Regulation of Acid/Base Balance

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Bicarbonate Excretion by the Kidney[edit]

  • The kidney filters 4,000 to 5,000 mmols/day bicarbonate
(calculated via plasma bicarbonate x GFR = 24 x 180 = 4,230 mmol/day)
  • Under normal conditions, filtered HCO3- is totally reabsorbed
  • 90% reabsorbed in the proximal tubule, the rest in the distal tubule
  • HCO3- reabsorption is crucial in acid-base balance as loss of bicarbonate has an acidifying effect
  • Reabsorption of HCO3- occurs indirectly:

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  • H2O and CO2 in the tubular endothelium combine to produce H+ (which is exchanged for Na+ into the tubules and combines with tubular HCO3-) and HCO3+ (which is transported back into the bloodstream)
  • CO2 formed in the tubule from HCO3- and H+ diffuses easily into the endothelium to continue the process of forming H+ and HCO3-
  • If plasma HCO3- is >28mmol/L, renal threshold is exceeded and bicarbonate appears in the urine
  • 4,000 - 5,000 mmol/day H+ ions are secreted each day, and without this accumulation and acidosis rapidly occurs

Metabolic Alkalosis

  • Acute metabolic alkalosis - the renal threshold for bicarbonate is exceeded and bicarbonate is rapidly excreted in the urine
  • This is rapidly corrected by the kidney which has a large capacity to excrete HCO3- by:
  • Allowing some filtered bicarbonate to pass through to the urine
  • Secreting bicarbonate via Type B intercalated cells.
  • Chronic metabolic alkalosis - there MUST be a mechanism present interfering with the kidney's ability to excrete bicarbonate, eg.
  • Reduced GFR, chloride depletion, potassium depletion and ECF volume depletion
  • Recovery requires correction of the primary cause, as well as correction of the abnormal factor maintaining the disorder

Excretion of Non-Bicarbonate Buffers[edit]

  • Every day 70–100mmol/day of fixed acids are excreted. This is the net excretion of H+ from the body, as secretion with bicarbonate doesn't actually remove H+ from the body
  • Mainly phosphoric acid, ketoacids, sulfuric acid
  • These arise from amino acid, carbohydrate metabolism and especially anaerobic metabolism
  • Non-bicarbonate buffers are either:
  • Filtered - most importantly phosphate
  • Not regulated for the purposes of acid excretion
  • Occurs in much the same way as bicarbonate is secreted
  • 40mmol/day of fixed acid is excreted this way
  • The ketones are important in buffering acidity in uncontrolled diabetes mellitus - they are filtered but because of their high concentration are only partially taken up, therefore are available in the distal nephron to buffer H+
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  • Synthesised - most importantly ammonia - is responsible for the balance of H+ excretion
  • Free ammonia is toxic → in liver converted to glutamine
  • The kidneys excrete acid by converting glutamine to bicarbonate and ammonium, excreting the ammonium, and returning the bicarbonate to the blood.
  • Crucial in renal acid secretion, as production can be increased in the presence of large acid loads
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