Countercurrent Mechanism and Concentration of Urine

Introduction

1. Countercurrent multiplier - the loop of Henle
2. Countercurrent exchanger - the vasa recta

Anatomical Basis

• About 20-25% of nephrons are juxtamedullary nephrons, with long loops of Henle that dip deep into the medulla, some reaching the tips of the renal papillae.
• Paralleling these loops are the vasa recta, the specialized peritubular capillaries that also form long U-shaped loops into the medulla.
• The collecting ducts pass through this hyperosmotic medulla en route to the renal pelvis and are essential to the final concentrating step.
• Guyton and Hall Textbook of Medical Physiology, p. ~372

Part 1: The Loop of Henle as a Countercurrent Multiplier

• High water permeability of the thin descending limb (via aquaporin-1), which is impermeable to solute.
• Active/secondary active transport of Na+, K+, Cl- out of the thick ascending limb, which is impermeable to water.
• Continuous flow of tubular fluid (inflow from proximal tubule, outflow to distal tubule).

1. Initially, fluid in both limbs and the interstitium is isosmotic (~300 mOsm/kg).
2. The Na+/Cl- pump in the thick ascending limb extrudes solute into the interstitium, raising it to about 400 mOsm/kg. This "single effect" creates only about a 200 mOsm/kg gradient between the ascending limb and the interstitium at any single horizontal level.
3. The thin descending limb, being water-permeable, equilibrates osmotically with this hypertonic interstitium - water leaves the descending limb, concentrating its contents.
4. As isotonic fluid continuously enters from the proximal tubule and flows down, and hypotonic fluid continuously leaves the ascending limb into the distal tubule, this process repeats along the length of the loop.
5. Because the two limbs are arranged in a countercurrent (hairpin) fashion, this single transverse effect is multiplied along the length of the loop, producing a much larger vertical (corticomedullary) gradient - progressively increasing osmolality from ~300 mOsm/kg at the cortex to 1200-1400 mOsm/kg at the papillary tip.

• Guyton and Hall Textbook of Medical Physiology, "Countercurrent Multiplier Mechanism"
• Ganong's Review of Medical Physiology, "The Countercurrent Mechanism"

1. Active transport of Na+ (with co-transport of K+, Cl-) out of the thick ascending limb into the interstitium.
2. Active transport of ions from the collecting ducts into the interstitium.
3. Facilitated diffusion of urea from the inner medullary collecting ducts into the interstitium (urea recycling contributes significantly to inner medullary osmolality).
4. Only limited diffusion of water into the interstitium relative to solute, so solute accumulates in excess of water.

Part 2: Vasa Recta as Countercurrent Exchangers

• Descending vasa recta: as blood descends into the increasingly hypertonic medulla, water diffuses out and solute (Na+, urea) diffuses in - blood becomes progressively more concentrated.
• Ascending vasa recta: as blood ascends back toward the cortex, the reverse occurs - solute diffuses out back into the interstitium and water re-enters the blood.
• Net effect: most of the solute that enters the descending limb is trapped and recycled back into the interstitium via the ascending limb rather than being carried away into systemic circulation. Blood flow through the vasa recta is also sluggish, which further limits solute washout.
• Only a small net amount of water and solute is actually removed from the medulla per unit time - just enough to balance what is added by the loops of Henle and collecting ducts, keeping the medullary gradient in a steady state.
• Guyton and Hall Textbook of Medical Physiology, "Countercurrent Exchange in Vasa Recta"
• Medical Physiology (Boron & Boulpaep) - "The vasa recta's countercurrent exchange and relatively low blood flow minimize washout of medullary hyperosmolality"

