, blood volume decreases. As the volume decreases, blood pressure drops. And as blood pressure drops, energy levels go down. It’s as simple as that.

It does not help that as soon as the kidneys detect dehydration and drop in pressure, they release hormones to provoke the contraction of the blood vessels in order to counter the pressure drop. This works to a great extent, but since the arteries and veins are constricted, blood flow throughout the body decreases, which in turn contributes significantly to our feeling increasingly heavy-footed and sleepy. With every passing minute, dehydration increases, pressure decreases, blood vessels contract more and our energy level drops further, to the point where we just want to sit down, or even better, lie down, right here on this park bench, and have a long nap.

Interesting, isn’t it? And here again there is nothing strange or paradoxical in this self-regulating mechanism that eventually puts us to sleep as we get increasingly dehydrated. It is simply the consequence of the kidneys doing their work in trying to maintain constant osmolarity and blood pressure. Avoiding dehydration in this case is even simpler: drink water.

If you’ve read this far, you know that both solutions to prevent dehydration are intimately linked: if we don’t drink enough water we get dehydrated, but if we drink too much water without eating salt we also get dehydrated. So let’s now ask another question:

Precisely how much water?

An adult human being needs on average a minimum of 3 litres of water per day to survive for more than a few days (Ref). This depends on climate and level of activity and a bunch of other factors, but in general the range is well established to be between 2 litres in cooler and 5 litres per day in the hottest climates. As suggested from our previous considerations, minimum water intake is also related to salt and food intake. And although this was obvious to me from my own experience of fasting rather regularly between 1 and 3 days at a time, I had not read about it. But as it turns out, the NRC and NAS both (independently) estimated minimum water intake as a function of food intake to be between 1 and 1.5 ml per calorie. For a diet of 2000 calories this would amount to between 2 and 3 litres. But this obviously does not mean that if we don’t eat anything, we don’t need any water! So, what is the very strict minimum amount of water the body needs before physiological functions break down? The short answer is 1.1 litres. For the slightly longer answer, here is a excerpt from page 45 of The Biology of Human Survival:

If obligatory losses are reduced to an absolute minimum and added up, the amounts are 600 milliliters of urine, 400 milliliters of insensible skin loss, and 200 milliliters of respiratory water loss, a total of 1.2 liters. Because maximum urine osmolarity is 1200 milliosmoles/liter, if diet is adjusted to provide the minimum solute excretion per day (about 600 mOsmol), minimum urine output may fall, in theory, to 500 milliliters per day and maitain solute balance. Hence, the absolute minimum water intake amounts to just more than 1 liter (1.1) per day.

(This is also taught in renal physiology lectures such as this one. If you are interested, you will learn a lot from this longer series of 13 segments on urine concentration and dilution here, as well as from this series of 7 segments on the renin-angiotension-aldosterone system here. I found all of them very instructive.)

Keep in mind that 1100 ml of water per day is the very bare minimum for survival, and that there are absolutely no other water losses: basically, you have to be lying, perfectly calm and unmoving at an ideal room temperature where you are neither hot nor cold, not even in the slightest. That’s not particularly realistic unless you’re in a coma. And to show just how extreme it is, let’s see how much of the water the kidneys need to reabsorb to make this happen:

For someone like me weighing 57 kg, the mass of blood is 57*7% = 4 kg. Since the density is almost equal to that of water, 4 kg corresponds to 4 litres. Of this, we know that plasma accounts for a little more than half (54.7%) by volume which makes 2.2 litres, and since plasma is 92% water, the volume of free water in the blood supply is almost exactly half: 2 litres. Blood flow through the kidneys is, on average, around 1.2 l/min. This amounts to more than 1700 litres per day, and means that for 4 litres of blood in the body, every drop of blood goes through the kidneys 425 times in 24 hours, each and every day.

In the kidneys the first step in filtration is the “mechanical”, particle-size-based separation of the blood’s solids from its liquid component. Water makes up half the blood volume, and therefore represents half the flow through the kidneys: 0.6 l or 600 ml/min (850 litres per day). But only 20% of the total flow goes through nephron filtration, which makes 120 ml/min. In the extreme case we are considering, urine output is taken to be 500 ml in 24 hours, equivalent to 20.83 ml/hour or 0.35 ml/min (500 ml/24 h/60 min). Therefore, to achieve this, the kidneys must reabsorb 119.65 ml of the 120 ml flowing through them every minute. This translates to an astounding 99.7% reabsorption efficiency! I’m very skeptical that your average person’s (generally compromised) kidneys could achieve this, but the point was to quantify how extreme this situation at the limit of human survival really is, and as you can see, it is indeed as extreme can be.

