Most explanations of the kidney start with a filter. Blood goes in, waste is separated out, and what is left over becomes urine.
That picture is convenient, and it is backwards.
A kidney filters on the order of 180 liters of plasma every day. It excretes roughly one to two liters of that as urine. So more than 99 percent of what it filters, it takes straight back.
The organ is not primarily a waste disposal machine. It is a recovery machine that discards a small remainder, and almost all of the work it does is reabsorption.
That single correction is what this post is about, because nearly everything the earlier version missed follows from having the picture the wrong way round.
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| Margherita. Urine, the small remainder: what is left after roughly 180 liters of filtrate has been filtered and almost all of it taken back. CC BY 4.0, via Wikimedia Commons. |
The Kidney Is Mostly a Recovery Machine
One hundred and eighty liters in, one or two out
Set the scale first, because every number in this subject is surprising only against it.
Blood flows into the kidney through the renal artery and is filtered under pressure into about a hundred and eighty liters of fluid a day. That fluid then travels down a tubule, and almost all of it is taken back into the blood before it reaches the end.
What leaves as urine is the small remainder, and it is the part the body has decided it can spare.
This is why the word waste is misleading here. The filtrate crossing the first barrier is not a dirty fluid with some clean liquid in it. It is plasma without its cells and its protein, and that plasma carries glucose, amino acids, bicarbonate and salts.
The kidney filters indiscriminately because it has to. It cannot tell at the point of filtration which molecules it will want back.
What the earlier version of this post got backwards
The previous version of this article said that waste products, excess water and toxins are separated from the blood, and that the remainder is urine.
It then described the filtrate moving through the loop of Henle and into the collecting ducts. That is the wrong order, and it skips the distal tubule entirely.
Filtrate runs the proximal tubule first, then down the descending limb, up the ascending limb, through the distal tubule, and only then into the collecting duct.
The segment it skipped is the only one that either of the two control hormones acts on. Leaving it out is why the post never got as far as explaining regulation.
What Actually Crosses the Barrier
Blood pressure does the filtering
Blood does not arrive at a glomerulus directly from the renal artery. It arrives through an arteriole called the afferent arteriole, and leaves through a second one, the efferent arteriole.
The tuft of capillaries between them is where filtration happens, and it happens because those capillaries sit under higher pressure than the tubule around them. The pressure difference forces water and small solutes out.
Having two arterioles instead of one artery to vein path is not incidental. It is what lets the kidney control how much it filters, and it is where the sensor for that control lives.
Three layers, and a size limit
The barrier is built in three layers. There is a fenestrated endothelium with pores in it, a basement membrane, and outside that the foot processes of podocytes, the cells that wrap the capillary.
What gets through is decided mainly by size and charge. Small solutes pass freely. Most protein does not, and blood cells do not.
The gap between adjacent foot processes is a real filter in its own right, and it is narrow enough to hold back albumin.
| What is in the blood | Crosses into the filtrate | Why |
| Water | All of it | Moves with pressure and with what is dissolved |
| Sodium, chloride, bicarbonate | All of it | Small and uncharged, so it passes |
| Glucose and amino acids | All of it | Passes, and is taken back almost entirely |
| Urea and other wastes | All of it | Passes, and most is taken back too |
| Blood cells | None | Far too large for any gap in the barrier |
| Albumin and most protein | Very little | Blocked by size and by charge |
Nothing at the barrier is classified as waste. The filtrate is simply plasma with the large parts left behind.
Once that table is the mental model, the recovery stops being a strange exception and becomes the normal case.
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| Lijun Yin, Lu Yu, John Cijiang He, and Anqun Chen. Podocyte foot processes on a glomerular capillary. These are the third layer of the filter, and what blocks protein is the slit between them. CC BY 4.0, via Wikimedia Commons. |
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| Uwe Gille. Kidney tissue under the microscope. Filtration, recovery and secretion are not separate organs but one continuous tubule, seen here in cross section. CC BY-SA 3.0, via Wikimedia Commons. |
Ninety-Nine Percent Comes Back
The proximal tubule takes most of it
Recovery starts immediately, in the first segment the filtrate enters, and it starts hard.
