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Physiology
Renal Tubular Physiology: Proximal Tubule, Loop of Henle, Distal Tubule, Collecting Duct — Ion Transport, Water Handling, Sites of Drug Action
Core Principle of Renal Tubular Physiology
The nephron transforms 180 L/day of glomerular filtrate into 1–2 L of urine through selective reabsorption and secretion along anatomically distinct segments.
Each segment has unique transporters, channels, and permeability characteristics that determine its function in maintaining fluid, electrolyte, and acid-base homeostasis.
The proximal tubule performs bulk reabsorption (65–70% of filtered Na⁺, water, glucose, amino acids), while distal segments fine-tune composition under hormonal control.
Understanding segmental transport mechanisms explains diuretic sites of action, genetic tubulopathies, and electrolyte disturbances.
Proximal Tubule: The Workhorse of Reabsorption
Reabsorbs 65–70% of filtered Na⁺, water, HCO₃⁻, glucose, amino acids, and phosphate — the bulk reabsorption segment.
Apical Na⁺/H⁺ exchanger (NHE3) drives Na⁺ reabsorption coupled to HCO₃⁻ reclamation via carbonic anhydrase.
Secondary active transport: Na⁺ gradient powers glucose (SGLT2), amino acid, and phosphate cotransporters on the apical membrane.
Water follows Na⁺ passively through aquaporin-1 channels — the proximal tubule is always highly permeable to water.
Board pearl: Proximal tubule dysfunction (Fanconi syndrome) → glucosuria, aminoaciduria, phosphaturia, and Type 2 RTA.
Na⁺/H⁺ Exchange and Bicarbonate Reabsorption
The proximal tubule reclaims 80–90% of filtered HCO₃⁻ through H⁺ secretion via NHE3.
Mechanism: H⁺ combines with filtered HCO₃⁻ → H₂CO₃ → CO₂ + H₂O (via luminal carbonic anhydrase IV).
CO₂ diffuses into the cell → H₂CO₃ → H⁺ + HCO₃⁻ (via intracellular carbonic anhydrase II).
HCO₃⁻ exits basolaterally via Na⁺/HCO₃⁻ cotransporter while H⁺ is recycled to the lumen.
Board clue: Acetazolamide inhibits carbonic anhydrase → ↓HCO₃⁻ reabsorption → proximal (Type 2) RTA pattern with bicarbonaturia.
Glucose and Amino Acid Transport
Glucose reabsorption occurs via SGLT2 (90%) and SGLT1 (10%) on the apical membrane, using the Na⁺ gradient established by Na⁺/K⁺-ATPase.
Transport maximum (Tm) for glucose ~375 mg/min — when exceeded, glucosuria occurs despite normal blood glucose (e.g., pregnancy).
SGLT2 inhibitors (gliflozins) block glucose reabsorption → glucosuria → osmotic diuresis and modest natriuresis.
Amino acids use multiple Na⁺-coupled transporters with overlapping specificity.
Board pearl: Isolated glucosuria with normal glucose = benign familial renal glucosuria (SGLT2 mutation); generalized aminoaciduria = Fanconi syndrome.
Proximal Tubule Secretion
Organic anion transporters (OAT1, OAT3) and organic cation transporters (OCT2) on the basolateral membrane uptake drugs and metabolites from blood.
These compounds exit into tubular fluid via apical transporters: organic anion transporter polypeptides (OATPs) and multidrug resistance proteins.
Examples of secreted substances: penicillin, probenecid, furosemide, metformin, creatinine (10–20% of total excretion).
Competition for transporters explains drug interactions — probenecid blocks penicillin secretion, prolonging its half-life.
Board distinction: Proximal secretion is active; distal K⁺ secretion is passive down electrochemical gradients.
Loop of Henle: Countercurrent Multiplication
The loop creates and maintains the corticomedullary osmotic gradient essential for urine concentration.
Descending limb: permeable to water (via AQP1) but not solutes → water exits → tubular fluid becomes concentrated.
Ascending limb: impermeable to water but actively transports NaCl → tubular fluid becomes diluted.
Thick ascending limb (TAL) contains the Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2) — the site of loop diuretic action.
This single effect of separating solute from water, multiplied along the loop length, generates the osmotic gradient.
NKCC2 and the Thick Ascending Limb
NKCC2 reabsorbs 25–30% of filtered Na⁺ using the electrochemical gradient for Na⁺ to drive K⁺ and Cl⁻ uptake.
K⁺ recycles back to lumen via ROMK channels, maintaining a lumen-positive voltage that drives paracellular Ca²⁺ and Mg²⁺ reabsorption.