Also, keeping in mind that these minimum vital physiological water losses in these circumstances would occur at a more or less uniform rate throughout the day, it would probably be much better to drink a little at regular intervals during our walking hours than to drink everything at once and nothing else during the remaining 24 hours. But what would be the ideal rate at which we should replenish our water in these extreme circumstances?

Assuming the theoretically minimum combined water losses of 1100 ml are lost evenly over the course of the 24 hours, this corresponds to a water loss rate of 0.76 ml/min (1100 ml/24 h/60 min). This is therefore the ideal rate at which to replenishing it. In practice, we may not have an IV system to do this for us, and we will probably be sleeping long nights as our heart rate and blood pressure will have hit rock bottom. Drinking 1100 ml in 11 hours (to work with round numbers) could be done by taking 100 ml, (half a small glass), every hour. This would be the simplest and most reasonable way to maintain solute balance as best we can.

Naturally, with such a minimal water intake, the kidneys are struggling to maintain osmolarity by retaining as much water as possible. Any additional intake of salt (or food) would make things worse in the sense that it would raise the concentration of sodium (and solutes) in the blood whose balance the kidneys will not be able to maintain without additional water. But remember that eating a 200 g cucumber, for example, supplies nearly no calories as it contains virtually no sugar, fat or protein, while proving almost 200 g (ml) of water. And that, conversely, any drink containing caffeine or alcohol will actually dehydrate as those substances are diuretic and cause the excretion of free water.

A somewhat more realistic scenario is one in which we are not eating, but very moderately active at comfortable temperatures. In this case, most experts would agree that the minimum water requirements would be around 2 litres per day. Since we are fasting, these additional water needs are due to greater water losses through evaporation and physiological activity; not to offsetting increased water needs due to food consumption. Consequently, we should ideally drink about 10 glasses of 200 ml, one approximately every hour from 7h to 19h, and we should not eat any salt.

More realistic but still not so common, is that you are doing a 24 hour fast. The purpose of the fast is to give a break to the digestive system, rehydrate bodily tissues, stimulate more fat burning and flush toxins out of the system. Say we drink 4 litres instead of the minimum of 2. In this case we should, in fact, eat some salt in order to ensure good hydration of tissues by supplying plenty of water through a well hydrated bloodstream without diluting the sodium and thus causing the kidneys to excrete more water. And this brings us back to the basic question that set us on this rather long  investigation:

Precisely how much salt?

But you already know the answer to this question: 1 teaspoon per litre in 2 of the 4 litres. Because we don’t drink during the night for about 12 hours, the body inevitably gets dehydrated. Therefore, the best strategy is to start with plain water to rehydrate the concentrated blood and bodily tissues dehydrated from the night, and end with a litre of plain water in preparation for the dry night coming. You should take the equivalent of 1 generous teaspoon of salt with each of the additional litres of water during the day. This will ensure proper hydration of tissues by preventing excessive dilution of blood sodium levels, and maximum urea excretion. Excess sodium, chloride and any other electrolyte will be readily excreted in the urine.

Finally, the far more realistic scenario and, in fact, the one that for most of us is the everyday, is that we are normally active and eating around 2000 calories a day, typically over the course of about 12 hours. In this case we need the basic 2 litres to offset minimum evaporation and physiological losses, and between 2 and 3 litres to offset the 2000 calories. This makes between 4 and 5 litres, 2 of which must be plain water, and 2 or 3 of which must be matched by a good teaspoon of salt per litre that will most naturally, and maybe also preferably, be taken with the food.

Keep in mind that this is the total salt requirements and many prepared foods contain quite a lot already. The hotter or drier the climate, the more water we need. The more we exercise, the more water and the more salt we need. The more we sweat, the more water and the more salt we need. The more stress we experience, the more water and the more salt we need. And in all of these cases, we also need a lot more magnesium.

By the way, it is interesting but not surprising that this conclusion on the amount of salt per day: about 10-15 g, is also the recommendation of the late Dr Batmanghelidj, the “Water doctor”, as well as that of Drs Volek and Phinney, the “Low-Carb doctors” (see References  for details), although the former emphasises the importance of an abundant water intake, while the latter hardly mention it if at all.

So this is it. We know how much water we should generally drink, and we know how much salt we should generally eat:

We should always drink the bare minimum of 2 litres per day. Ideally we should drink 4-5 litres every day. If for some reason we drink 2 litres or less, we should not take any salt (or food for that matter!). If we drink more than 2 litres, we should match each additional litre of water with 1 teaspoon of salt, taking into account the salt contained in the food we eat. It is always better physiologically to drink more than to drink less. And remember that we hydrate most effectively on an empty stomach by drinking 30 minutes before meals.