Renal Physiology

The kidneys are the body's master chemists. Each of your two kidneys is barely the size of a fist, yet together they process the entire blood volume roughly every five minutes, filter about 180 litres of plasma a day, and hand back all but one or two of those litres perfectly adjusted for salt, acid, water, and dozens of dissolved substances. The remarkable part is not that they excrete waste — many organs do that — but that they decide, moment to moment, exactly what the internal environment of every other cell in your body should look like. Understand renal physiology and you understand how the body keeps its own ocean constant.

This page teaches the functional unit (the nephron), the three core operations (filtration, reabsorption, secretion), and how these combine to regulate volume, electrolytes, acid-base status, and blood pressure. We build from structure to function to clinical meaning, so that by the end the numbers on a chemistry panel start to tell you a story.

Learning Objectives

• Describe the structure of the nephron and the vascular arrangement that makes glomerular filtration possible.
• Explain glomerular filtration, the Starling forces that drive it, and what glomerular filtration rate (GFR) measures.
• Trace reabsorption and secretion segment by segment, from proximal tubule to collecting duct.
• Explain how the kidney regulates water, sodium, potassium, acid-base balance, and blood pressure.
• Connect renal physiology to bedside findings, common lab abnormalities, and pharmacology.

Quick Answer

The nephron is the working unit of the kidney, and there are about a million per kidney. It performs three jobs. First, filtration: high pressure in the glomerulus pushes a protein-free, cell-free filtrate of plasma into Bowman's capsule (about 125 mL/min, the GFR). Second, reabsorption: as this filtrate flows through the tubule, roughly 99% of the water and nearly all the useful solutes (glucose, amino acids, most sodium and bicarbonate) are recovered back into the blood. Third, secretion: the tubule actively adds unwanted substances (potassium, hydrogen ions, drugs, toxins) into the forming urine. By varying reabsorption and secretion under hormonal control, the kidney fine-tunes the volume and composition of body fluids and helps regulate blood pressure and acid-base balance.

Where It Came From

For most of history the kidney was a mystery of plumbing — clearly it made urine, but how? The breakthrough was anatomical and it came with the microscope. In 1666 the Italian anatomist Marcello Malpighi, one of the founders of microscopic anatomy, injected the renal arteries and saw tiny bead-like tufts of blood vessels scattered through the kidney tissue. These "Malpighian corpuscles" (the glomeruli) were the first hint that the kidney was a filtering machine built from countless small units rather than a single sponge.

Nearly two centuries later, in 1842, the English surgeon and anatomist Sir William Bowman worked out the crucial connection. He showed that Malpighi's vascular tuft sits cupped inside a hollow capsule — now called Bowman's capsule — which drains directly into the urinary tubule. Bowman proposed that the glomerulus filters watery fluid from the blood while the tubule secretes the specific constituents of urine. His structural insight set up the central debate of nineteenth-century renal physiology: is urine made by filtration, by secretion, or both?

The German physiologist Carl Ludwig argued in the 1840s for filtration driven by blood pressure followed by concentration. The definitive answer came in the 1920s and 1930s when Alfred Newton Richards micro-punctured single frog nephrons and directly sampled the fluid — proving that Bowman's capsule truly contains a filtrate of plasma, and that the tubule then modifies it. Homer Smith then turned clearance measurements into the quantitative science we use today. The motivation throughout was intensely practical: dropsy (oedema), kidney failure, and "Bright's disease" killed people, and physicians needed to know what a failing kidney could and could not do.

The Nephron: Anatomy That Explains Function

A nephron has two parts: a renal corpuscle (glomerulus plus Bowman's capsule) where filtration happens, and a long tubule where the filtrate is processed. The tubule runs in a defined sequence:

1. Proximal convoluted tubule (PCT)
2. Loop of Henle (thin descending limb, thin and thick ascending limbs)
3. Distal convoluted tubule (DCT)
4. Collecting duct

The vascular arrangement is unusual and is the key to everything. Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole — an artery-to-artery bridge, with a capillary bed in between held at high pressure. Because the kidney can independently constrict the afferent or efferent arteriole, it can raise or lower the pressure inside the glomerulus and thereby control filtration directly.

