Renal Regulation Of Acid-Base Balance
RENAL REGULATION OF ACID-BASE BALANCE
H+ SECRETION & HCO3– REABSOPRTION:
- HCO3– reabsorption involves H+ tubular secretion.
- One HCO3– reabsorbed→ One H+must be secreted.
- Occurs in all parts of tubules.
- Except descending & ascending thin limbs of Henle loop.
- Rate varies with different tubular segments.
- In Proximal Tubule – 80%-90% HCO3– reabsorption.
- In thick ascending limb of Henle loop, distal tubule & collecting duct – 10 %.
MECHANISM:
1. IN EARLY TUBULAR SYSTEM:
- H+ tubular secretions –
- Mainly in PCT, thick ascending limb of Henle loop & early DCT.
- Mediated by Na2+ – H+ exchanger (“Sodium & hydrogen countertransport”).
- Energy – From Na2+-K+-ATPase on basolateral membrane.
- Hence, “Secondary active transport”.
STEPS:
- H2 CO3 dissociates into CO2 & H2O by carbonic anhydrase.
- Doesn’t result in very high tubular fluid H+conc.
- Very acidic tubular fluid only in collecting tubules & ducts.
ENZYME INVOLVED:
Carbonic anhydrase:
- Present in brush border of PCT, thick ascending limb & early DCT.
- Also in gastric mucosa, exocrine pancreas, ciliary body of eye, erythrocytes, & brain.
- Maximum – In Erythrocytes.
- Absent – In plasma.
IN DISTAL TUBULES:
- In late distal tubule & collecting ducts,
- H+ ions secreted by “Primary active transport”.
- Mainly by “Hydrogen -Transporting ATPase”.
- Occurs in “Intercalated (I) cells”.
Steps:
Main difference bt. H+ secretion & HCO3– reabsorption in proximal & distal tubule:
1. H+ moves across luminal membrane by “Primary active transport”.
- Instead of counter-transport (as in early parts).
2. H+ secretion in late distal & collecting tubules – 5% of total H+ secreted.
- Mechanism is important in maximally acidic urine.
3. H+ concentrating ability can be increased as much as 900-fold in collecting tubules.
- Decreases tubular fluid pH to 4.5.
- Lower pH limit achieved in normal kidney.
FATE OF H+ IN TUBULAR FLUID:
- Removed by urinary buffers in tubular fluid.
Important buffers:
1. Bicarbonate buffer:
- Present in proximal segments (Proximal tubules & Thick ascending limb).
- Most important buffer in proximal tubules.
2. Phosphate buffer:
- In distal tubules.
3. Ammonia buffers:
- Most important active form of buffering mechanism in distal tubule.
- Ammonia & phosphate buffers are excreted, in exchange for HCO–3 absorption.
Mechanism:
NEW BICARBONATE GENERATION:
- Every H+ secreted → HCO3– is either reabsorbed/new HCO3– generated & absorbed.
Exam Question
RENAL REGULATION OF ACID-BASE BALANCE
- In Proximal Tubule – 80%-90% HCO–3 reabsorption.
- H+ is secreted into the tubular fluid by Na2+ – H+ exchanger.
- H2 CO3 dissociates into CO2 & H2O by carbonic anhydrase.
- Carbonic anhydrase:
- Present in brush border of PCT, thick ascending limb, early DCT, & brain.
- Maximum – In Erythrocytes; Absent – In plasma.
- In late distal tubule & collecting ducts, H+ ions are secreted by primary active transport in “Intercalated (I) cells”.
- This mechanism is important in maximally acidic urine.
- H+ concentrating ability can be increased as much as 900-fold in the collecting tubules.
- Decreases tubular fluid pH to about 4.5, which is lower pH limit achieved in normal kidney.
- Bicarbonate buffer – Most important buffer in proximal tubule.
- Ammonia buffers – Most important buffer indistal tubule.
- Ammonia & phosphate buffers are excreted, in exchange for HCO–3 absorption.
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