The proximal tubule reclaims roughly 65 percent of the filtered load, about 80 percent of the filtered bicarbonate, and essentially all of the filtered glucose and amino acids.
Glucose is the interesting one, because it goes back up a hill. It is reabsorbed against a concentration gradient, into blood that already holds more glucose than the tubule fluid does.
Glucose has a ceiling
Going up a hill costs something, and the cost is a limit on how fast the carriers can work.
Because the carriers have a maximum rate, glucose recovery is saturable. Below that ceiling, every glucose molecule that is filtered comes back.
Above it, they cannot keep pace, and glucose begins to appear in the urine. That is not a separate disease of the kidney. It is the same ceiling, reached.
The same limit governs amino acids, and it is why the phrase the earlier version used, that nutrients are simply reabsorbed, leaves out the part that matters.
Salt up, water left behind
Further along, the tubule stops being a recovery line and starts building a machine.
The loop of Henle is a hairpin. The descending limb lets water out and does not let salt through. The ascending limb pushes salt out into the surrounding tissue and is impervious to water.
The two limbs run in opposite directions, and only one of them leaks. So salt gets deposited faster at the bottom of the hairpin than at the top, over and over.
The result is a gradient. Tissue fluid inside the kidney is saltier deeper in than near the cortex, and the difference is what a urine sample eventually gets made against.
Urine can end up a hundred times more concentrated than blood. Not because the kidney concentrated it in one step, but because it assembled a gradient in advance and then collected against it.
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| Pashute. Countercurrent multiplication in the loop of Henle. This is the machine that builds the gradient the collecting duct then draws water out of. CC0, via Wikimedia Commons. |
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| Henry Vandyke Carter. The loop of Henle in section. The ascending limb carries salt up and lets no water follow, and that asymmetry is the whole trick. Public domain, via Wikimedia Commons. |
| Segment | What happens there | Roughly how much |
| Glomerulus | Filtration under pressure | Everything above is filtered here |
| Proximal tubule | Bulk reabsorption of salt, water and bicarbonate | About 65 percent of the filtered load |
| Descending limb | Water leaves, salt stays | Salty fluid, carried down |
| Ascending limb | Salt leaves, water cannot | Builds the gradient |
| Distal tubule | Fine adjustment by hormone | The regulated step |
| Collecting duct | Water collected against the gradient | The concentrated remainder |
One continuous tubule. Most of it is recovery, and only two segments are under direct hormonal control.
Two Hormones, Two Taps
Everything above is fixed plumbing. The number of nephrons does not change, and as far as is known they cannot be replaced.
So all day-to-day control has to come from turning settings. Two hormones do most of it, and they work on different sides of the same problem.
Vasopressin moves water channels into place
Vasopressin is made in the hypothalamus and released from the posterior pituitary. It has been covered in more detail in the post on what waste costs in water, linked at the end.
Where it acts here is the collecting duct, the very last part of the nephron. By default that segment is relatively impermeable to water.
Vasopressin changes that. It binds a receptor on the cell surface, and the signal it starts moves water channels from inside the cell out to the membrane facing the tubule.
More channels at the surface means more water reabsorbed back into the blood, against the gradient the loop of Henle built. Less vasopressin means the channels come back inside, and more water stays in the tubule.
That is the whole mechanism for making urine more or less concentrated, and it is a relocation of existing channels rather than a change in how many are made.
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| Vossman. A water channel set in a membrane. Vasopressin moves channels like this one into the collecting duct surface, and takes them back out again. CC BY-SA 3.0, via Wikimedia Commons. |
Aldosterone sets the sodium side
The other tap handles salt. Aldosterone comes from the adrenal gland and acts on the same distal region.
It raises the number of sodium channels in the membrane, so more sodium is pulled out of the tubule and back into the blood.
Water follows the sodium, whether or not anyone asks it to. So raising aldosterone expands blood volume and raises pressure. Lowering it does the reverse.