Two nephron populations exist. Cortical nephrons (about 85%) have short loops and handle routine reabsorption. Juxtamedullary nephrons have long loops of Henle that plunge deep into the medulla; these build the osmotic gradient that lets the kidney concentrate urine. Wrapped around the tubules are peritubular capillaries, which recover reabsorbed fluid, and the specialised vasa recta around the long loops.

A small but mighty structure, the juxtaglomerular apparatus (JGA), sits where each nephron's own distal tubule brushes past its glomerulus. Its macula densa cells sense the salt concentration of the fluid leaving the loop, and its granular (JG) cells in the afferent arteriole secrete renin. The JGA is the sensor and control hub for both single-nephron filtration and whole-body blood pressure.

Filtration: The First Pass

Filtration is passive and driven by pressure. Think of it as a set of competing Starling forces:

• Glomerular capillary hydrostatic pressure (~55 mmHg) pushes fluid out — the main driver.
• Bowman's capsule hydrostatic pressure (~15 mmHg) pushes back.
• Glomerular oncotic pressure (~30 mmHg), from plasma proteins that cannot cross, pulls fluid back in.

Net filtration pressure is roughly 55 − 15 − 30 = 10 mmHg outward. Multiply by the filtration coefficient (a measure of membrane permeability and surface area) and you get the GFR, about 125 mL/min or ~180 L/day.

The filtration barrier has three layers: fenestrated capillary endothelium, the glomerular basement membrane, and the podocytes with their interdigitating foot processes and slit diaphragms. This barrier is selective by size and charge — water, ions, glucose, and small molecules pass freely; albumin and larger proteins are held back, partly because the barrier carries a negative charge that repels negatively charged albumin. When podocytes are damaged (as in nephrotic syndrome), the charge and size barrier fails and protein floods into the urine.

GFR is autoregulated. Over a wide range of blood pressures (roughly 80–180 mmHg mean arterial pressure), GFR stays remarkably constant through two mechanisms: the myogenic response (the afferent arteriole constricts when stretched by higher pressure) and tubuloglomerular feedback (when the macula densa senses high salt delivery, it signals the afferent arteriole to constrict, reducing filtration). This is why you do not spill your entire plasma volume every time your blood pressure spikes.

Reabsorption and Secretion: Tuning the Filtrate

The 180 litres filtered per day would be catastrophic if excreted, so the tubule reclaims nearly all of it. Reabsorption and secretion happen in characteristic patterns along the tubule.

Proximal convoluted tubule — the bulk workhorse. The PCT reabsorbs about 65% of filtered sodium and water, essentially all glucose and amino acids, and most bicarbonate. Sodium reabsorption here is the engine: the basolateral Na/K-ATPase pumps sodium out into the blood, keeping intracellular sodium low, and this gradient powers cotransporters (SGLT2 for glucose, others for amino acids and phosphate) on the luminal side. Water follows osmotically. This is the site of the transport maximum for glucose — normally all glucose is reabsorbed, but above a plasma glucose of roughly 180–200 mg/dL the transporters saturate and glucose "spills" into the urine, which is why uncontrolled diabetes causes glycosuria.

Loop of Henle — building the gradient (countercurrent multiplier). The thin descending limb is permeable to water but not salt, so water leaves and the fluid concentrates. The thick ascending limb is the opposite: impermeable to water but actively pumping salt out via the Na-K-2Cl cotransporter (NKCC2) — the target of loop diuretics like furosemide. This separation of salt and water reabsorption creates a hyperosmotic medullary interstitium (up to ~1200 mOsm/kg deep in the medulla), the osmotic engine that later lets the collecting duct extract water. Fluid leaving the loop is dilute.

Distal convoluted tubule — fine sodium control. The DCT reabsorbs sodium via the Na-Cl cotransporter (NCC), the target of thiazide diuretics. It is also where parathyroid hormone drives calcium reabsorption.

Collecting duct — the final decision. Here two hormones act. Aldosterone stimulates principal cells to reabsorb sodium (through ENaC channels) and secrete potassium. Antidiuretic hormone (ADH/vasopressin) inserts aquaporin water channels, allowing water to be pulled out into the hyperosmotic medulla — producing concentrated urine when the body needs to conserve water. Intercalated cells here handle the final secretion of hydrogen ions and reabsorption of bicarbonate to fine-tune acid-base balance.