Part 3: Role of the Collecting Duct and ADH in Final Urine Concentration

• In the presence of ADH (vasopressin): Aquaporin-2 water channels are inserted into the luminal membrane of the principal cells of the cortical and medullary collecting ducts. As tubular fluid flows through the corticomedullary gradient, water is progressively reabsorbed by osmosis into the hypertonic interstitium and carried away by the vasa recta. Final urine osmolality can approach 1200-1400 mOsm/kg (concentrated urine, antidiuresis).
• In the absence of ADH: The collecting ducts remain relatively impermeable to water; dilute fluid delivered from the ascending limb (which is hypotonic, having lost solute without water) passes through largely unmodified, producing large volumes of dilute urine (as low as 50 mOsm/kg) - diuresis.
• Urea recycling: Facilitated urea transporters (UT-A1, UT-A3) in the inner medullary collecting duct, upregulated by ADH, allow urea to move into the medullary interstitium, adding to the osmotic gradient and being trapped/recycled via the thin descending limb (which is urea-permeable), further amplifying the concentrating capacity.
• Katzung's Basic and Clinical Pharmacology, 16th Ed., Fig 15-6
• Campbell-Walsh-Wein Urology, "Water"

Summary Table

Clinical Correlation

• Diabetes insipidus (central or nephrogenic) - failure of ADH action/AQP2 insertion, leading to large volumes of dilute urine.
• Loop diuretics (furosemide) - inhibit the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb, abolishing the single effect and hence the entire medullary gradient, producing marked diuresis.
• Medullary washout - occurs with excessive fluid intake, rapid diuresis, or loss of the vasa recta's exchange function (e.g., in renal papillary damage), impairing maximal urine concentrating ability.

Recent evidence check

Countercurrent Mechanism and Concentration of Urine

Introduction

1. Countercurrent multiplier - the loop of Henle
2. Countercurrent exchanger - the vasa recta

Anatomical Basis

• About 20-25% of nephrons are juxtamedullary nephrons, with long loops of Henle that dip deep into the medulla, some reaching the tips of the renal papillae.
• Paralleling these loops are the vasa recta, the specialized peritubular capillaries that also form long U-shaped loops into the medulla.
• The collecting ducts pass through this hyperosmotic medulla en route to the renal pelvis and are essential to the final concentrating step.
• Guyton and Hall Textbook of Medical Physiology, p. ~372

Part 1: The Loop of Henle as a Countercurrent Multiplier

• High water permeability of the thin descending limb (via aquaporin-1), which is impermeable to solute.
• Active/secondary active transport of Na+, K+, Cl- out of the thick ascending limb, which is impermeable to water.
• Continuous flow of tubular fluid (inflow from proximal tubule, outflow to distal tubule).

1. Initially, fluid in both limbs and the interstitium is isosmotic (~300 mOsm/kg).
2. The Na+/Cl- pump in the thick ascending limb extrudes solute into the interstitium, raising it to about 400 mOsm/kg. This "single effect" creates only about a 200 mOsm/kg gradient between the ascending limb and the interstitium at any single horizontal level.
3. The thin descending limb, being water-permeable, equilibrates osmotically with this hypertonic interstitium - water leaves the descending limb, concentrating its contents.
4. As isotonic fluid continuously enters from the proximal tubule and flows down, and hypotonic fluid continuously leaves the ascending limb into the distal tubule, this process repeats along the length of the loop.
5. Because the two limbs are arranged in a countercurrent (hairpin) fashion, this single transverse effect is multiplied along the length of the loop, producing a much larger vertical (corticomedullary) gradient - progressively increasing osmolality from ~300 mOsm/kg at the cortex to 1200-1400 mOsm/kg at the papillary tip.

• Guyton and Hall Textbook of Medical Physiology, "Countercurrent Multiplier Mechanism"
• Ganong's Review of Medical Physiology, "The Countercurrent Mechanism"

1. Active transport of Na+ (with co-transport of K+, Cl-) out of the thick ascending limb into the interstitium.
2. Active transport of ions from the collecting ducts into the interstitium.
3. Facilitated diffusion of urea from the inner medullary collecting ducts into the interstitium (urea recycling contributes significantly to inner medullary osmolality).
4. Only limited diffusion of water into the interstitium relative to solute, so solute accumulates in excess of water.

