18.08.02 · organismal-bio / renal

Nephron function: filtration, tubular reabsorption, secretion, and the countercurrent multiplier

stub3 tiersLean: nonepending prereqs

Anchor (Master): Guyton, A. C. & Hall, J. E. — Textbook of Medical Physiology, 14th ed. (2021), Ch. 26-31

Intuition Beginner

Kidneys filter blood to remove waste and excess water. Each kidney contains about a million tiny filters called nephrons. Blood enters a knot of capillaries called the glomerulus, where blood pressure forces water and small dissolved substances through a filtration barrier into Bowman's capsule. The resulting fluid, called filtrate, has nearly the same composition as blood plasma minus the large proteins and blood cells.

The filtrate then travels through a long, winding tube (the renal tubule) where two key processes occur. Reabsorption moves useful substances -- glucose, amino acids, sodium, water -- back into the blood. Secretion moves additional waste substances from the blood into the tubule. By the end, about 99% of the water and useful solutes have been reclaimed, and the remaining 1% becomes urine.

A hormone called ADH (antidiuretic hormone) controls how much water the collecting duct reabsorbs. When you are dehydrated, ADH levels rise, the collecting duct becomes more permeable to water, and you produce a small volume of concentrated urine. When you have excess water, ADH levels fall, less water is reabsorbed, and you produce a large volume of dilute urine.

Visual Beginner

The nephron can be drawn as a long tube with specialised segments. The proximal convoluted tubule (PCT) reabsorbs the bulk of useful substances. The loop of Henle creates a concentration gradient in the kidney's medulla: the descending limb is permeable to water (fluid concentrates), and the ascending limb actively pumps out salt without allowing water to follow (fluid dilutes). This counter-current arrangement builds an osmotic gradient from 300 mOsm in the cortex to 1200 mOsm deep in the medulla. The collecting duct passes through this gradient, and ADH determines whether water is drawn out (concentrated urine) or retained (dilute urine).

Worked example Beginner

Calculate the filtration fraction and daily urine output from typical values.

Given: Renal plasma flow (RPF) = 625 mL/min, GFR = 125 mL/min, 99% of filtrate is reabsorbed.

Step 1. Filtration fraction -- the proportion of plasma that is filtered:

F F = \frac{GFR}{RPF} = \frac{125}{625} = 0.20 = 20%.

Step 2. Daily filtrate volume:

180 L/day = 125 mL/min \times 1440 min/day \times \frac{1 L}{1000 mL} .

Step 3. If 99% is reabsorbed, 1% is excreted:

Urine output = 180 \times 0.01 = 1.8 L/day .

The kidney reabsorbs 178.2 of the 180 litres filtered each day. This means that for every drop of urine produced, about 99 drops of filtrate were reclaimed.

Check your understanding Beginner

Formal definition Intermediate+

Glomerular filtration rate and Starling forces

Glomerular filtration rate (GFR) is the volume of plasma filtered per unit time, approximately 125 mL/min in a healthy adult. Filtration is driven by the balance of Starling forces across the glomerular capillary wall:

GFR = K_{f} \times P_{n e t},

P_{n e t} = P_{GC} - P_{B S} - π_{GC} + π_{B S},

where $K_{f}$ is the filtration coefficient (hydraulic permeability times surface area), $P_{GC}$ is glomerular capillary hydrostatic pressure (~~55 mmHg), $P_{B S}$ is Bowman's capsule hydrostatic pressure (~~15 mmHg), $π_{GC}$ is glomerular capillary oncotic pressure (~~30 mmHg, rising along the capillary as protein-free filtrate is removed), and $π_{B S}$ is Bowman's capsule oncotic pressure (~~0 mmHg, because proteins are not filtered). Net filtration pressure averages about 10 mmHg.

The oncotic pressure $π_{GC}$ rises progressively along the glomerular capillary from approximately 20 mmHg at the afferent end to approximately 35 mmHg at the efferent end, because filtration concentrates the plasma proteins remaining in the capillary. Filtration therefore occurs predominantly in the first half of the capillary; toward the efferent end, filtration equilibrium may be reached where $P_{n e t}$ approaches zero.

