How the Nephron Works: Structure, Function, and Regulation

Your kidneys clean your blood through roughly a million tiny units on each side. Each unit, a nephron, is a filter attached to a long, folded tube. The work happens in two broad moves: filter almost everything small out of the blood, then reclaim what the body still needs. What’s left becomes urine, and its makeup is adjusted minute by minute by pressure sensors and hormones to keep your blood pressure, salt and water steady. The number of nephrons is set before birth and never grows, which is why protecting them matters across a lifetime.1PubMed Central. Developmental Origins and Nephron Endowment in Hypertension

The Parts of a Nephron

Following the path of fluid from start to finish, a nephron has five main parts:

Inside the kidney, the filters and early tubes sit in the outer layer, the cortex. The loops of Henle and collecting ducts dip into the inner layer, the medulla. Collecting ducts merge as they go and drain urine toward the bladder.

A tube follows the filter because the filter can’t choose what to keep beyond going by size. The tube does the choosing, segment by segment. Each segment has its own set of pumps, so a medicine can target one segment and leave the others alone.

How the Filter Keeps Protein in the Blood

The filter’s barrier has three layers: a vessel lining full of holes, a basement membrane, and wrap-around cells called podocytes.6PubMed Central. Structural and functional insights of the podocyte slit diaphragm complex Podocytes have interlocking “feet” joined by a thin slit membrane, which works as the final, finest sieve.7PubMed Central. Structural and functional insights of the podocyte slit diaphragm complex Water, salts, glucose and small wastes pass through, while proteins and larger molecules stay in the blood.8PubMed Central. Structural and functional insights of the podocyte slit diaphragm complex

Blood pressure inside the capillaries pushes fluid out into the cup.9PubMed. Visualizing filtration: a hands-on model for understanding Starling forces in glomerular filtration rate Proteins left in the blood pull water back, and fluid already in the cup pushes back too.10PubMed. Visualizing filtration: a hands-on model for understanding Starling forces in glomerular filtration rate The filtering rate depends on the balance of these three pushes and pulls. Your lab report may show “eGFR,” an estimate of how much blood all your filters clean each minute. The full name is glomerular filtration rate (GFR), and the “e” means estimated.

Healthy filters let almost no albumin, a main blood protein, through. When a urine test shows albumin, it often means the podocyte layer is strained. Damage to the slit-membrane proteins is a known cause of heavy protein leaks. Because exercise or fever can briefly raise the result, doctors often repeat the test before drawing conclusions.

Taking Back What the Body Needs

About 66% of filtered sodium is taken back in the proximal tubule.11PubMed Central. Tubular effects of sodium–glucose cotransporter 2 inhibitors: intended and unintended consequences Glucose rides back into the body alongside sodium through a carrier called SGLT2, packed into the first part of the tube.12PubMed. The human kidney low affinity Na+/glucose cotransporter SGLT2. Delineation of the major renal reabsorptive mechanism for D-glucose. SGLT2 handles most glucose recovery.13PubMed Central. SGLT2 Mediates Glucose Reabsorption in the Early Proximal Tubule When blood sugar runs very high, the carriers are overwhelmed and sugar spills into the urine. Normally urine has almost no sugar, so finding some usually means blood sugar exceeded what the carriers can recover.

Proteins get rescued too. Two receptors, megalin and cubilin, grab filtered proteins and pull them back into the tube cells.14PubMed. Megalin and cubilin in proximal tubule protein reabsorption: from experimental models to human disease They rescue albumin, vitamin-carrying proteins and hormones.15PubMed. Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule Losing these receptors causes proteins and vitamins to leak into the urine.16PubMed. Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule

All this nonstop pumping is why tubule cells are among the most energy-hungry in the body.17PubMed Central. The multifaceted role of kidney tubule mitochondrial dysfunction in kidney disease development They need steady oxygen and blood flow, so low blood pressure or dehydration hits them hard. That’s one reason kidney injury often follows severe illness.

