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Renal Regulation of Acid Base Balance

Renal regulation of acid base balance

  • The kidney has two main functions in relation to maintaining the acid base balance of the body
    • It reabsorbs filtered HCO3 (4000-5000 mmol/day)
    • It excretes fixed acids (1 mmol/kg/day)
  • Renal mechanism works in two aspects; the PCT mechanisms and the DCT mechanisms

 

PCT mechanisms

1) Bicarbonate reabsorption

  • Almost 85-90% of bicarbonate that is filtered with urine from the glomerulus is reabsorbed into the peritubular capillary pleuxes
  • Since bicarbonate reabsorption and hydronium ion secretion provide the same effect, ythey will be studied together
    • H+ leaves the PCT cell and enters the lumen of the Proximal convoluted tubule, under influence of aldosterone, in two ways – the Na+ H+ antiporter (RED) and the H+ ATPase proton pump (the Na+ H+ antiporter is more widely used in the PCT mechanism)
    • HCO3 in the glomerular filtrate cannot directly cross the cell membrane of the PCT cells. Therefore it combines with H+ to form CO2 and H2O (catalyzed by carbonic anhydrase enzyme). CO2 which is lipid soluble, enters the PCT cells. Inside the cells it joins with OH to forms bicarbonate again. This bicarbonate is sent out to the blood cell via a Na+ HCO3 symporter (GREEN).
    • At the same time, to keep intracellular Na+ concentration low, a Na+ K+ ATPase (BLUE) acts in the opposite direction. This maintains the concentration gradient for the Na+ H+ antiporter.

(The Na+ H+ antiporter is not electrogenic, meaning it transports equal amounts to both sides. But the Na+ HCO3 symporter is electrogenic, pumping one Na+ for every three HCO3. The Na+ K+ ATPase is also electrogenic in nature)

 

  • This system provides a net result of one bicarbonate ion reabsorbed and one Na+ ion reabsorbed for every one H+ ion secreted
  • Bicarbonate reabsorption also increases with
    1. Luminal bicarbonate concentration
    2. Luminal flow rate
  • Partial pressure of CO2 in arteries
  1. Angiotensin II levels
  • Bicarbonate reabsorption drops when parathormone levels increase. This is because parathormone can reduce angiotensin II levels

 

2) Ammonium production

  • Ammonium is mainly produced in the PCT cells from glutamine which is converted to ammonium by enzyme glutaminase
  • 80% of glutamate is obtained from the peritubular capillaries while the remaining 20% is obtained from the glomerular filtrate
  • Ammonium has a pH of 9.2. since this is a very high value, it is present entirely as ammonium in both extra cellular and inter cellular environments

 

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DCT mechanisms

  • Three main functions occur in the DCT to maintain acid base balance
    • Formation of titrable acidity : represents the H+ which is buffered by phosphates, creatinine and ketoacids.
    • Addition of ammonium to luminal fluid
    • Reabsorption of remaining bicarbonate
  • In the DCT, H+ is secreted mainly by a H+ ATPase (rather than a Na+ H+ antiport) while bicarbonate is absorbed by a HCO3 Cl- exchanger (rather than a Na+ HCO3 symporter)
  • Here too, the system provides a net result of one bicarbonate ion reabsorbed and one Na+ ion reabsorbed for every one H+ ion secreted
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Categorised in: Medicine, Nephrology, Physiology

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