Each human kidney contains roughly one million nephrons, and every one of them performs the same remarkable feat: filtering blood, reclaiming what the body needs, and discarding what it does not. The nephron is the kidney’s functional unit, responsible for producing urine while simultaneously regulating blood pressure, electrolyte balance, acid-base status, and even hormone production. Understanding how a nephron works is really understanding how your kidneys keep you alive, because almost everything the kidney does happens at the level of this single microscopic structure.
How Filtration Begins at the Glomerulus
Blood enters the nephron at the glomerulus, a tight ball of capillaries nestled inside a cup-shaped structure called Bowman’s capsule. The capillary walls here are uniquely leaky compared with capillaries elsewhere in the body, and for good reason: their job is to let water, salts, glucose, amino acids, and small waste molecules pass through while holding back larger components like blood cells and most proteins. Three layers make up this filtration barrier: the capillary endothelium (which has small pores), a basement membrane, and a layer of specialized cells called podocytes. Podocytes wrap finger-like extensions around the capillaries, leaving narrow gaps between them. These gaps are bridged by a structure called the slit diaphragm, which is critical for determining what can and cannot pass through the filter.1PubMed. Cell biology of the glomerular podocyte Key proteins in that slit diaphragm, including one called nephrin, physically create the selective barrier. When these proteins are defective, protein leaks into the urine, a hallmark of kidney disease.2PubMed. Unraveling the molecular make-up of the glomerular podocyte slit diaphragm
What actually pushes fluid out of the blood and into Bowman’s capsule is a set of opposing pressures. The blood pressure inside the glomerular capillaries drives fluid outward. Working against that are the protein-rich plasma’s tendency to pull water back (oncotic pressure) and the fluid pressure already sitting in Bowman’s capsule. In a healthy kidney, the oncotic pressure within Bowman’s capsule itself is so small it can be ignored.3PubMed. Visualizing filtration: a hands-on model for understanding Starling forces in glomerular filtration rate The net result of these competing forces is the glomerular filtration rate, or GFR, which in a healthy adult produces about 180 liters of filtrate per day. That enormous volume is not all lost as urine, of course. More than 99 percent of it gets reclaimed downstream.
The Proximal Tubule Does the Heavy Lifting
Immediately after filtrate leaves Bowman’s capsule, it enters the proximal tubule, which is the nephron’s workhorse. This segment reabsorbs roughly two-thirds of the filtered sodium and water, along with nearly all of the filtered glucose, amino acids, and bicarbonate.4PubMed Central. Tubular effects of sodium-glucose cotransporter 2 inhibitors: intended and unintended consequences The cells lining the proximal tubule are packed with mitochondria and have a densely folded surface (the brush border) to maximize the area available for transport.
Sodium reabsorption here depends heavily on a transporter called the sodium-hydrogen exchanger (NHE3), which swaps sodium from the filtrate for hydrogen ions moving the other direction. When the flow of filtrate increases, the microvilli of the brush border sense the higher drag and ramp up NHE3 activity, pulling more sodium and bicarbonate back into the blood.5PubMed Central. Regulation of glomerulotubular balance: flow-activated proximal tubule function This automatic scaling, called glomerulotubular balance, ensures that the fraction of filtrate reabsorbed stays roughly constant even when the filtration rate swings up or down. Research has shown that changes in the protein concentration of blood leaving the glomerulus also help drive this coupling: when the filtration fraction rises, the blood downstream becomes more protein-rich, and the higher oncotic pressure pulls more fluid back across the tubular wall.6Journal of Clinical Investigation. Postglomerular vascular protein concentration: evidence for a causal role in governing fluid reabsorption and glomerulotubular balance by the renal proximal tubule
The proximal tubule is also the site that modern diabetes drugs target. SGLT2 inhibitors block the sodium-glucose cotransporter in this segment, causing glucose and sodium to stay in the tubular fluid rather than being reabsorbed. Blocking SGLT2 also partially inhibits NHE3, contributing to the drugs’ ability to lower blood pressure by promoting sodium and fluid loss.7Nephrology Dialysis Transplantation. How can inhibition of glucose and sodium transport in the early proximal tubule protect the cardiorenal system? In diabetes, high blood glucose drives excess glucose and sodium reabsorption through these same transporters, which feeds back through the macula densa to increase GFR. This glomerular hyperfiltration is now understood as a tubule-driven process, not just a vascular one.8PubMed Central. The tubular hypothesis of nephron filtration and diabetic kidney disease
The Loop of Henle and the Concentrating Gradient
After the proximal tubule, the filtrate dips into the loop of Henle, a hairpin-shaped structure that plunges into the kidney’s medulla and then loops back toward the cortex. This architecture is not decorative. It creates the osmotic gradient that ultimately allows the kidney to produce concentrated urine.