The filtration fraction is the ratio of GFR to renal plasma flow:

F F = \frac{GFR}{RPF} = \frac{125}{625} \approx 0.20.

Autoregulation of GFR

GFR is maintained nearly constant over a range of mean arterial pressure from 80 to 180 mmHg through two mechanisms:

Myogenic response: increased transmural pressure stretches the afferent arteriolar smooth muscle, triggering voltage-gated calcium channels and constriction, which dissipates the excess pressure upstream of the glomerulus.
Tubuloglomerular feedback (TGF): the macula densa cells in the thick ascending limb sense NaCl concentration in the tubular fluid. Increased NaCl delivery (signalling elevated GFR) triggers adenosine release, which constricts the afferent arteriole, reducing GFR. Decreased NaCl delivery dilates the afferent arteriole and stimulates renin release.

Tubular reabsorption

The proximal convoluted tubule reabsorbs approximately 65% of filtered Na+, water (isosmotically, following sodium), K+, Ca2+, HCO3-, and 100% of filtered glucose and amino acids under normal conditions. The driving force is the basolateral Na+/K+ ATPase, which maintains a low intracellular Na+ concentration (~15 mM vs ~140 mM in plasma). This sodium gradient powers secondary active transport:

SGLT2 (early proximal tubule) and SGLT1 (late proximal tubule): sodium-glucose cotransport reclaims filtered glucose.
NHE3 (Na+/H+ exchanger 3): couples sodium reabsorption to H+ secretion, driving bicarbonate reabsorption.
Amino acid cotransporters recover the filtered organic nitrogen load.

Water follows sodium osmotically through aquaporin-1 (AQP1) and the paracellular route.

Loop of Henle: the countercurrent multiplier

The loop of Henle establishes the medullary osmotic gradient through three segments with distinct permeability properties:

Descending limb: permeable to water but not to solutes. As tubular fluid descends into the increasingly hypertonic medulla, water exits by osmosis and the fluid concentrates to ~1200 mOsm at the papillary tip.
Thin ascending limb: impermeable to water but permeable to NaCl. NaCl diffuses passively down its concentration gradient into the interstitium, diluting the tubular fluid.
Thick ascending limb: impermeable to water; actively transports NaCl out via the NKCC2 (Na-K-2Cl) cotransporter, powered by the basolateral Na+/K+ ATPase. This active transport constitutes the "single effect" that separates solute from water at any given medullary level. The counter-current flow arrangement multiplies this single effect along the loop's length, building a gradient from 300 mOsm at the corticomedullary junction to 1200 mOsm at the inner medulla.

Secretion

Secretion moves substances from peritubular blood into the tubular fluid for excretion. The proximal tubule secretes organic anions via OAT1/3 (organic anion transporters) and organic cations via OCT2 (organic cation transporter). PAH (para-aminohippuric acid) is nearly completely cleared from plasma by secretion and is used to estimate renal plasma flow. The distal convoluted tubule and collecting duct secrete K+ (via ROMK channels, regulated by aldosterone) and H+ (via H+-ATPase in intercalated cells).

Clearance

The renal clearance of a substance X is the volume of plasma completely cleared of X per unit time:

C_{x} = \frac{U _{x} \times V}{P _{x}},

where $U_{x}$ is urine concentration, $V$ is urine flow rate, and $P_{x}$ is plasma concentration. Key clearance markers:

Inulin: freely filtered, neither reabsorbed nor secreted. $C_{in} = GFR$ .
PAH: freely filtered and actively secreted. At low plasma concentrations, $C_{P A H} \approx RPF$ (effective renal plasma flow).
Creatinine: endogenous marker, freely filtered with slight secretion. Used clinically to estimate GFR via the eGFR equation.