Loop diuretics (furosemide, bumetanide) block NKCC2 → massive natriuresis, K⁺ wasting, and impaired Ca²⁺/Mg²⁺ reabsorption.
Bartter syndrome: loss-of-function mutations in NKCC2, ROMK, or ClC-Kb → loop diuretic-like phenotype.
Board pearl: Loop diuretics cause hypokalemia, hypocalcemia, hypomagnesemia, and metabolic alkalosis.
Distal Convoluted Tubule: Fine-Tuning Begins
The early DCT reabsorbs 5–10% of filtered Na⁺ via the thiazide-sensitive Na⁺-Cl⁻ cotransporter (NCC).
Unlike the TAL, the DCT is impermeable to water — further diluting tubular fluid (diluting segment).
Active Ca²⁺ reabsorption occurs via TRPV5 channels (apical) and NCX/PMCA (basolateral) under PTH control.
Thiazides block NCC → mild natriuresis but enhanced Ca²⁺ reabsorption (opposite of loops).
Board distinction: Thiazides increase Ca²⁺ reabsorption (used for hypercalciuria); loops increase Ca²⁺ excretion (used for hypercalcemia).
Gitelman vs. Bartter Syndrome
Gitelman syndrome: NCC mutations → thiazide-like effect → hypokalemia, metabolic alkalosis, hypomagnesemia, hypocalciuria.
Bartter syndrome: NKCC2/ROMK/ClC-Kb mutations → loop diuretic-like effect → similar electrolytes but with hypercalciuria.
Key distinction: Check urinary calcium — low in Gitelman (like thiazides), high in Bartter (like loops).
Both present with salt wasting, volume depletion, and secondary hyperaldosteronism with paradoxically low/normal blood pressure.
Board pearl: Young patient with unexplained hypokalemia and metabolic alkalosis → check urine chloride and calcium to differentiate.
Collecting Duct: Final Adjustments
Principal cells: reabsorb Na⁺ via ENaC, secrete K⁺ via ROMK/BK channels — both processes stimulated by aldosterone.
α-intercalated cells: secrete H⁺ via H⁺-ATPase and H⁺/K⁺-ATPase, reabsorb K⁺ — acidify urine.
β-intercalated cells: secrete HCO₃⁻ via pendrin, reabsorb Cl⁻ — alkalinize urine in alkalosis.
Water permeability is regulated by ADH-dependent insertion of AQP2 channels in principal cells.
Board concept: The collecting duct determines final urine concentration, K⁺ balance, and urine pH.
ENaC and Potassium Secretion
ENaC creates a lumen-negative voltage by reabsorbing Na⁺ → drives K⁺ secretion through ROMK channels.
Factors increasing K⁺ secretion: ↑aldosterone, ↑distal Na⁺ delivery, ↑flow rate, ↑serum K⁺, alkalosis.
Amiloride and triamterene directly block ENaC → K⁺-sparing diuresis.
Spironolactone and eplerenone block mineralocorticoid receptors → indirect ENaC inhibition.
Board pearl: Any diuretic acting proximal to ENaC (loops, thiazides) causes K⁺ wasting by increasing distal Na⁺ delivery.
Aldosterone Actions in the Collecting Duct
Aldosterone binds cytoplasmic mineralocorticoid receptors → nuclear translocation → gene transcription.
Genomic effects (hours): ↑ENaC synthesis, ↑Na⁺/K⁺-ATPase, ↑ROMK channels, ↑mitochondrial enzymes.
Non-genomic effects (minutes): ↑ENaC membrane insertion, ↑channel open probability.
Net effect: ↑Na⁺ reabsorption, ↑K⁺ secretion, ↑H⁺ secretion → volume expansion, hypokalemia, metabolic alkalosis.
Board clue: Primary hyperaldosteronism → hypertension, hypokalemia, metabolic alkalosis with low plasma renin.
ADH and Water Reabsorption
ADH (vasopressin) binds V2 receptors on principal cells → cAMP → PKA → AQP2 phosphorylation and apical insertion.
Without ADH: collecting duct impermeable to water → dilute urine (50 mOsm/kg).
With maximal ADH: water reabsorption into hypertonic medulla → concentrated urine (1200 mOsm/kg).
Central DI: no ADH production; nephrogenic DI: kidney unresponsive to ADH (V2 receptor or AQP2 mutations, lithium).
Board pearl: Psychogenic polydipsia has low urine osmolality that corrects with water deprivation; DI does not correct.