Part 2: Vasa Recta as Countercurrent Exchangers

• Descending vasa recta: as blood descends into the increasingly hypertonic medulla, water diffuses out and solute (Na+, urea) diffuses in - blood becomes progressively more concentrated.
• Ascending vasa recta: as blood ascends back toward the cortex, the reverse occurs - solute diffuses out back into the interstitium and water re-enters the blood.
• Net effect: most of the solute that enters the descending limb is trapped and recycled back into the interstitium via the ascending limb rather than being carried away into systemic circulation. Blood flow through the vasa recta is also sluggish, which further limits solute washout.
• Only a small net amount of water and solute is actually removed from the medulla per unit time - just enough to balance what is added by the loops of Henle and collecting ducts, keeping the medullary gradient in a steady state.
• Guyton and Hall Textbook of Medical Physiology, "Countercurrent Exchange in Vasa Recta"
• Medical Physiology (Boron & Boulpaep) - "The vasa recta's countercurrent exchange and relatively low blood flow minimize washout of medullary hyperosmolality"

Part 3: Role of the Collecting Duct and ADH in Final Urine Concentration

• In the presence of ADH (vasopressin): Aquaporin-2 water channels are inserted into the luminal membrane of the principal cells of the cortical and medullary collecting ducts. As tubular fluid flows through the corticomedullary gradient, water is progressively reabsorbed by osmosis into the hypertonic interstitium and carried away by the vasa recta. Final urine osmolality can approach 1200-1400 mOsm/kg (concentrated urine, antidiuresis).
• In the absence of ADH: The collecting ducts remain relatively impermeable to water; dilute fluid delivered from the ascending limb (which is hypotonic, having lost solute without water) passes through largely unmodified, producing large volumes of dilute urine (as low as 50 mOsm/kg) - diuresis.
• Urea recycling: Facilitated urea transporters (UT-A1, UT-A3) in the inner medullary collecting duct, upregulated by ADH, allow urea to move into the medullary interstitium, adding to the osmotic gradient and being trapped/recycled via the thin descending limb (which is urea-permeable), further amplifying the concentrating capacity.
• Katzung's Basic and Clinical Pharmacology, 16th Ed., Fig 15-6
• Campbell-Walsh-Wein Urology, "Water"

Summary Table

Clinical Correlation

• Diabetes insipidus (central or nephrogenic) - failure of ADH action/AQP2 insertion, leading to large volumes of dilute urine.
• Loop diuretics (furosemide) - inhibit the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb, abolishing the single effect and hence the entire medullary gradient, producing marked diuresis.
• Medullary washout - occurs with excessive fluid intake, rapid diuresis, or loss of the vasa recta's exchange function (e.g., in renal papillary damage), impairing maximal urine concentrating ability.

Recent evidence check

Countercurrent Mechanism and Concentration of Urine

Introduction

1. Countercurrent multiplier - the loop of Henle
2. Countercurrent exchanger - the vasa recta

Anatomical Basis

• About 20-25% of nephrons are juxtamedullary nephrons, with long loops of Henle that dip deep into the medulla, some reaching the tips of the renal papillae.
• Paralleling these loops are the vasa recta, the specialized peritubular capillaries that also form long U-shaped loops into the medulla.
• The collecting ducts pass through this hyperosmotic medulla en route to the renal pelvis and are essential to the final concentrating step.
• Guyton and Hall Textbook of Medical Physiology, p. ~372

Part 1: The Loop of Henle as a Countercurrent Multiplier

• High water permeability of the thin descending limb (via aquaporin-1), which is impermeable to solute.
• Active/secondary active transport of Na+, K+, Cl- out of the thick ascending limb, which is impermeable to water.
• Continuous flow of tubular fluid (inflow from proximal tubule, outflow to distal tubule).