ADH action and the collecting duct

ADH (vasopressin) binds V2 receptors on the basolateral membrane of principal cells in the collecting duct, activating adenylate cyclase, raising cAMP, and triggering insertion of aquaporin-2 (AQP2) water channels into the apical membrane. Water enters the cell through AQP2 and exits basolaterally through AQP3 and AQP4 into the hypertonic medullary interstitium. Without ADH, the collecting duct is impermeable to water and dilute urine is produced.

Counter-current exchange in the vasa recta

The vasa recta are hairpin-shaped capillaries that descend into the medulla alongside the loop of Henle. As blood descends, it gains solute and loses water, equilibrating with the hypertonic interstitium. As blood ascends back toward the cortex, the reverse occurs. The net effect is that blood flowing through the vasa recta picks up relatively little net solute, preserving the medullary gradient. High medullary blood flow (as in osmotic diuresis) overwhelms this mechanism and washes out the gradient.

Key theorem with proof Intermediate+

Theorem (Counter-current multiplier amplification). If the thick ascending limb can establish a single-effect osmolarity difference of $Δ$ at any level of the medulla, then the steady-state medullary osmotic gradient from cortex to papillary tip is substantially larger than $Δ$ due to the multiplication of the single effect along the counter-current flow. The final gradient exceeds what any single-stage separation could produce.

Proof. Consider the loop of Henle as two parallel tubes (descending and ascending limbs) with opposing flows, separated by the medullary interstitium. At any axial position $x$ along the medulla:

The thick ascending limb at position $x$ actively pumps NaCl into the interstitium, raising local interstitial osmolarity by an increment $δ (x)$ (the single effect).
The descending limb at position $x$ is permeable to water. It equilibrates osmotically with the local interstitium, so its fluid concentrates to match the local interstitial osmolarity.
This concentrated fluid then flows deeper into the medulla to position $x^{'} > x$ , where the ascending limb pumps out NaCl from even more concentrated fluid.

The key insight is step 3: because the ascending limb flows upward (toward the cortex), at position $x$ it receives fluid that was concentrated at a deeper position $x^{'} > x$ . This fluid is already more concentrated than the interstitium at $x$ , so the NaCl pump at $x$ has a larger gradient to work with than it would in a single-stage system. Each increment of the single effect builds on all the increments below it.

Formally, if we denote the interstitial osmolarity at the corticomedullary junction as $C_{0} = 300$ mOsm, then at depth $x$ :

C_{in t} (x) \approx C_{0} + \int_{0}^{x} δ (x^{'}) d x^{'},

where $δ (x^{'})$ itself depends on the tubular fluid concentration at $x^{'}$ , which depends on $C_{in t}$ at all deeper positions. This positive feedback -- the single effect at each level is amplified by the accumulation of single effects at deeper levels -- is the multiplication. The total gradient $C_{in t} (L) - C_{0}$ is therefore substantially larger than the maximum single effect $Δ$ at any one level. $□$

Bridge. This theorem builds toward 18.08.03 pending, where the nephron's handling of hydrogen and bicarbonate ions in the proximal tubule and collecting duct extends the segment-specific transport framework established here. The interstitial gradient maintained by the countercurrent multiplier also influences the driving force for hydrogen ion secretion in the medullary collecting duct.

Exercises Intermediate+

Exercise 2 (medium, short answer).

Explain why inulin clearance equals GFR, but PAH clearance estimates renal plasma flow.

Hint

Consider what happens to each substance after it is filtered. Is it reabsorbed? Is it secreted?

Answer

Inulin is freely filtered at the glomerulus and is neither reabsorbed nor secreted by any nephron segment. The amount filtered ( $GFR \times P_{in}$ ) equals the amount excreted ( $U_{in} \times V$ ), so $C_{in} = (U_{in} \times V) / P_{in} = GFR$ . Inulin is the gold standard for GFR measurement.

PAH is freely filtered and also actively secreted by the proximal tubule (via OAT1/3). At low plasma concentrations, nearly all PAH is removed from the blood in a single pass through the kidney (filtration plus secretion clears about 90% of the PAH from renal plasma). Therefore $C_{P A H}$ approximates effective renal plasma flow (ERPF). True RPF can be estimated as $RPF = C_{P A H} /0.9$ to account for the ~10% that escapes clearance.