Meanwhile, secretion actively dumps substances into the tubule: potassium and hydrogen ions for homeostasis, and organic acids and bases including many drugs (penicillin, diuretics themselves must be secreted into the lumen to reach their targets) and toxins (creatinine is partly secreted, which is why it slightly overestimates GFR).

A worked example: what happens after a salty meal and no water

Plasma osmolality rises. Osmoreceptors in the hypothalamus trigger thirst and ADH release. ADH inserts aquaporins in the collecting duct; water is reabsorbed into the concentrated medulla; a small volume of concentrated urine is produced and plasma osmolality falls back toward 285–295 mOsm/kg. If instead you drank a litre of plain water, ADH would fall to near zero, the collecting duct would stay water-impermeable, and you would pass a large volume of dilute urine within an hour. Same nephron, opposite output — that is regulation.

How the Kidney Regulates the Whole Body

• Fluid volume and blood pressure: The renin-angiotensin-aldosterone system (RAAS) is central. Low blood pressure or low salt delivery triggers renin from the JGA, which generates angiotensin II (a vasoconstrictor that also preferentially constricts the efferent arteriole to preserve GFR) and aldosterone (salt and water retention). This is why RAAS drugs — ACE inhibitors, ARBs — are cornerstones of hypertension and heart-failure treatment.
• Osmolality: ADH and thirst keep plasma osmolality within a tight window.
• Potassium: Mostly by aldosterone-driven secretion in the collecting duct; small errors are dangerous because potassium controls cardiac excitability.
• Acid-base: The kidney reabsorbs filtered bicarbonate, generates new bicarbonate, and excretes acid (buffered by ammonia and phosphate). Together with the lungs, this holds blood pH near 7.4.
• Calcium, phosphate, and bone: The kidney activates vitamin D (1-alpha-hydroxylation) and responds to parathyroid hormone.
• Red blood cells: The kidney senses oxygen and secretes erythropoietin; this is why chronic kidney disease causes anaemia.

Real-World Applications

Renal physiology is not abstract — it is read directly off routine tests and drives everyday prescribing. An estimated GFR on a metabolic panel tells a clinician how much filtering capacity remains and guides the dosing of drugs cleared by the kidney (many antibiotics, metformin, opioids). Diuretics are physiology weaponised: loop diuretics block NKCC2 in the thick ascending limb for powerful fluid removal in heart failure; thiazides block NCC in the DCT for hypertension; spironolactone blocks aldosterone. SGLT2 inhibitors, now major drugs for diabetes and heart failure, simply prevent glucose reabsorption in the PCT. Understanding the countercurrent mechanism explains why a patient with damaged medulla cannot concentrate urine and passes large dilute volumes. And knowing that the kidney makes erythropoietin explains why dialysis patients need EPO injections.

Common Mistakes

• "GFR measures kidney blood flow." No. GFR measures the volume filtered into Bowman's capsule per minute. Renal blood flow is much larger (~1.1 L/min); only about a fifth of the plasma is filtered (the filtration fraction). A kidney can have adequate blood flow but low GFR if glomerular pressure or membrane integrity is impaired.
• "Most regulation happens in the collecting duct because that's where the hormones act." The collecting duct does the fine-tuning, but the PCT does the heavy lifting — 65% of everything is reabsorbed there before any hormone gets involved. Bulk reabsorption is largely automatic; hormonal control adjusts the last few percent, which is nonetheless what makes precise homeostasis possible.
• "Glucose in the urine means the kidney is failing." Glycosuria usually means the plasma glucose exceeded the transport maximum (as in diabetes), overwhelming healthy PCT transporters — not that the filter is broken. A failing filter more typically leaks protein (albumin), not glucose.

Comparison and Connections

Segment	Main reabsorbs	Key transporter	Hormone / drug
Proximal tubule	Na, water, glucose, HCO3, amino acids	Na/K-ATPase, SGLT2	SGLT2 inhibitors
Thick ascending limb	Na, K, Cl (not water)	NKCC2	Loop diuretics (furosemide)
Distal tubule	Na, Cl, Ca	NCC	Thiazides, PTH
Collecting duct	Na, water	ENaC, aquaporins	Aldosterone, ADH

Do not confuse reabsorption (moving substances from filtrate back into blood) with secretion (moving them from blood into filtrate) — they are opposite directions and both shape the final urine. Also distinguish ADH (controls water, so osmolality) from aldosterone (controls sodium, so volume); they are often muddled but answer different questions.