How the Loop of Henle Makes Urine Concentrated

The thick rising side of the loop pumps salt out but won’t let water follow.18PubMed Central. Urine-Concentrating Mechanism in the Inner Medulla: Function of the Thin Limbs of the Loops of Henle So the tissue around the loop gets saltier the deeper it goes.19PubMed. A better explanation of countercurrent multiplication in the formation of the corticopapillary osmotic gradient in the outer medulla Looping blood vessels nearby trap that salt and urea instead of washing it away.20PubMed. Countercurrent exchange in the renal medulla

Fluid runs down one side of the hairpin and up the other. Salt pumped out on the way up raises the saltiness around the way down, and repeating this along the loop multiplies a small difference into a large one.

Day to day, dark, scanty urine on a hot day uses this gradient fully. Pale, plentiful urine means the body chose not to use it. Loop water pills disrupt the gradient, which is part of why they’re so strong.

How the Body Sets Your Water Balance

ADH (vasopressin), released when you’re low on water, signals collecting-duct cells.21PubMed. Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models The cells then move water channels to their surface.22PubMed. Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models Water flows out of the urine into the salty surrounding tissue built by the loop.23PubMed. Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models Over longer periods the cells also make more or fewer of these channels.24PubMed. Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models

Alcohol suppresses ADH, which is why it causes extra urination. Without ADH or its channels, people pass large amounts of dilute urine.

After you drink a lot of water, your blood becomes slightly diluted. The water-keeping hormone falls and the channels are pulled back, so urine turns pale and plentiful within about an hour.

Hormones That Control Salt, Potassium and Blood Pressure

Renin is released by cells in the small artery feeding each filter.25PubMed. Regulation of renin secretion by renal juxtaglomerular cells Release rises with low pressure in that artery, nerve signals, and low salt detected at a sensing patch of the tubule.26PubMed Central. Macula Densa Sensing and Signaling Mechanisms of Renin Release Renin starts production of angiotensin II, a powerful vessel-tightener.27PubMed Central. Macula Densa Sensing and Signaling Mechanisms of Renin Release Angiotensin II also makes the kidney and gut absorb more salt and water, and triggers aldosterone from the adrenal glands.28PubMed Central. Macula Densa Sensing and Signaling Mechanisms of Renin Release

Aldosterone opens more sodium channels in collecting-duct cells.29PubMed Central. Aldosterone-mediated regulation of ENaC alpha, beta, and gamma subunit proteins in rat kidney More sodium returns to the blood, which drives potassium out into the urine.30The FASEB Journal. Aldosterone (aldo) and epithelial Na channel (ENaC)‐ dependent K secretion by BK‐α/β4 in the distal nephron (DN) So high levels of aldosterone, the salt-keeping hormone, can lower blood potassium, and blocking it can raise potassium.

This chain responds to what you eat. A low-salt diet switches it on and a high-salt diet turns it down. Long-term high activity contributes to high blood pressure.

It also explains common blood pressure pills. ACE inhibitors and ARBs interrupt this chain, so doctors check potassium and creatinine after you start them. A small creatinine rise early on can be expected, but report big changes.

How the Nephron Protects Itself From Pressure Swings

Kidney blood flow and filtering stay roughly steady between about 80 and 180 mmHg of pressure.31PubMed Central. Renal Autoregulation in Health and Disease One reason is that the feeding artery’s muscle tightens when stretched by higher pressure.32PubMed Central. Renal Autoregulation in Health and Disease It reacts beat by beat, shielding the delicate filter from pressure spikes.33PubMed. Renal myogenic response: kinetic attributes and physiological role A second reflex adds to this: a salt sensor in the tubule tells the same artery to tighten when too much salt arrives, 34PubMed. Paracrine factors in tubuloglomerular feedback: adenosine, ATP, and nitric oxide

Outside the safe range, protection fails. Below about 80 mmHg, filtering falls, as can happen in shock or severe dehydration. Long-standing high pressure overwhelms the protection and scars filters, which is why blood pressure control is central to kidney care.