Exercise 3 (medium, short answer).

Explain why the thick ascending limb of the loop of Henle is called the "diluting segment" and why its function is necessary for producing both concentrated and dilute urine.

Hint

The thick ascending limb removes solute without water. How does this create the option to either reabsorb water later (concentrated urine) or not (dilute urine)?

Answer

The thick ascending limb actively pumps NaCl out via NKCC2 but is impermeable to water. This removes solute without water, producing dilute tubular fluid (~100 mOsm) entering the distal tubule. Simultaneously, the pumped NaCl builds the medullary osmotic gradient. The diluting segment is essential for both outcomes: to produce concentrated urine, ADH makes the collecting duct permeable to water, which is drawn out by the medullary gradient; to produce dilute urine, absence of ADH keeps the collecting duct impermeable and the already-dilute fluid passes through unchanged. Without the diluting segment, neither option would be possible.

Exercise 5 (hard, symbolic).

Show that the free water clearance $C_{H_{2} O} = V - C_{os m}$ is negative when the kidney is producing concentrated urine, where $C_{os m} = (U_{os m} \times V) / P_{os m}$ .

Hint

Free water clearance measures whether the kidney is excreting or retaining solute-free water. What does the sign tell you about urine versus plasma osmolarity?

Answer

$C_{H_{2} O} = V - C_{os m} = V - \frac{U _{os m} \times V}{P _{os m}} = V (1 - \frac{U _{os m}}{P _{os m}})$ .

For concentrated urine, $U_{os m} > P_{os m}$ (urine osmolarity exceeds plasma osmolarity, e.g., 800 mOsm vs 300 mOsm). Therefore $U_{os m} / P_{os m} > 1$ , so $(1 - U_{os m} / P_{os m}) < 0$ , and $C_{H_{2} O} < 0$ . A negative free water clearance means the kidney is retaining solute-free water in the body (by reabsorbing it from the collecting duct), producing urine that is more concentrated than plasma. This requires both an intact medullary gradient and adequate ADH.

Conversely, for dilute urine, $U_{os m} < P_{os m}$ , and $C_{H_{2} O} > 0$ , indicating the kidney is excreting solute-free water.

Exercise 6 (hard, short answer).

A patient takes furosemide (a loop diuretic that blocks NKCC2). Explain why this drug abolishes the kidney's ability to concentrate urine and why it increases urinary calcium excretion.

Hint

NKCC2 is the active pump in the thick ascending limb that creates the single effect of the countercurrent multiplier. What happens when you remove the single effect? Also consider what drives paracellular calcium reabsorption in the thick ascending limb.

Answer

Furosemide blocks NKCC2 in the thick ascending limb, abolishing the active NaCl transport that constitutes the single effect of the countercurrent multiplier. Without the single effect, the multiplication fails and the medullary osmotic gradient collapses. Even with maximal ADH, there is no gradient to draw water from the collecting duct, so the kidney cannot concentrate urine.

Calcium reabsorption in the thick ascending limb is paracellular, driven by the lumen-positive transepithelial potential created by potassium recycling through ROMK channels (K+ enters the lumen via NKCC2 and exits through ROMK, creating a positive charge). When NKCC2 is blocked, this lumen-positive potential is lost, and the electrical driving force for paracellular Ca2+ reabsorption disappears. Urinary calcium excretion therefore increases.

Clinical nephrology: acute kidney injury, chronic kidney disease, and diuretic mechanisms Master

Acute kidney injury (AKI)

Acute kidney injury is defined by a rapid decline in GFR over hours to days, classified by the KDIGO criteria (increase in serum creatinine by $\geq$ 0.3 mg/dL within 48 hours, or $\geq$ 1.5 times baseline within 7 days, or urine output < 0.5 mL/kg/hr for 6 hours). AKI is divided into three categories by mechanism:

Prerenal AKI results from reduced renal perfusion without intrinsic parenchymal damage. Causes include hypovolaemia (haemorrhage, dehydration, burns), decreased cardiac output (heart failure, cardiogenic shock), systemic vasodilation (sepsis, anaphylaxis), and renal artery stenosis. The kidney responds by retaining sodium and water (RAAS activation, ADH release). The urine sodium concentration is low (< 20 mEq/L) because intact tubules reabsorb sodium avidly. The BUN-to-creatinine ratio is typically elevated (> 20:1) because urea reabsorption increases proportionally more than creatinine when tubular flow is slow. Prerenal AKI is reversible if the underlying perfusion deficit is corrected before tubular injury occurs.