Practice Questions

Recall

Q: Name the three basic processes the nephron uses to form urine. A: Glomerular filtration, tubular reabsorption, and tubular secretion.

Understanding

Q: Why does the thick ascending limb being impermeable to water matter for the whole kidney? A: By pumping salt out without water, it dilutes the tubular fluid and deposits salt in the medullary interstitium, building the hyperosmotic gradient. That gradient is what later allows the collecting duct, under ADH, to pull water out and concentrate the urine. Without this step the kidney cannot make concentrated urine.

Application

Q: A patient with heart failure has fluid overload. Which diuretic gives the strongest response and why? A: A loop diuretic (e.g., furosemide), because it blocks NKCC2 in the thick ascending limb, the segment responsible for a large fraction of salt reabsorption and for building the concentrating gradient. Blocking it produces powerful salt and water loss — hence "high-ceiling" diuretics.

Analysis

Q: A patient's blood pressure drops. Explain step by step how the kidney tries to preserve GFR and restore volume. A: Reduced pressure and salt delivery are sensed by the JGA, releasing renin. Renin generates angiotensin II, which constricts the efferent arteriole (raising glomerular pressure to defend GFR) and stimulates aldosterone (sodium and water retention to restore volume). ADH also rises if osmolality or volume warrants it. The myogenic response and tubuloglomerular feedback simultaneously adjust afferent tone. The net effect conserves salt and water and supports blood pressure.

FAQ

Why do the kidneys filter 180 litres a day only to reabsorb 99% of it — isn't that wasteful? It looks wasteful but it is the price of precision. High-volume filtration lets the kidney clear waste rapidly and gives the tubule a large stream to selectively pick from. Regulating output by adjusting reabsorption of a huge filtrate is far more flexible and fast than trying to secrete each waste product individually.

What is the difference between GFR and creatinine clearance? GFR is the true filtration rate. Creatinine clearance is a practical estimate of it, because creatinine is freely filtered and mostly not reabsorbed. It slightly overestimates GFR because a little creatinine is also secreted, but it is convenient and widely used clinically.

How can the kidney make urine more concentrated than blood? Through the countercurrent multiplier of the loop of Henle, which builds a very salty medullary interstitium, combined with ADH opening water channels in the collecting duct. Water is then osmotically drawn out of the collecting duct into that salty interstitium, concentrating the urine.

Why does kidney disease cause anaemia? The kidney senses blood oxygen and produces erythropoietin, the hormone that tells bone marrow to make red blood cells. Diseased kidneys make too little, so red cell production falls.

Does drinking lots of water "flush out" the kidneys and help them? Adequate hydration helps the kidneys work and can reduce stone risk, but there is no evidence that drinking excessive amounts "detoxes" or improves healthy kidney function — the kidney already regulates precisely. Overdrinking can even dangerously dilute blood sodium (hyponatraemia). Balance, not excess, is the goal.

Quick Revision

• Nephron = renal corpuscle (glomerulus + Bowman's capsule) + tubule; ~1 million per kidney.
• Three processes: filtration, reabsorption, secretion.
• GFR ~125 mL/min (~180 L/day); net filtration pressure ~10 mmHg; autoregulated 80–180 mmHg.
• PCT reabsorbs ~65% of everything; glucose transport maximum ~180–200 mg/dL.
• Thick ascending limb (NKCC2) builds the medullary gradient; loop diuretics act here.
• DCT: NCC (thiazides); collecting duct: aldosterone (Na/K) and ADH (water).
• Regulates volume/BP (RAAS), osmolality (ADH), potassium, acid-base, calcium/vitamin D, and red cells (erythropoietin).
• History: Malpighi (1666) saw the glomeruli; Bowman (1842) linked capsule to tubule and proposed filtration + secretion.

Related Topics

Prerequisites

• Nephrology Overview
• Cardiovascular physiology and blood pressure control (see ../../2._Physiology/index.md)

Related Topics

• Acid-base balance and electrolyte disorders (see ../../2._Physiology/index.md)
• Diuretics and renal pharmacology (see ../../5._Pharmacology/index.md)

Next Topics

• Glomerular diseases and nephrotic/nephritic syndromes
• Acute kidney injury and chronic kidney disease