The same channels set up potassium excretion. Sodium moving out leaves the cell interior negative, and potassium leaves through that negative charge. The kidneys handle over 90 percent of dietary potassium this way.
This is a tidy arrangement, and it explains one clinical habit. Diuretics are commonly paired with drugs that block aldosterone, because the two act on the same segment from opposite directions.
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| Ben Mills. Aldosterone, the hormone that sets the sodium side of the nephron. Every drop of water the body retains follows the salt it keeps. Public domain, via Wikimedia Commons. |
Renin, and the brake on it
Aldosterone is downstream of a sensor. The sensor is a cluster of cells sitting at the tip of each afferent arteriole.
When blood pressure or circulating volume falls, those cells release renin into the blood. Renin chops up a protein the liver made, and the fragments that result include angiotensin II.
Angiotensin II constricts vessels, which lifts pressure, and it stimulates aldosterone release, which holds fluid in. Together those two effects reverse the fall that triggered them.
And then the loop stops, because the same cells read rising intracellular calcium as a signal that the job is done and stop releasing renin.
So the answer to the earlier version's question about blood pressure is not that the kidney balances salt and water. It is that it runs a feedback loop with a brake on it, and the brake is as much a part of the design as the sensor.
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| Yan, Y., Zhou, A., Carrell, R.W., Read, R.J.. Angiotensinogen, the liver-made precursor renin chops up to start the pressure loop. Falling blood pressure is what sets the cascade off. CC0, via Wikimedia Commons. |
Three things the kidney makes, not one
The earlier version listed one hormone, erythropoietin, and called the kidney an endocrine organ on that basis.
Erythropoietin is real. When tissue oxygen falls, sensors in the kidney respond and peritubular cells release this signal, which drives red blood cell production in the bone marrow.
It is also only part of the story, because failing kidneys stop making enough of it. Inflammation and iron shortage sit alongside that failure, so correcting the hormone alone does not always correct the anemia.
The kidney also performs the final step of making calcitriol, the active form of vitamin D, by adding a hydroxyl group in the proximal tubule. That is not a minor role in bone biology.
And it releases renin, which is the third of its endocrine outputs and the one described above.
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| AndrewGNF. Erythropoietin, made by peritubular cells when the kidney senses too little oxygen. It is one of three things this organ releases. CC BY-SA 3.0, via Wikimedia Commons. |
| What the kidney releases | Made where | What it is for |
| Renin | Cells at the tip of the afferent arteriole | Starts the cascade that lifts falling pressure |
| Erythropoietin | Peritubular interstitial cells | Drives red blood cell production when oxygen is short |
| Calcitriol, from a precursor | Proximal tubule | The active form of vitamin D, finished off here |
Three outputs, not one. The earlier version of this post listed the second of them.
Where It Goes Wrong
Stones
Kidney stones are the commonest urological problem there is, and the earlier version described them too thinly to be useful.
Calcium oxalate is the commonest stone, and it recurs readily. It does not form from scratch in an open channel.
It grows on something already there. Calcium phosphate deposits form under the surface of the renal papillae, and those deposits give a stone a surface to attach to.
The earlier version listed excess dietary calcium as a cause. The relevant variable is calcium in the urine, which is high in a substantial minority of stone formers and low in plenty of people who never form one.
Low fluid intake and low urinary citrate are more consistently involved. Which is worth noting, because the previous version also recommended drinking more water without ever questioning the fixed eight-glass figure attached to that advice.
On treatment, most stones pass on their own. Shock wave lithotripsy is aimed at larger stones that will not move, and it is extracorporeal, meaning the shock waves are generated outside the body and focused on the stone.
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| J3D3. A calcium oxalate crystal, the commonest kind of kidney stone. They grow on a mineral deposit already sitting in the papilla. CC BY-SA 4.0, via Wikimedia Commons. |
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| Bill Rhodes. A radiograph showing stones in both kidneys. The commonest cause is too little fluid, not too much calcium. CC BY 2.0, via Wikimedia Commons. |
Anti-inflammatory painkillers
Ibuprofen and its relatives are not dangerous in the way a general warning implies, and the reason they can be is instructive.