1. Initially, fluid in both limbs and the interstitium is isosmotic (~300 mOsm/kg).
2. The Na+/Cl- pump in the thick ascending limb extrudes solute into the interstitium, raising it to about 400 mOsm/kg. This "single effect" creates only about a 200 mOsm/kg gradient between the ascending limb and the interstitium at any single horizontal level.
3. The thin descending limb, being water-permeable, equilibrates osmotically with this hypertonic interstitium - water leaves the descending limb, concentrating its contents.
4. As isotonic fluid continuously enters from the proximal tubule and flows down, and hypotonic fluid continuously leaves the ascending limb into the distal tubule, this process repeats along the length of the loop.
5. Because the two limbs are arranged in a countercurrent (hairpin) fashion, this single transverse effect is multiplied along the length of the loop, producing a much larger vertical (corticomedullary) gradient - progressively increasing osmolality from ~300 mOsm/kg at the cortex to 1200-1400 mOsm/kg at the papillary tip.

• Guyton and Hall Textbook of Medical Physiology, "Countercurrent Multiplier Mechanism"
• Ganong's Review of Medical Physiology, "The Countercurrent Mechanism"

1. Active transport of Na+ (with co-transport of K+, Cl-) out of the thick ascending limb into the interstitium.
2. Active transport of ions from the collecting ducts into the interstitium.
3. Facilitated diffusion of urea from the inner medullary collecting ducts into the interstitium (urea recycling contributes significantly to inner medullary osmolality).
4. Only limited diffusion of water into the interstitium relative to solute, so solute accumulates in excess of water.

Part 2: Vasa Recta as Countercurrent Exchangers

• Descending vasa recta: as blood descends into the increasingly hypertonic medulla, water diffuses out and solute (Na+, urea) diffuses in - blood becomes progressively more concentrated.
• Ascending vasa recta: as blood ascends back toward the cortex, the reverse occurs - solute diffuses out back into the interstitium and water re-enters the blood.
• Net effect: most of the solute that enters the descending limb is trapped and recycled back into the interstitium via the ascending limb rather than being carried away into systemic circulation. Blood flow through the vasa recta is also sluggish, which further limits solute washout.
• Only a small net amount of water and solute is actually removed from the medulla per unit time - just enough to balance what is added by the loops of Henle and collecting ducts, keeping the medullary gradient in a steady state.
• Guyton and Hall Textbook of Medical Physiology, "Countercurrent Exchange in Vasa Recta"
• Medical Physiology (Boron & Boulpaep) - "The vasa recta's countercurrent exchange and relatively low blood flow minimize washout of medullary hyperosmolality"

Part 3: Role of the Collecting Duct and ADH in Final Urine Concentration

• In the presence of ADH (vasopressin): Aquaporin-2 water channels are inserted into the luminal membrane of the principal cells of the cortical and medullary collecting ducts. As tubular fluid flows through the corticomedullary gradient, water is progressively reabsorbed by osmosis into the hypertonic interstitium and carried away by the vasa recta. Final urine osmolality can approach 1200-1400 mOsm/kg (concentrated urine, antidiuresis).
• In the absence of ADH: The collecting ducts remain relatively impermeable to water; dilute fluid delivered from the ascending limb (which is hypotonic, having lost solute without water) passes through largely unmodified, producing large volumes of dilute urine (as low as 50 mOsm/kg) - diuresis.
• Urea recycling: Facilitated urea transporters (UT-A1, UT-A3) in the inner medullary collecting duct, upregulated by ADH, allow urea to move into the medullary interstitium, adding to the osmotic gradient and being trapped/recycled via the thin descending limb (which is urea-permeable), further amplifying the concentrating capacity.
• Katzung's Basic and Clinical Pharmacology, 16th Ed., Fig 15-6
• Campbell-Walsh-Wein Urology, "Water"

Summary Table

Clinical Correlation

• Diabetes insipidus (central or nephrogenic) - failure of ADH action/AQP2 insertion, leading to large volumes of dilute urine.
• Loop diuretics (furosemide) - inhibit the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb, abolishing the single effect and hence the entire medullary gradient, producing marked diuresis.
• Medullary washout - occurs with excessive fluid intake, rapid diuresis, or loss of the vasa recta's exchange function (e.g., in renal papillary damage), impairing maximal urine concentrating ability.

Recent evidence check

Renal Tubular Function - Long Answers

1. Acidification of Urine (15 marks)

Introduction

1. Reclamation of filtered HCO3- (mainly proximal tubule) - does not by itself add new base to blood, just prevents loss of filtered bicarbonate.
2. Excretion of the fixed acid load / generation of new HCO3- (distal nephron), buffered in the urine as titratable acid and ammonium.