Diabetes ties in too. Early diabetes can push filters into overworking, and the salt-sensing feedback is involved. SGLT2 drugs restore more salt to the sensor, easing that overwork.

How Common Kidney Medicines Act on the Nephron

Each medicine class blocks a carrier in a specific segment. Loop diuretics such as furosemide block a salt carrier in the thick rising limb.35PubMed Central. Diuretics and the kidney. Because that segment reabsorbs so much salt, they cause strong urine output.36PubMed Central. Diuretics and the kidney. A diuretic, or water pill, from the thiazide group, such as hydrochlorothiazide, blocks a different salt carrier in the early distal tubule, with a milder effect.37PubMed Central. Diuretics and the kidney. SGLT2 drugs block glucose return in the first part of the tube.38PubMed Central. SGLT2 Mediates Glucose Reabsorption in the Early Proximal Tubule Sugar and some salt then leave in the urine.

In practice, water pills raise urination within hours, so taking doses earlier in the day helps sleep. Both loop and thiazide pills can lower potassium, and blood tests check it. SGLT2 drugs cause sugar in urine on tests, which is expected. Genital yeast infections are a known SGLT2 side effect, so keep the area clean.

Call a doctor if you feel dizzy on standing, which may mean too much fluid lost. Muscle cramps or weakness may signal low potassium. During a vomiting or diarrhea illness, ask your doctor whether to pause doses.

Acid Balance and the Kidney’s Other Jobs

Specialized collecting-duct cells pump out acid when blood is too acidic.39Diseases. Intercalated Cells: More than pH Regulation Other cells release bicarbonate, a base, when blood is too alkaline.40PubMed Central. Kidney metabolism and acid–base control: back to the basics They adjust within minutes and build more pumps over days.41PubMed. Adaptation of intercalated cells along the collecting duct to systemic acid/base changes

The kidney also makes hormones. Kidney tissue makes erythropoietin when oxygen is low, prompting red blood cell production.42MedReport Foundation. The Kidney as an Endocrine Organ: Renin, Erythropoietin, and Vitamin D in Health and Disease The proximal tubule converts vitamin D into its active form.43MedReport Foundation. The Kidney as an Endocrine Organ: Renin, Erythropoietin, and Vitamin D in Health and Disease So failing kidneys often bring anemia and weak bones.

On a lab report, low bicarbonate (sometimes listed as CO2) can signal the kidneys struggling with acid. Low hemoglobin with kidney disease often reflects less erythropoietin. Doctors may prescribe active vitamin D, since plain vitamin D needs kidney conversion.

How Many Nephrons You Have and What Happens When Some Are Lost

Nephrons start forming around week 9 of pregnancy and finish by about week 36.44PubMed Central. Developmental Origins and Nephron Endowment in Hypertension No new nephrons form after that.45PubMed Central. Developmental Origins and Nephron Endowment in Hypertension People range from about 200,000 to over 2 million nephrons in total.46PubMed Central. Kidney volume-to-birth weight ratio as an estimate of nephron endowment in extremely low birth weight preterm infants Premature birth and low birth weight are linked to fewer nephrons.47PubMed. Developmental programming of a reduced nephron endowment: more than just a baby’s birth weight Babies born early may therefore face more kidney trouble later in life.48PubMed Central. Kidney volume-to-birth weight ratio as an estimate of nephron endowment in extremely low birth weight preterm infants

When nephrons are lost with age or scarring, the remaining ones filter more, keeping total function steady.49PubMed Central. Single-Nephron Glomerular Filtration Rate in Healthy Adults But higher pressure in the remaining filters wears down podocytes and lets protein leak.50PubMed Central. Glomerular hyperfiltration as a therapeutic target for CKD Overworking can drive further loss over years.51PubMed Central. Physiology and Pathophysiology of Compensatory Adaptations of a Solitary Functioning Kidney

Yearly checks of blood pressure and urine protein catch strain early. Avoid frequent high-dose anti-inflammatory painkillers like ibuprofen. If you were born prematurely, mention it to your doctor as a long-term kidney factor.