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DISTINCTION-LEVEL NOTES: MBBS 1st Year Physiology - Kidney

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PART 1: ROLE OF COUNTERCURRENT MECHANISM IN THE FORMATION OF CONCENTRATED URINE

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Introduction

1. A high level of ADH (antidiuretic hormone / vasopressin)
2. A hyperosmotic renal medullary interstitium - created by the countercurrent mechanism

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Anatomical Basis

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A. Countercurrent Multiplier Mechanism (Loop of Henle)

The key insight: Each "single effect" (200 mOsm gradient) is multiplied by the countercurrent flow of fluid to generate a far greater axial gradient along the medulla. This is the "multiplier."

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B. Role of Urea in Concentrating Urine

• Cortex/outer medulla tubules are impermeable to urea
• High ADH increases water reabsorption in collecting ducts → urea becomes concentrated
• The inner medullary collecting duct has UT-A1 and UT-A3 urea transporters (upregulated by ADH) → urea diffuses into medullary interstitium
• Urea recycles: diffuses into thin descending limb of loop of Henle and re-enters collecting ducts → "urea recycling" builds up medullary urea

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C. Countercurrent Exchange Mechanism (Vasa Recta)

• Blood descending into medulla: encounters progressively hypertonic interstitium → solutes enter, water exits → blood becomes concentrated (~1200 mOsm/L at tip)
• Blood ascending back to cortex: solutes exit, water re-enters → blood becomes dilute again

• Medullary blood flow is only <5% of total renal blood flow - sluggish flow minimizes washout
• Increased medullary blood flow (e.g., prostaglandins, calcium channel blockers) reduces concentrating ability

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D. Role of ADH in Forming Concentrated Urine

• V2 receptors on principal cells of collecting duct
• Activates adenylyl cyclase → ↑cAMP → PKA activation
• Inserts aquaporin-2 (AQP-2) water channels into apical membrane
• Aquaporin-3 and -4 are constitutively present on basolateral side

1. Cortical collecting duct becomes permeable to water → large water reabsorption here (most water is absorbed in cortex, preserving medullary gradient)
2. Medullary collecting duct: fluid equilibrates with ~1200 mOsm/L interstitium → small but concentrated urine produced
3. ADH also increases UT-A1/A3 urea transporter expression → more urea recycling

• Glomerular filtrate: 300 mOsm/L
• End of proximal tubule: 300 mOsm/L (isosmotic)
• Tip of descending loop of Henle: ~1200 mOsm/L
• End of ascending loop: ~100-140 mOsm/L (always dilute regardless of ADH!)
• End of early distal tubule: ~100 mOsm/L
• Final urine (collecting duct): ~1200 mOsm/L (concentrated)

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E. Summary: Requirements for Concentrated Urine

1. Intact loop of Henle (especially long loops of juxtamedullary nephrons)
2. ADH secretion and functional ADH receptors (V2)
3. Functional AQP-2 insertion
4. Intact urea recycling (UT-A1, UT-A3)
5. Low medullary blood flow through vasa recta
6. Adequate GFR and tubular flow

• Lack of ADH → Diabetes insipidus (central)
• V2 receptor mutations → Nephrogenic diabetes insipidus
• Loss of medullary architecture (chronic pyelonephritis, sickle cell disease)
• Loop diuretics (furosemide blocks NKCC2) → ablate medullary gradient
• High medullary blood flow → wash out gradient

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PART 2: MECHANISM OF ACIDIFICATION OF URINE

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Introduction

1. Reabsorbing filtered HCO₃⁻ (predominantly proximal tubule)
2. Excreting H⁺ ions (as titratable acid and as ammonium)

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Sites of H⁺ Secretion

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A. Mechanism in the Proximal Tubule (HCO₃⁻ Reabsorption)

1. CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ (catalyzed by intracellular carbonic anhydrase II)
2. H⁺ is secreted into the lumen via the Na⁺/H⁺ exchanger (NHE3) - driven by Na⁺ gradient maintained by basolateral Na⁺/K⁺-ATPase
3. In the lumen, secreted H⁺ combines with filtered HCO₃⁻: H⁺ + HCO₃⁻ → H₂CO₃ → CO₂ + H₂O (catalyzed by brush border carbonic anhydrase IV)
4. CO₂ diffuses back into the cell
5. Intracellular HCO₃⁻ exits via basolateral Na⁺-HCO₃⁻ cotransporter (NBC-1) into peritubular capillaries