Proximal Tubular Bicarbonate Reabsorption

• The Na+-H+ exchanger (NHE3) on the luminal membrane secretes H+ into the lumen in exchange for Na+.
• Secreted H+ combines with filtered HCO3- to form H2CO3, rapidly converted to CO2 + H2O by carbonic anhydrase IV on the brush border.
• CO2 and H2O diffuse into the cell, where carbonic anhydrase II catalyzes their recombination to H2CO3, which dissociates to H+ and HCO3-.
• The regenerated intracellular HCO3- exits across the basolateral membrane via the Na+/HCO3- cotransporter (NBC-1) into the peritubular capillary blood.
• The H+ generated intracellularly recycles back through NHE3 to secrete another H+.

Distal Urinary Acidification (Generation of New HCO3-)

• H+ secretion here is mediated by H+-ATPase (and H+-K+-ATPase) on the luminal membrane of alpha-intercalated cells.
• Intracellular water dissociates to H+ and OH-; OH- combines with CO2 (via carbonic anhydrase II) to form HCO3-, which exits via the basolateral Cl-/HCO3- exchanger (AE1/band 3) into blood as "new" bicarbonate.
• For every H+ secreted and excreted, one new HCO3- ion is added to plasma.
• Beta-intercalated cells do the opposite (secrete HCO3- via a luminal exchanger, reabsorb H+) and predominate during alkalosis, allowing excretion of excess base.

Why Urinary Buffers Are Essential

(a) Titratable acid - phosphate buffer

• Dibasic phosphate (HPO4^2-), pKa 6.8, is filtered (30-40 mEq/day) and buffers H+ in the tubule fluid to form H2PO4-.
• Accounts for roughly half the daily fixed acid excretion.
• Called "titratable acid" because it is measured by titrating urine back to pH 7.4 with NaOH.
• Capacity is limited because filtered phosphate load is relatively fixed.

(b) Ammonium (NH4+) buffer

• NH4+ is generated in proximal tubule cells by deamination of glutamine -> glutamate -> alpha-ketoglutarate, yielding 2 NH4+ and 2 new HCO3- per glutamine molecule.
• NH4+ substitutes for K+ on the NKCC2 transporter and enters the medullary interstitium in the thick ascending limb; it dissociates to NH3, which diffuses (via the Rhesus glycoprotein RhCG on alpha-intercalated cells) into the inner medullary collecting duct lumen, where it is trapped as NH4+ (because low luminal pH favors NH4+ over NH3 - "diffusion trapping").
• Normally excretes 30-40 mEq/day but can increase up to 200 mEq/day during metabolic acidosis - this is the kidney's major adaptive reserve for handling acid loads.
• Falls in chronic kidney disease (GFR < 45 mL/min) contributing to the non-anion-gap metabolic acidosis of CKD.

Quantifying Net Acid Excretion

Regulation of Acidification

• Pco2 / pH: acidosis stimulates H+-ATPase activity and ammoniagenesis; alkalosis suppresses them and increases beta-intercalated cell activity.
• Aldosterone: stimulates H+-ATPase in alpha-intercalated cells (and Na+ reabsorption via ENaC, which favors electrogenic H+ secretion).
• Potassium status: hypokalemia stimulates ammoniagenesis and H+ secretion; hyperkalemia suppresses both.
• Angiotensin II: activates NHE3, enhancing proximal Na+/HCO3- reabsorption independent of serum bicarbonate level (e.g., in low effective arterial blood volume states).