They block an enzyme that stops the kidney making prostaglandins. Those prostaglandins exist to hold the afferent arteriole open, and without them that vessel narrows.
So the afferent arteriole constricts, filtration pressure falls, and filtration falls with it.
The risk concentrates rather than spreading. It matters most in someone already dehydrated, or already taking a drug that acts on the other arteriole.
It matters as well in someone who began life with a reduced nephron endowment, because there is less of the organ to lose.
The eight-glass rule, which is not a rule
The earlier version advised drinking at least eight glasses of water a day to protect the kidneys.
A review that went looking for evidence behind that advice, published in 2002, traced the recommendation back and found no scientific basis for it.
What the authorities actually publish are adequate intake values for total water, and total includes water in food as well as in drinks. They are reference values, not targets, and they are lower than eight glasses.
For a healthy adult with normal kidneys, thirst and ordinary food supply most of what is needed. The useful version of the advice is much duller: drink enough that your urine stays pale, and take extra water when you are sweating, vomiting or febrile.
The number you cannot replace
This is the fact that ties the whole post together, and the one the earlier version never mentioned.
A human kidney carries on average about 900,000 nephrons, with wide variation between one person and the next.
In a baby born at term that number is reached at around 36 weeks of gestation. After that the count does not rise. It holds steady until nephrons are lost to ageing or disease.
And as far as is known they are not replaced. There is no regeneration, no slow rebuilding after damage, no recruitment of new units under demand.
So the honest answer to what this organ does is that it runs a fixed amount of hardware extremely well for a lifetime. Every adjustment described above is a setting on hardware that was chosen before birth.
A person born early gets fewer units, and spends the rest of their life with the ones they got. That is why the architecture in the companion post about excretory organs is not an academic question.
For how kidney disease is actually found, which is a separate subject and has its own post on this blog, see the link at the end.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Glomerulus | The capillary tuft where blood pressure forces filtrate out into the tubule. |
| Afferent arteriole | The vessel that brings blood into a glomerulus, and where the pressure sensor sits. |
| Efferent arteriole | The vessel that carries blood out again, which is what makes pressure control possible. |
| Podocyte | A cell whose foot processes wrap a glomerular capillary and help hold protein back. |
| Filtrate | What the filter produces. Plasma without its cells and most of its protein, and nearly all of it gets taken back. |
| Reabsorption | Moving something out of the tubule and back into the blood. It is where most of the kidney's work is. |
| Secretion | Moving something from the blood into the tubule, against the direction filtration went. |
| Loop of Henle | The hairpin that builds the salt gradient inside the kidney. |
| Countercurrent multiplication | How the hairpin concentrates salt deeper in than out, by running its two limbs in opposite directions. |
| Osmotic gradient | A difference in how concentrated a fluid is from one place to another. Here, saltier deep inside. |
| Vasopressin | Also called antidiuretic hormone. Made in the hypothalamus and released from the posterior pituitary. Sets water reabsorption. |
| Aquaporin-2 | The water channel vasopressin moves into the collecting duct membrane. |
| Aldosterone | The hormone that sets the sodium side, and with it blood volume and potassium loss. |
| Renin | Released when pressure falls. Starts the cascade that restores it, and that raises aldosterone. |
| Erythropoietin | The red blood cell signal, made by peritubular cells when oxygen is short. |
| Calcitriol | The active form of vitamin D. Its final step happens in the proximal tubule. |
| Nephron | One filtering unit. About 900,000 per human kidney, fixed for life. |
Terms used above, in roughly the order they first appear.
Key Takeaways
- A kidney filters roughly 180 liters of plasma a day and excretes one or two. More than 99 percent of what it filters, it takes straight back.
- Filtration is not selective about waste. Glucose, amino acids and salts all cross into the filtrate, and the tubule recovers them afterward.
- The proximal tubule reclaims about 65 percent of the filtered load, and essentially all of the filtered glucose and amino acids.
- Glucose recovery has a ceiling. It is carried out against a gradient, and running past the ceiling is what puts glucose in urine.
- The loop of Henle builds a salt gradient in advance by countercurrent multiplication, and urine is then made against that gradient.