The key mechanism is countercurrent multiplication. In the thick ascending limb of the loop, the wall is impermeable to water but actively pumps sodium and chloride out into the surrounding tissue. Because the descending limb running alongside it is permeable to water, that water gets drawn out by the increasing saltiness of the surrounding medulla. The result is a progressive buildup of solute concentration from the outer medulla toward the inner tip of the kidney.9PubMed. A better explanation of countercurrent multiplication in the formation of the corticopapillary osmotic gradient in the outer medulla In the outer medulla, this gradient is clearly generated by the active salt pumping of the thick ascending limb.10PubMed Central. Urine-concentrating mechanism in the inner medulla: function of the thin limbs of the loops of Henle
Urea plays a surprisingly important role in maintaining this gradient, especially in the deeper medulla. The kidney recycles urea through a set of dedicated urea transporters in different nephron segments and blood vessels. One family of these transporters, in the inner medullary collecting duct, is upregulated when urea levels are low, boosting urea reabsorption to keep the medullary concentration high. Others, in the descending thin limbs and the vasa recta (the blood vessels running alongside the loops), increase when medullary urea is already abundant, preventing it from washing away into the general circulation.11PubMed Central. Long-Term Regulation of Renal Urea Transporters during Antidiuresis Modeling studies predict massive urea recycling, particularly in the upper inner medulla, with urea that is deposited deep in the papilla by collecting ducts eventually traveling back to the cortex through short vasa recta in the outer medulla.12PubMed. Cycles and separations in a model of the renal medulla
Fine-Tuning in the Distal Tubule
By the time fluid reaches the distal convoluted tubule, about 90 percent of the filtered sodium has already been reclaimed. What remains is a comparatively small but physiologically critical fraction. The distal convoluted tubule handles roughly 5 to 10 percent of filtered sodium chloride through a transporter called the sodium-chloride cotransporter, or NCC.13PubMed. Redistribution of distal tubule Na+-Cl- cotransporter (NCC) in response to a high-salt diet NCC sits on the cell’s luminal surface and is the target of thiazide diuretics, one of the most widely prescribed classes of blood pressure medication.14PubMed Central. Structure-function relationships in the sodium chloride cotransporter
What makes NCC especially interesting is how responsive it is to the body’s salt status. When you eat a low-salt diet, the body upregulates NCC and the hormone aldosterone to hold on to every bit of sodium it can. Switch to a high-salt diet, and NCC abundance drops.15PubMed. Redistribution of distal tubule Na+-Cl- cotransporter (NCC) in response to a high-salt diet This segment marks the beginning of the aldosterone-sensitive nephron, where hormonal signals start to exert much tighter control over exactly how much sodium and potassium end up in the final urine.16PubMed Central. The thiazide-sensitive Na+-Cl- cotransporter: molecular biology, functional properties, and regulation by WNKs
The Collecting Duct Decides How Concentrated Your Urine Gets