Clinical Correlation - Renal Tubular Acidosis (RTA)

• National Kidneys Foundation Primer on Kidney Diseases, 8e, "Acid-Base Homeostasis and the Kidney"
• Smith and Tanagho's General Urology, 19th Ed., "Classic Renal Tubular Acidosis Type I"

2. Reabsorption of Sodium (Na+) (15 marks)

Introduction

Segment-wise Mechanism

Proximal Convoluted Tubule (~65-67% of filtered Na+)

• Early PCT: Na+ enters via cotransport with glucose (SGLT), amino acids, phosphate, and via the Na+-H+ exchanger (NHE3) (linked to HCO3- reclamation).
• Na+-K+-ATPase pumps Na+ out basolaterally; Cl- and water follow passively (paracellular, via tight junctions and AQP-1) to maintain electroneutrality and osmotic equilibrium - isosmotic reabsorption.
• A small amount of Na+, solute, and water re-enters the lumen ("backleak") through leaky tight junctions.
• Glomerulotubular balance: the fraction of filtered Na+ reabsorbed here stays constant (~65-67%) despite changes in GFR, due to peritubular capillary oncotic/hydrostatic pressure changes and load-dependent transport.
• Carbonic anhydrase inhibitors (acetazolamide) reduce Na+/HCO3- reabsorption here.

Thick Ascending Limb of Loop of Henle (~25%)

• Na+, K+, and 2Cl- enter together via the Na-K-2Cl cotransporter (NKCC2) on the luminal membrane - electroneutral but generates a lumen-positive potential (via K+ backleak through ROMK) that drives paracellular reabsorption of Mg2+ and Ca2+.
• This segment is water-impermeable, so solute is removed without water - this is the "single effect" that underlies the countercurrent multiplier and generates the medullary hypertonicity.
• Loop diuretics (furosemide) inhibit NKCC2, causing the most potent diuresis of any diuretic class; genetic loss of function causes Bartter syndrome.

Distal Convoluted Tubule (~5%)

• Na+ and Cl- enter via the electroneutral Na+-Cl- cotransporter (NCC).
• Inhibited by thiazide diuretics; loss-of-function mutation causes Gitelman syndrome.
• This segment is also impermeable to water, further diluting tubular fluid.

Collecting Duct - Principal Cells (~2-3%, the "fine-tuning" segment)

• Na+ enters through the epithelial sodium channel (ENaC), an electrogenic process that makes the lumen electronegative, which in turn drives K+ secretion (via ROMK) and H+ secretion by adjacent intercalated cells.
• ENaC expression/trafficking to the membrane is upregulated by aldosterone (acting via mineralocorticoid receptors) and inhibited by atrial natriuretic peptide (ANP).
• Amiloride/triamterene (potassium-sparing diuretics) block ENaC directly; spironolactone/eplerenone block the mineralocorticoid receptor.

Hormonal and Physical Regulation of Overall Na+ Reabsorption

• National Kidneys Foundation Primer on Kidney Diseases, 8e, "The Cellular Basis for Tubular Sodium Reabsorption"
• Guyton and Hall Textbook of Medical Physiology, "Passive Water Reabsorption Coupled to Sodium Reabsorption"

3. Reabsorption of Glucose (15 marks)

Introduction

Site and Mechanism

1. Luminal (apical) membrane: Na+-glucose cotransporters (SGLT).
   • SGLT2 - early proximal tubule (S1 segment): low affinity, high capacity, 1 Na+ : 1 glucose - reabsorbs about 90% of filtered glucose.
   • SGLT1 - later proximal tubule (S3 segment): high affinity, low capacity, 2 Na+ : 1 glucose - mops up the remaining ~10%.
   • Two Na+ (or one, for SGLT2) bind with glucose to the carrier; the protein rotates and releases Na+ and glucose into the cell - glucose moves against its own concentration gradient using the energy of the downhill Na+ gradient.
2. Basolateral membrane: glucose exits into peritubular blood by facilitated diffusion (down its gradient, no energy needed) via GLUT2 (paired with SGLT2 cells) and GLUT1 (paired with SGLT1 cells).
3. Na+-K+-ATPase on the basolateral membrane maintains the low intracellular Na+ that powers the whole process - hence this is termed secondary active transport.

