Fluid and Electrolyte Balance Question And Answers

Water, Electrolyte, And Acid-Base Balance

Explain the role of the kidney in acid-base balance
Answer:

Role of Kidney in Acid-Base Balance:

• The renal regulation of pH occurs by the following mechanism

1. Excretion of H+ ions

• Carbonic acid is formed from carbon dioxide & water by carbonic anhydrase
• Carbonic acid then dissociates to H+ & bicarbonate ions
• H+ ions are secreted into the tubular lumen in exchange for Na+
• Na+ along with bicarbonate ions is reabsorbed
• Bicarbonate ions add up to the alkali reserve, while H+ combines with a noncarbonate base & is excreted in urine

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2. Reabsorption of bicarbonate

• Bicarbonate diffuses from the plasma into the tubular lumen
• Here it combines with H+ to form carbonic acid
• By carbonic anhydrase enzyme, it dissociates to carbon dioxide & water
• Carbon dioxide diffuses into the tubular cells
• Here it again combines with water & forms carbonic acid
• Then it dissociates into H+ & bicarbonate ions
• H+ is secreted into the lumen in exchange for Na+
• The bicarbonate ion is reabsorbed into plasma along with Na+

3. Excretion of titrable acid

• H+ ion is secreted into the tubular lumen in exchange for Na+ ion
• Na+ is formed from disodium hydrogen phosphate
• This combines with H+ & forms sodium dihydrogen phosphate
• As the tubular fluid moves down the renal tubules, more & more H+ ions are added
• This causes acidification of urine

4. Excretion of ammonium ions

• H+ ions secreted into tubular fluid combines with NH3 to form an ammonium ion
• In the tubular cells, deamidation of glutamine to glutamate & NH3 occurs by the enzyme glutaminase
• NH3 diffuses into the tubular lumen where it combines with H+ to form an ammonium ion
• Ammonium ions cannot diffuse back into tubular cells & therefore are excreted into the urine.