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B. Mechanism in the Distal Tubule and Collecting Duct (True Acidification)

TUBULAR LUMEN     |  INTERCALATED CELL  |  PERITUBULAR BLOOD
                  |                     |
                  | CO₂ + H₂O           |
                  |    ↓ (CA II)        |
                  | H₂CO₃              |
                  |    ↓                |
       H⁺ ←——— | H⁺  + HCO₃⁻         |
    (H⁺-ATPase)  |                     | HCO₃⁻ exits via
                  |                     | Cl⁻/HCO₃⁻ exchanger (AE1)

1. Inside Type A intercalated cells: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ via carbonic anhydrase II
2. H⁺-ATPase (electrogenic proton pump) on apical membrane actively secretes H⁺ against steep gradient into tubular lumen
3. HCO₃⁻ exits via Cl⁻/HCO₃⁻ exchanger (AE1/Band 3 protein) on basolateral membrane into the blood
4. H⁺/K⁺-ATPase can also secrete H⁺ (relevant in hypokalemia and potassium retention states)

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C. Buffers that Carry Excreted H⁺ (Titratable Acidity and Ammonium)

1. Phosphate Buffer (Titratable Acid)

• Filtered phosphate: HPO₄²⁻ (pKa = 6.8)
• In the tubular lumen: H⁺ + HPO₄²⁻ → H₂PO₄⁻
• H₂PO₄⁻ cannot be reabsorbed and is excreted in urine
• This accounts for ~30-40 mEq of H⁺ excreted per day
• Called "titratable acidity" because it can be measured by back-titrating urine to pH 7.4

2. Ammonium (NH₄⁺) - Quantitatively Most Important

• Glutamine → 2NH₄⁺ + 2HCO₃⁻ (via glutaminase and glutamate dehydrogenase)
• HCO₃⁻ enters blood (replenishes plasma HCO₃⁻ - the "new bicarbonate")
• NH₄⁺ is secreted into tubular lumen via NHE3 (NH₄⁺ substitutes for H⁺)

• NH₄⁺ is reabsorbed from the loop of Henle (substitutes for K⁺ on NKCC2)
• NH₄⁺ accumulates in the medullary interstitium
• NH₃ (lipid-soluble) diffuses into collecting duct lumen

• H⁺ secreted by Type A intercalated cells combines with NH₃ in lumen: NH₃ + H⁺ → NH₄⁺ (trapped - cannot diffuse back, "diffusion trapping")
• NH₄⁺ is excreted in urine

• Low pH (direct stimulation of glutaminase)
• Angiotensin II (stimulates proximal tubule NH₄⁺ production)
• Aldosterone (stimulates distal H⁺ secretion)

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D. Net Acid Excretion - Summary Formula

Net Acid Excretion = Titratable Acid + NH₄⁺ - HCO₃⁻ in urine

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E. Factors Regulating Urine Acidification

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F. Renal Tubular Acidosis (Clinical Correlate)

• Urine pH >5.5 despite systemic acidosis = hallmark of Type 1 RTA (failed distal acidification)

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Quick Revision - Key Points for Exams

• Multiplier = Loop of Henle; Exchanger = Vasa recta
• Thick ascending limb: NKCC2 pumps NaCl, impermeable to water → creates 200 mOsm gradient
• Descending limb: permeable to water only → equilibrates with interstitium
• Countercurrent flow multiplies the gradient to 1200 mOsm/L
• ADH inserts AQP-2 in collecting duct → water follows gradient → concentrated urine
• Urea (via UT-A1/A3) contributes 40-50% of medullary osmolarity

• Proximal tubule: NHE3 secretes H⁺, reclaims ~80% filtered HCO₃⁻ (carbonic anhydrase IV on brush border)
• Collecting duct: H⁺-ATPase in type A intercalated cells → true acidification
• Buffers: Phosphate (titratable acid) + NH₃/NH₄⁺ (ammonium trapping)
• Maximum urine pH = 4.5
• Ammonium is the most important and regulatable buffer system
• Aldosterone stimulates both distal H⁺ secretion and K⁺ excretion

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