Tubular Maximum (Tm) and the Glucose Titration Curve

• Reabsorption is transporter-limited. As plasma glucose rises, the filtered load rises proportionally, and reabsorption increases linearly - until the SGLT carriers become saturated at the transport maximum (TmG), approximately 375 mg/min in men, 300 mg/min in women.
• Beyond TmG, any additional filtered glucose is excreted unchanged - glucose appears in urine (glucosuria) and excretion rises linearly with plasma glucose.
• Splay: in reality, glucose starts appearing in urine at a plasma level somewhat below the theoretical Tm/GFR value, because not all nephrons have identical Tm and some glomerulotubular mismatch occurs - this rounding of the titration curve near Tm is called "splay."

Renal Threshold

• Theoretical threshold = TmG / GFR = 375/125 = 300 mg/dL.
• Actual/practical renal threshold is much lower, about 180-200 mg/dL arterial plasma (≈160-180 mg/dL venous), because of splay and because some nephrons with lower Tm start spilling glucose earlier.

Glucosuria

1. Plasma glucose exceeds the renal threshold - classically uncontrolled diabetes mellitus, the commonest cause.
2. Reduced Tm despite normal plasma glucose - renal glucosuria (familial renal glycosuria), due to inherited SGLT2 (or rarely SGLT1) mutations; benign, glucose spills at normal or even low plasma levels.
3. Pregnancy (increased GFR, mild reduction in Tm), Fanconi syndrome (generalized proximal tubular dysfunction).

Clinical Relevance - SGLT2 Inhibitors

• Ganong's Review of Medical Physiology, 26th Ed., "Glucose Reabsorption"
• Costanzo Physiology, 7th Ed., "Cellular Mechanism for Glucose Reabsorption" and "Glucose Titration Curve and Tm"

4. Reabsorption of Water (H2O) (15 marks)

Introduction

Segment-wise Water Handling

Proximal Tubule (~65%)

• Highly water permeable at all times because of abundant aquaporin-1 (AQP-1) in luminal and basolateral membranes, plus leaky paracellular tight junctions.
• Water reabsorption is essentially obligatory and isosmotic - it follows Na+/solute reabsorption so closely that tubular fluid osmolality barely changes along this segment.
• Solvent drag also carries some solutes with the bulk water flow.

Descending Thin Limb of Loop of Henle

• Also highly water permeable (AQP-1), impermeable to solute - equilibrates osmotically with the increasingly hypertonic medullary interstitium as it descends, so water leaves and luminal fluid becomes progressively more concentrated (up to ~1200 mOsm/kg at the papillary tip).

Ascending Limb of Loop of Henle

• Essentially water-impermeable (thin and thick portions) regardless of ADH - active/passive NaCl extrusion here without water removal is the "single effect" of the countercurrent multiplier, producing dilute fluid that enters the distal tubule (hyposmotic, ~100 mOsm/kg).

Distal Tubule and Collecting Duct - the "Facultative" Segment

• Water permeability here is variable, controlled by antidiuretic hormone (ADH/vasopressin).
• ADH binds V2 receptors on principal cells, triggering a cAMP-mediated cascade that inserts aquaporin-2 (AQP-2) channels into the luminal membrane (basolateral membrane already has constitutive AQP-3/AQP-4).
• With high ADH, water moves by osmosis from the dilute tubular fluid into the hypertonic medullary interstitium (built by the countercurrent multiplier) and is carried away by the vasa recta - urine becomes concentrated (up to 1200-1400 mOsm/kg).
• Without ADH, the collecting duct remains water-impermeable, and the dilute fluid delivered from the ascending limb passes through largely unmodified - large volumes of dilute urine (as low as 50 mOsm/kg) result.

Role of the Countercurrent System (covered previously in detail)

• The loop of Henle acts as a countercurrent multiplier, generating the medullary osmotic gradient (300 -> 1200-1400 mOsm/kg).
• The vasa recta act as countercurrent exchangers, preserving this gradient by minimizing solute washout.
• This gradient is the "osmotic sink" into which water moves from the collecting duct under ADH's influence - water reabsorption in the final segment is entirely dependent on the gradient generated upstream.