Intrinsic AKI involves damage to the renal parenchyma. The most common form is acute tubular necrosis (ATN), which can result from prolonged ischaemia (ischaemic ATN, the natural progression of untreated prerenal AKI) or direct tubular toxicity (nephrotoxic ATN from contrast agents, aminoglycosides, cisplatin, myoglobin in rhabdomyolysis, or haemoglobin in haemolysis). ATN is characterised by muddy brown casts in the urine, a high urine sodium (> 40 mEq/L, because damaged tubules cannot reabsorb sodium), and a BUN-to-creatinine ratio < 15:1. Other causes of intrinsic AKI include glomerulonephritis, acute interstitial nephritis (often drug-induced, with eosinophiluria), and vascular causes (malignant hypertension, thrombotic microangiopathy).

Postrenal AKI results from urinary tract obstruction (bilateral ureteral obstruction, bladder outlet obstruction, urethral stricture). It is the least common cause but the most immediately reversible if diagnosed promptly. Postrenal AKI should be suspected in any patient with AKI and a history of prostate disease, pelvic malignancy, or neurogenic bladder.

Chronic kidney disease (CKD)

CKD is defined as GFR < 60 mL/min/1.73 m $^{2}$ for more than 3 months, or evidence of kidney damage (proteinuria, haematuria, or structural abnormalities) regardless of GFR. The KDOQI staging system classifies CKD by GFR:

Stage	GFR (mL/min/1.73 m $^{2}$ )	Description
G1	$\geq$ 90	Normal or high GFR with other evidence of kidney damage
G2	60-89	Mildly decreased
G3a	45-59	Mildly to moderately decreased
G3b	30-44	Moderately to severely decreased
G4	15-29	Severely decreased
G5	< 15	Kidney failure (dialysis or transplantation required)

The two leading causes of CKD are diabetes mellitus (diabetic nephropathy) and hypertension (hypertensive nephrosclerosis). Diabetic nephropathy progresses through glomerular hyperfiltration, microalbuminuria (30-300 mg/day), macroalbuminuria (> 300 mg/day), and declining GFR. ACE inhibitors and ARBs slow progression by reducing intraglomerular pressure.

CKD produces systemic consequences through the loss of multiple renal functions: sodium and water retention (oedema, hypertension), potassium retention (hyperkalaemia), metabolic acidosis (impaired ammonium excretion), renal osteodystrophy (impaired phosphate excretion and calcitriol production, leading to secondary hyperparathyroidism), anaemia (insufficient erythropoietin), and accelerated cardiovascular disease.

Nephrotic vs nephritic syndromes

Nephrotic syndrome is defined by proteinuria > 3.5 g/day, hypoalbuminaemia, oedema, and hyperlipidaemia. The underlying pathology is podocyte damage leading to loss of the glomerular filtration barrier's size selectivity. Causes include minimal change disease (most common in children, responsive to corticosteroids), focal segmental glomerulosclerosis (FSGS), membranous nephropathy, and diabetic nephropathy. Complications include increased susceptibility to infection (loss of immunoglobulins in urine), thromboembolism (loss of antithrombin III, increased fibrinogen), and hyperlipidaemia (hepatic response to hypoalbuminaemia increases lipoprotein synthesis).

Nephritic syndrome is characterised by haematuria (dysmorphic red cells and red cell casts), proteinuria (< 3.5 g/day), hypertension, and elevated serum creatinine. The underlying pathology is glomerular inflammation causing disruption of the filtration barrier. Causes include post-streptococcal glomerulonephritis, IgA nephropathy (Berger's disease, the most common primary glomerulonephritis worldwide), membranoproliferative glomerulonephritis, and rapidly progressive glomerulonephritis (RPGN, characterised by crescents on biopsy).