The collecting duct is where the kidney makes its final call on water balance. Tubular fluid arriving here passes through the medulla, where that osmotic gradient established by the loop of Henle is waiting. Whether the collecting duct lets water follow that gradient depends almost entirely on one hormone: vasopressin (also called antidiuretic hormone, or ADH). When you are dehydrated, your brain releases vasopressin, which travels to the collecting duct and triggers the insertion of water channels called aquaporin-2 (AQP2) into the cell membrane facing the tubular lumen. With those channels in place, water floods out of the collecting duct into the concentrated medullary tissue, and urine becomes much more concentrated.17PubMed Central. Molecular mechanisms regulating aquaporin-2 in kidney collecting duct When you are well hydrated, vasopressin levels drop, AQP2 channels are pulled back inside the cell, the collecting duct becomes relatively waterproof, and you produce dilute urine.18PubMed. Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models
The collecting duct is not just about water, though. Interspersed among the principal cells that handle water and sodium are intercalated cells, which are the nephron’s acid-base specialists. These cells secrete hydrogen ions or bicarbonate depending on whether the blood is too acidic or too alkaline, and different subtypes of intercalated cells handle different directions of that exchange. They appear throughout the collecting duct from the cortex down into the beginning of the inner medulla.19PubMed Central. Collecting duct intercalated cell function and regulation
How the Nephron Protects Itself Through Autoregulation
Blood pressure fluctuates throughout the day, but the nephron cannot afford wild swings in filtration. Two mechanisms keep things stable across a wide range of blood pressures, roughly 80 to 180 mmHg.20PubMed Central. Renal autoregulation in health and disease
The first is the myogenic response. When blood pressure rises, the smooth muscle cells in the afferent arteriole (the vessel feeding the glomerulus) sense the stretch and contract, narrowing the vessel to keep flow constant. Research suggests this response is fast enough to track individual heartbeats, serving primarily to shield the delicate glomerular capillaries from pressure spikes during systole.21PubMed. Renal myogenic response: kinetic attributes and physiological role When this myogenic mechanism fails, as it does in certain salt-sensitive forms of hypertension, the glomerulus is left exposed to elevated pressures and can sustain damage over time.22PubMed Central. Mechanism of impaired afferent arteriole myogenic response in Dahl salt-sensitive rats: role of 20-HETE
The second mechanism is tubuloglomerular feedback (TGF). A cluster of specialized cells called the macula densa sits where the thick ascending limb of the loop of Henle passes right next to the glomerulus of its own nephron. These cells monitor the chloride concentration of the tubular fluid flowing past them. If too much salt is arriving, it means filtration is too high, and the macula densa sends a chemical signal to constrict the afferent arteriole and bring GFR back down.23PubMed. Tubuloglomerular feedback and the control of glomerular filtration rate That chemical messenger turns out to involve ATP released from the macula densa, which is broken down in the surrounding tissue into adenosine. Adenosine then acts on receptors on the arteriole wall to trigger constriction.24PubMed. Role of macula densa adenosine triphosphate (ATP) in tubuloglomerular feedback Mice engineered to lack adenosine receptors on arteriolar smooth muscle show a dramatically blunted TGF response, confirming adenosine’s central role.25PubMed Central. Renal afferent arteriolar and tubuloglomerular feedback reactivity in mice with conditional deletions of adenosine 1 receptors
The Nephron as a Hormone Factory
People usually think of the kidney as a filter, but the nephron and its surrounding cells are also deeply involved in hormonal regulation.