Acid Secretion by Intercalated Cells
Type A (α) intercalated cells secrete H⁺ via apical H⁺-ATPase and H⁺/K⁺-ATPase.
For each H⁺ secreted, one HCO₃⁻ exits basolaterally via Cl⁻/HCO₃⁻ exchanger (AE1) → new bicarbonate generation.
Type 1 (distal) RTA: defective H⁺-ATPase → cannot acidify urine below pH 5.5 → hypokalemia, nephrolithiasis.
Type 4 RTA: aldosterone deficiency/resistance → ↓H⁺ and K⁺ secretion → hyperkalemia, mild acidosis.
Board distinction: Type 1 RTA = hypokalemia + alkaline urine; Type 4 RTA = hyperkalemia + mild acidosis.
Diuretic Mechanisms and Sites of Action
Acetazolamide: carbonic anhydrase inhibitor → ↓HCO₃⁻ reabsorption in proximal tubule → mild diuresis, metabolic acidosis.
Loop diuretics: block NKCC2 in TAL → profound natriuresis, K⁺/Ca²⁺/Mg²⁺ wasting, metabolic alkalosis.
Thiazides: block NCC in DCT → moderate natriuresis, K⁺ wasting, Ca²⁺ retention, metabolic alkalosis.
K⁺-sparing: block ENaC (amiloride) or aldosterone (spironolactone) in collecting duct → mild natriuresis, K⁺ retention.
Board pearl: Diuretic resistance in CHF → use sequential nephron blockade (loop + thiazide).
Tubular Transport Defects and Syndromes
Fanconi syndrome: global proximal tubule dysfunction → glucosuria, aminoaciduria, phosphaturia, Type 2 RTA.
Bartter syndrome: NKCC2 pathway defects → mimics chronic loop diuretic use.
Gitelman syndrome: NCC defects → mimics chronic thiazide use.
Liddle syndrome: gain-of-function ENaC mutation → Na⁺ retention, K⁺ wasting, hypertension with low aldosterone.
Gordon syndrome (PHAII): WNK kinase mutations → ↑NCC activity → hypertension, hyperkalemia, metabolic acidosis.
Integration: From Filtration to Final Urine
Glomerulus filters 180 L/day → proximal tubule reabsorbs 70% (iso-osmotic) → loop concentrates medulla and dilutes fluid → distal nephron fine-tunes under hormonal control.
Volume depletion → ↑renin → ↑aldosterone → ↑Na⁺ reabsorption in collecting duct.
Hyperosmolality → ↑ADH → ↑water reabsorption → concentrated urine.
Hyperkalemia → direct aldosterone release + ↑K⁺ secretion by principal cells.
Acidosis → ↑H⁺ secretion by intercalated cells + ↑NH₃ synthesis in proximal tubule.
Clinical Correlations and Calculations
Fractional excretion of Na⁺ (FENa) = (UNa × PCr)/(PNa × UCr) × 100 — distinguishes prerenal (<1%) from intrinsic renal (>2%) failure.
Transtubular K⁺ gradient (TTKG) = (UK/PK)/(Uosm/Posm) — assesses collecting duct K⁺ secretion.
Urine anion gap = UNa + UK − UCl — positive in RTA (impaired NH₄⁺ excretion), negative in diarrhea.
Free water clearance = V − (UNa + UK) × V/PNa — negative means net water reabsorption.
Board tip: These calculations help localize tubular dysfunction and guide diagnosis.
Board Question Stem Patterns
Polyuria + hyponatremia + low urine osmolality → psychogenic polydipsia (corrects with water restriction).
Polyuria + hypernatremia + low urine osmolality despite water deprivation → diabetes insipidus.
Hypokalemia + metabolic alkalosis + low urine Ca²⁺ → Gitelman syndrome or thiazide use.
Hypokalemia + metabolic alkalosis + high urine Ca²⁺ → Bartter syndrome or loop diuretic use.
Hyperkalemia + metabolic acidosis + hypertension → Gordon syndrome.
Hypokalemia + metabolic acidosis + alkaline urine → Type 1 RTA.
Glucosuria with normal blood glucose → SGLT2 defect or Fanconi syndrome.
One-Line Recap
The nephron's sequential segments — proximal tubule (bulk reabsorption via NHE3, SGLT2), loop of Henle (countercurrent multiplication via NKCC2), distal tubule (fine-tuning via NCC), and collecting duct (hormonal control via ENaC, AQP2) — transform glomerular filtrate through specific transporters that serve as drug targets and mutation sites, ultimately determining urine composition and volume.
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