Diuretic mechanisms

Diuretics increase urine output by inhibiting specific sodium reabsorptive transporters at defined nephron segments:

Loop diuretics (furosemide, bumetanide, torsemide) block NKCC2 in the thick ascending limb. This abolishes the countercurrent multiplier's single effect, collapses the medullary gradient, and produces massive natriuresis and diuresis. They also increase urinary calcium and magnesium excretion (loss of the lumen-positive potential). Loop diuretics are the most potent diuretics and are used for pulmonary oedema, heart failure, and hypercalcaemia.

Thiazide diuretics (hydrochlorothiazide, chlorthalidone) block the Na-Cl cotransporter NCC in the distal convoluted tubule. Unlike loop diuretics, thiazides decrease urinary calcium excretion (making them first-line for calcium nephrolithiasis prevention). They cause potassium depletion by increasing sodium delivery to the collecting duct (enhancing Na+/K+ exchange via ENaC and ROMK).

Potassium-sparing diuretics act on the collecting duct. Aldosterone antagonists (spironolactone, eplerenone) block the mineralocorticoid receptor. ENaC blockers (amiloride, triamterene) directly close the epithelial sodium channel. Both reduce potassium secretion and are used with loop or thiazide diuretics to prevent hypokalaemia.

Osmotic diuretics (mannitol) are freely filtered but not reabsorbed. Their osmotic presence in the tubule opposes water reabsorption, and they increase medullary blood flow, washing out the gradient. Used for reducing intracranial pressure and preventing AKI in rhabdomyolysis.

SGLT2 inhibitors

SGLT2 inhibitors (empagliflozin, dapagliflozin, canagliflozin) block the sodium-glucose cotransporter 2 in the early proximal tubule, preventing glucose reabsorption and causing glycosuria. Originally developed as glucose-lowering agents for type 2 diabetes, they have demonstrated significant cardiovascular and renal protective benefits independent of their glucose-lowering effect. Mechanisms of renoprotection include: reduction in glomerular hyperfiltration (the osmotic diuresis and natriuresis reduce tubuloglomerular feedback-mediated afferent arteriolar constriction, lowering intraglomerular pressure), reduction in sodium-glucose cotransport-associated tubular metabolic stress, and anti-inflammatory and anti-fibrotic effects. Risks include genital mycotic infections, volume depletion, and euglycaemic diabetic ketoacidosis.

Renin-angiotensin-aldosterone system in the nephron

The RAAS acts on multiple nephron segments. Angiotensin II preferentially constricts the efferent arteriole, maintaining glomerular capillary pressure and GFR when systemic pressure falls. Aldosterone acts on principal cells of the distal convoluted tubule and collecting duct, upregulating ENaC (apical sodium entry), Na+/K+ ATPase (basolateral sodium exit), and ROMK (apical potassium exit). The net effect is increased Na+ reabsorption, increased K+ secretion, and increased H+ secretion. ACE inhibitors and ARBs reduce intraglomerular pressure by dilating the efferent arteriole, which is renoprotective in CKD but can precipitate AKI in bilateral renal artery stenosis.

Fanconi syndrome

Fanconi syndrome is a generalised proximal tubule dysfunction resulting in impaired reabsorption of glucose (glycosuria at normal blood glucose), amino acids (generalised aminoaciduria), phosphate (hypophosphataemia and bone disease), bicarbonate (proximal renal tubular acidosis), urate (hypouricaemia), and low-molecular-weight proteins. Causes include inherited disorders (cystinosis, Wilson's disease, Lowe syndrome), acquired conditions (multiple myeloma with light-chain cast nephropathy, ifosfamide, lead nephropathy, tenofovir), and idiopathic forms. Treatment targets the underlying cause and supplements losses (phosphate, bicarbonate, vitamin D).