Regulation of ADH / Water Reabsorption

• Osmoreceptors in the hypothalamus (most sensitive regulator) - a rise of just 1% in plasma osmolality stimulates ADH release.
• Baroreceptors/volume receptors - a fall in blood volume/pressure of >10% stimulates ADH (less sensitive but overrides osmotic control in severe hypovolemia).
• Other stimuli: pain, stress, nausea, certain drugs (nicotine) increase ADH; alcohol suppresses it.

Clinical Correlate

• Osmotic diuretics (mannitol) reduce water reabsorption by raising tubular fluid osmotic pressure, opposing the normal gradient.
• Failure of this system (ADH deficiency or resistance) is the basis of diabetes insipidus, discussed below.
• Guyton and Hall Textbook of Medical Physiology, "Passive Water Reabsorption Coupled to Sodium Reabsorption"
• Brenner and Rector's The Kidney, "Aquaporin-2: The Vasopressin-Sensitive Collecting Duct Water Channel"

5. Short Note: Diabetes Insipidus

Definition

Types and Causes

• Causes: head trauma, pituitary/hypothalamic tumors, surgery, hemorrhage, infection, granulomatous disease, idiopathic, congenital/genetic forms.

• Causes: lithium (most common acquired cause - interferes with AQP-2 expression/trafficking), hypercalcemia, hypokalemia, chronic kidney disease, genetic mutations in the V2 receptor (AVPR2) or AQP2 gene.

Pathophysiology

• In central DI, no ADH is available to act on V2 receptors of principal cells, so AQP-2 is not inserted into the luminal membrane - collecting duct stays water-impermeable.
• In nephrogenic DI, ADH is present but the receptor or channel machinery fails to respond.
• Either way, dilute tubular fluid delivered from the ascending limb passes through the distal nephron largely unmodified, producing large volumes of dilute urine and progressive dilution as fluid descends further; urine osmolality can fall to 50-100 mOsm/kg.

Clinical Features

• Polyuria (often >3 L/day, occasionally 15-20 L/day) and compensatory polydipsia.
• Low urine specific gravity/osmolality.
• If thirst mechanism is intact and water intake keeps pace, plasma osmolality stays near normal; if thirst is impaired or water is inaccessible, dehydration and hypernatremia (with risk of irreversible brain injury) can develop.

Diagnosis

• Confirm hypotonic polyuria (low urine osmolality despite normal/high plasma osmolality).
• Water deprivation test: fluid restriction fails to concentrate urine appropriately in DI.
• Desmopressin (DDAVP) challenge: distinguishes the two types -
  • Central DI: urine osmolality rises significantly (>50%) after DDAVP (kidney responds normally once ADH is supplied).
  • Nephrogenic DI: little or no rise in urine osmolality after DDAVP (kidney cannot respond regardless of ADH).
• Plasma copeptin (a stable ADH surrogate) is increasingly used in modern diagnostic algorithms.
• Must be differentiated from primary (psychogenic) polydipsia, where excessive water intake itself suppresses ADH - here plasma osmolality tends to be low-normal rather than high-normal, and water deprivation does allow urine concentration (with more prolonged testing).

Treatment

• Central DI: desmopressin (DDAVP), a synthetic ADH analog with selective V2 activity, given intranasally, orally, or parenterally.
• Nephrogenic DI:
  • Treat/remove the underlying cause (stop lithium if possible).
  • Thiazide diuretics (paradoxically reduce urine volume) - cause mild volume contraction, enhancing proximal Na+/water reabsorption and reducing distal delivery of fluid to the diluting segments; also partially restore AQP-2 expression in lithium-induced NDI.
  • Dietary sodium/protein restriction reduces solute load and osmotic diuresis.
  • Amiloride can be added in lithium-induced NDI (blocks lithium entry into collecting duct cells via ENaC).
  • NSAIDs (indomethacin) can reduce urine volume by inhibiting renal prostaglandins that normally antagonize ADH action; acetazolamide has also shown benefit.
• Katzung's Basic and Clinical Pharmacology, 16th Ed., "Diabetes Insipidus"
• Goldman-Cecil Medicine, International Edition, "Diabetes Insipidus"