The most famous product is renin. Specialized cells in the wall of the afferent arteriole, called juxtaglomerular cells, store renin in granules and release it into the bloodstream when blood pressure drops or when the macula densa senses low chloride in the tubular fluid.26PubMed. Renin release: sites, mechanisms, and control Renin kicks off the renin-angiotensin-aldosterone system (RAAS), a hormonal cascade that raises blood pressure and promotes sodium retention. The macula densa, already discussed as a salt sensor for TGF, also participates in renin regulation, and recent work shows these cells respond to metabolic and chemical signals beyond just sodium and chloride.27PubMed Central. Macula densa sensing and signaling mechanisms of renin release Renin release is boosted through pathways involving cyclic AMP and is paradoxically inhibited by calcium-related signals, which is the opposite of what most secretory cells do.28American Journal of Hypertension. Control of Renin Synthesis and Secretion
The kidney is also the primary site where vitamin D gets activated. The liver converts vitamin D into an intermediate form, but it is the proximal tubule cells that perform the final step, using an enzyme called 1-alpha-hydroxylase to produce calcitriol, the hormonally active version of vitamin D that regulates calcium absorption in the gut.29PubMed. The vitamin D receptor in the proximal renal tubule is a key regulator of serum 1α,25-dihydroxyvitamin D₃ The same cells also express a deactivating enzyme, so the proximal tubule acts as both the on-switch and off-switch for circulating vitamin D.30PubMed Central. Regulation of 1 and 24 hydroxylation of vitamin D metabolites in the proximal tubule This is a major reason chronic kidney disease so often leads to bone problems and calcium imbalances.
Erythropoietin (EPO), the hormone that stimulates red blood cell production, also comes primarily from the kidney. The producing cells are not tubular cells themselves but fibroblasts sitting in the cortical tissue between tubules. These cells sense low oxygen through a protein called HIF-2-alpha, and more than 90 percent of EPO-expressing cells in the kidney’s outer cortex coexpress this oxygen sensor.31PubMed. Hypoxia-inducible factor-2alpha-expressing interstitial fibroblasts are the only renal cells that express erythropoietin under hypoxia-inducible factor stabilization When kidneys fail, EPO production plummets, which is why anemia is almost universal in advanced kidney disease.
How the Nephron Handles Acid
Your blood needs to stay within a narrow pH range, and the nephron is one of the main systems that makes that happen. The proximal tubule is the primary site of ammoniagenesis, a process that breaks down the amino acid glutamine to produce ammonium and bicarbonate. The ammonium eventually gets excreted in the urine, carrying acid out of the body. The bicarbonate enters the blood and acts as a buffer. Critically, ammonia that does not make it into the urine ends up back in the liver, where its processing consumes bicarbonate, canceling out the benefit. So the kidney’s acid-handling only works to the extent that ammonia actually leaves via the urine.32PubMed Central. Renal ammonia metabolism and transport The intercalated cells in the collecting duct add another layer by directly secreting hydrogen ions, as described earlier, offering fine control over the final urine pH.
Nephron Numbers and What Happens with Age
Not everyone starts with the same number of nephrons. Estimates for a healthy adult kidney range widely, from fewer than 200,000 to over a million nephrons per kidney, depending on birth weight, genetics, and prenatal nutrition. And nephrons are not replaceable: mammals cannot grow new ones after birth. Healthy aging brings a steady decline in nephron number, and research on kidney donors and autopsy specimens confirms that lower nephron counts in older adults reflect both fewer nephrons to begin with and nephrons lost over a lifetime.33PubMed Central. The Substantial Loss of Nephrons in Healthy Human Kidneys with Aging This is why GFR gradually declines with age even in people who have no kidney disease. Remaining nephrons compensate by filtering more per unit, but that compensatory hyperfiltration can itself contribute to further damage over decades.
Why Kidney Architecture Varies Across Species
The nephron’s design is not identical across mammals. Desert-dwelling species that need to conserve every drop of water tend to have longer loops of Henle, a higher fraction of nephrons with long loops, and more elaborate vascular bundles in the medulla, all of which deepen the osmotic gradient and allow the production of extremely concentrated urine.34PubMed. Urinary concentrating ability: insights from comparative anatomy In placental mammals more broadly, the kidney took on an outsized role in urine concentration because, unlike reptiles and birds, mammals lack nasal salt glands and have fully separated urinary and rectal tracts, leaving the kidney as the sole organ responsible for adjusting the final composition of urine.35Nature Reviews Urology. Effects of the environment on the evolution of the vertebrate urinary tract Human kidneys sit somewhere in the middle of the mammalian range, capable of concentrating urine to about 1,200 milliosmoles per kilogram, enough for terrestrial life but far short of what a desert rodent can achieve.