Bartter and Gitelman syndromes

Bartter syndrome results from mutations in transporters of the thick ascending limb (NKCC2 in type I, ROMK in type II, CLCNKB in type III, BSND in type IV). The functional effect mimics chronic loop diuretic use: impaired NaCl reabsorption, loss of the medullary gradient, polyuria, salt wasting, hypokalaemic metabolic alkalosis, and hypercalciuria (with nephrocalcinosis in some types). Renin and aldosterone are elevated (secondary hyperaldosteronism from volume depletion) but blood pressure is normal or low because the underlying defect prevents sodium retention in the thick ascending limb.

Gitelman syndrome results from mutations in the Na-Cl cotransporter NCC in the distal convoluted tubule (the thiazide-sensitive transporter). It mimics chronic thiazide diuretic use: hypokalaemic metabolic alkalosis, hypomagnesaemia, and hypocalciuria (the opposite of Bartter syndrome in calcium handling). Gitelman syndrome is more common and generally milder than Bartter syndrome, often presenting in adolescence or adulthood with muscle cramps, fatigue, and hypokalaemia.

Renal calculi

Kidney stones form when urinary solute concentration exceeds the solubility product, allowing crystal nucleation and growth. The five major stone types are:

Calcium oxalate (most common, ~70%): risk factors include hypercalciuria, hyperoxaluria, hypocitraturia, and low urine volume. Thiazide diuretics reduce stone recurrence by decreasing urinary calcium excretion.
Uric acid (~10%): forms in acidic urine (pH < 5.5). Treatment includes alkalinisation (potassium citrate) and allopurinol in hyperuricaemic patients.
Struvite (magnesium ammonium phosphate) (~10%): associated with urinary tract infection by urease-producing organisms (Proteus, Klebsiella). The urease splits urea to ammonia, alkalinising the urine and promoting struvite precipitation. Staghorn calculi can develop.
Calcium phosphate (~10%): associated with renal tubular acidosis and hyperparathyroidism.
Cystine (~1%): occurs in cystinuria, an inherited defect in proximal tubule cystine reabsorption. Cystine is poorly soluble at physiological pH.

Prevention centres on increasing fluid intake (> 2.5 L/day), dietary sodium restriction (reduces urinary calcium), and targeted pharmacological therapy based on stone composition.

Dialysis principles

When GFR falls below ~10-15 mL/min (stage G5 CKD), renal replacement therapy is required. Haemodialysis uses an artificial membrane (dialyser) to exchange solutes between blood and dialysate across a semipermeable membrane. Solute removal follows first-order kinetics: the clearance depends on blood flow rate, dialysate flow rate, and membrane surface area. Peritoneal dialysis uses the peritoneal membrane as the dialysis surface, with dialysate instilled into the peritoneal cavity. Small solutes diffuse from blood into the dialysate; ultrafiltration is achieved by osmotic agents (glucose or icodextrin) in the dialysate.

Connections Master

Renal physiology -- homeostasis and the nephron 18.08.01 is the prerequisite covering nephron anatomy, the basic concepts of filtration-reabsorption-secretion, RAAS, and acid-base balance. This unit deepens the mechanistic treatment of each nephron segment's transport processes and extends to clinical pathology.
Cardiovascular physiology 18.02.01 determines renal perfusion pressure and cardiac output, which drive GFR. The RAAS creates a feedback loop connecting renal sodium handling to systemic blood pressure regulation. Prerenal AKI is fundamentally a cardiovascular problem (insufficient perfusion).
Endocrine hormones 18.07.01 regulate nephron function through ADH (water reabsorption), aldosterone (sodium reabsorption and potassium secretion), parathyroid hormone (calcium and phosphate handling), and erythropoietin (red cell production). The endocrine control of the nephron is an instance of the general hormone-receptor-signalling principles covered in that unit.
Membrane transport 17.02.02 provides the molecular basis for every nephron transport process: the Na+/K+ ATPase as the primary active transporter, SGLT cotransporters, NKCC2, NCC, ENaC, ROMK, aquaporins, and the paracellular pathway through tight junctions.
Acid-base chemistry is connected through the nephron's role in bicarbonate reabsorption, titratable acid formation, and ammonium excretion, which maintain blood pH at 7.40.

