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.

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