Renal Regulation Of Acid-Base Balance

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 HCO3 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% HCO3 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 HCO3 absorption.
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