Historical & philosophical context Master

The understanding of nephron function evolved through a series of experimental breakthroughs spanning more than a century. Carl Ludwig (1844) proposed that glomerular filtration is a physical process driven by blood pressure, but his hypothesis could not be tested experimentally at the time. William Bowman (1842) ^{[Bowman 1842]} had described the anatomical relationship between the glomerular capillaries and the renal tubule, but the functional relationship remained speculative.

The filtration-reabsorption model was confirmed by A. N. Richards in the 1930s ^{[Richards 1938]}, who used micropuncture techniques to sample fluid from Bowman's capsule and individual nephron segments in living animals. He demonstrated that glomerular filtrate is an ultrafiltrate of plasma (identical in non-protein composition to plasma) and that tubular fluid is progressively modified along the nephron -- proving that both filtration and reabsorption occur.

The countercurrent mechanism was proposed by Werner Kuhn and colleagues in the 1950s ^{[Kuhn & Ramel 1959]}, drawing on principles from chemical engineering. Kuhn recognised that the loop of Henle operates as a counter-current multiplier analogous to industrial heat exchangers and distillation columns: a small single-stage separation, repeated along a hairpin geometry with opposing flows, generates a gradient far exceeding what any single stage could produce. Wirz and colleagues (1951) ^{[Wirz et al. 1951]} provided the experimental confirmation by demonstrating the medullary osmotic gradient using tissue freezing-point depression measurements.

The clearance concept was formalised by Donald van Slyke and colleagues in the 1930s and developed by Homer Smith, who established inulin as the gold standard for GFR measurement and PAH for renal plasma flow estimation. Smith's 1951 textbook The Kidney: Structure and Function in Health and Disease unified the clearance approach with the emerging understanding of tubular transport.

The development of diuretics illustrates how basic renal physiology translates into therapeutics. The thiazide diuretics were discovered in 1957 by Karl Beyer and colleagues at Merck, who were searching for carbonic anhydrase inhibitors more potent than acetazolamide and serendipitously found compounds that acted on the distal tubule rather than the proximal tubule. Furosemide was developed in the 1960s as a more potent agent acting on the thick ascending limb. The aldosterone antagonist spironolactone was introduced in 1960. Each diuretic class maps directly onto a specific nephron segment and transporter, and understanding the mechanism of action requires understanding the segmental transport architecture described in this unit.

Bibliography Master

Bowman, W., "On the structure and use of the Malpighian bodies of the kidney", Phil. Trans. Roy. Soc. 132 (1842), 57-80.
Richards, A. N., "Processes of urine formation", Proc. Roy. Soc. B 126 (1938), 398-432.
Kuhn, W. & Ramel, A., "Aktiver Salztransport als moeglicher (und wahrscheinlicher) Einzeleffekt bei der Harnkonzentrierung in der Niere", Helv. Chim. Acta 42 (1959), 628-660.
Wirz, H., Hargitay, B. & Kuhn, W., "Location of net fluid transport in the kidney of the rat", Helv. Physiol. Acta 9 (1951), 196-207.
Smith, H. W., The Kidney: Structure and Function in Health and Disease (Oxford UP, 1951).
Guyton, A. C. & Hall, J. E., Textbook of Medical Physiology, 14th ed. (Elsevier, 2021), Ch. 26-31.
Sherwood, L., Human Physiology: From Cells to Systems, 9th ed. (Cengage, 2016), Ch. 14.
Silverthorn, D. U., Human Physiology: An Integrated Approach, 8th ed. (Pearson, 2019), Ch. 19.
Boron, W. F. & Boulpaep, E. L., Medical Physiology, 3rd ed. (Elsevier, 2017), Ch. 33-40.
KDIGO Clinical Practice Guideline for Acute Kidney Injury, Kidney Int. Suppl. 2 (2012), 1-138.
KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease, Kidney Int. 105 (2024), S117-S314.