- Vasopressin sets the water side and aldosterone sets the sodium side. Neither adds a filter; both move settings on hardware that already exists.
- The kidney releases renin, erythropoietin and calcitriol. It is an endocrine organ, not just a filter.
- Nephrons are made before birth, number around 900,000 per kidney, and are not replaced afterward.
Frequently Asked Questions
Can an adult kidney grow new nephrons?
No. In a baby born at term the count is reached at around 36 weeks of gestation, and after that it does not rise.
As far as is known nephrons are not replaced afterward either. This is the specific point earlier versions of this subject tend to reach for and get wrong.
Is the kidney really a filter?
It filters, and it recovers far more than it discards. Calling it a filter is like calling a bank a hole in the ground.
The filter is the easy part. Almost all of the interesting behavior is in what happens to the filtrate afterward.
How much water do I actually need to drink?
There is no fixed number, and the eight-glass figure has no evidence behind it. Adequate intake values for total water include water from food, not only drinks.
For a healthy adult, thirst and normal food cover most of the need. Drink more when you are losing water faster, and keep an eye on how pale your urine is.
Does drinking more water prevent kidney disease?
It prevents one specific thing, which is stones forming because the urine is too concentrated. It is not a general protection.
Why does diabetes damage the kidneys?
Glucose recovery has a ceiling. Run past it for years and the tubule cells are exposed to more glucose than they are built for.
That is also why the filtered load rises, so the same ceiling is passed sooner rather than later.
Key references
• Zhuo & Johnson 2013, proximal nephron, Compr Physiol 3(3):1079-123 -- what the first tubule reclaims, and how much
• Pediatr Nephrol 2014, nephron number and its determinants in early life, 29(12):2299-2308 -- about 900,000 nephrons per kidney, and no replacement
• Nat Rev Nephrol 2023, intrauterine growth restriction, prematurity and nephron endowment, 19(4):218-228 -- the number is set by about 36 weeks
• Kidney Dis 2017, the urine concentrating defect, 3(2):57-65 -- active salt uptake in the thick ascending limb
• Adv Physiol Educ 2026, countercurrent multiplication revisited, 50(3):739-746 -- how the corticomedullary gradient is built
• Cell Physiol Biochem 2025, epithelial impermeability to water, 59(5):730-735 -- why the ascending limb admitting no water is what makes the multiplication work
• Electrolyte Blood Press 2015, vasopressin-regulated aquaporin-2 in collecting duct cells, 13(1):1-6 -- the water channel and the V2 receptor
• J Intern Med 2023, ion and water transport in the distal nephron, 293(1):4-22 -- equilibrating outputs to inputs
• Circ Res 2025, calcium oscillations in juxtaglomerular cell clusters, 137(8):1051-1068 -- renin release and the feedback that limits it
• Nat Rev Nephrol 2015, anemia in kidney disease, 11(7):394-410 -- HIF-2 and the peritubular cells that make erythropoietin
• Exp Biol Med 2022, hydroxylation of vitamin D metabolites in the proximal tubule, 247(13):1103-1111 -- the kidney makes calcitriol
• Sheng Li Xue Bao 2023, kidney regulation of potassium balance, 75(2):216-230 -- principal cells, ENaC, and over 90 percent of dietary potassium
• Am J Physiol 2002, is there scientific evidence for "8 x 8", 283(5):R993-1004 -- the eight-glass rule has none
• Drug Saf 1991, drug-induced nephrotoxicity, 6(2):131-147 -- why anti-inflammatory painkillers constrict the afferent arteriole
• Zool Res 2026, calcium oxalate kidney stone research, 47(3):706-726 -- the commonest stone, and how it recurs
• Int J Mol Med 2026, Randall's plaque formation, 58(5) -- the mineral deposit in the papilla that a stone grows on
• Companion post: what waste chemistry costs in water
• Companion post: why excretory organs exist at all
• Companion post: kidney disease, how it is found, and what helps
Figures are from Wikimedia Commons, with the author and license named in each caption. Figures and values are from the cited papers.
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