How does the kidney compensate for acidosis?
The body tries to minimize pH changes and responds to acid-base disturbances with body buffers, compensatory responses by the lungs and kidney (to metabolic and respiratory disturbances, respectively) and by the kidney attempting to correct for a metabolic disturbances. Show
Metabolic disturbancesIn primary metabolic acid-base disorders, the body responds initially through plasma and interstitial or extracellular buffers. Bicarbonate is the dominant buffer in a primary metabolic acidosis. It takes a little longer (hours) for intracellular buffering with hydrogen shifts on proteins to occur, but this starts to happen as well. We would also expect to find immediate compensatory changes in alveolar ventilation and pCO2 (i.e. secondary respiratory compensation). With time, the kidney should act to correct the metabolic disturbance, particularly after we fix the primary problem or if the primary problem was self-limiting or mild. There is some evidence that cats cannot correct a primary metabolic disturbance like dogs can (Di Bartola). Note, respiratory compensation will not return pH to normal in a primary metabolic acidosis. This requires cessation of the originating cause and medical or surgical intervention, as required, followed by renal correction. The following can be used as a guide to estimate the expected respiratory compensatory response to primary metabolic acid-base disturbances in dogs (it is unclear if this formula works for other species, but it is all we have): pCO2 (expected) = pCO2 (normal) + ([HCO3–(measured) – HCO3–(normal)] x 0.7) ± X where, X=3 for a metabolic acidosis or X=2 for a metabolic alkalosis. Note that some authors use a correction factor of 1.2 versus 0.7. Alternatively, the maximum respiratory compensation expected in a primary metabolic acidosis in dogs is: pCO2 (expected) = 1.5 [HCO3–] + 8. Response to a primary metabolic acidosisThe body’s response to a primary metabolic acidosis consists of body buffers and a compensatory respiratory alkalosis. Renal acid excretion (ammoniagenesis, vacuolar-type H+-ATPase activity) serves to attempt to correct the acidosis, but requires time (days) and a functioning kidney, as well as cessation of the primary problem (and respiratory compensatory response). There are also species differences in the body’s response to a metabolic acidosis. As indicated above, the kidney in cats, in general, are not as good as responding to the acidosis as well as dogs.
Response to a primary metabolic alkalosisRegulation of acid-base through renal ammoniagenesis Reclaimation of filtered bicarbonate A primary metabolic alkalosis is initially offset by body buffers (not bicarbonate) and a compensatory respiratory acidosis. Increased renal excretion of bicarbonate (through increased filtration of the bicarbonate through the glomeruli and active excretion of bicarbonate by type B intercalated cells in the collecting tubules of the distal nephron) and can rapidly correct the alkalosis, particularly when mild or self-limiting. This corrective response of the kidney to a primary metabolic alkalosis is frequently impaired, as a consequence of the primary disease process causing the alkalosis, which results in acid and not base excretion by the kidney, thus worsening the alkalosis.
The kidney and perpetuation of metabolic alkalosis These factors (which result in a paradoxic aciduria) are:
Respiratory disturbancesThe body’s response to primary respiratory acid-base disorders involves intracellular buffers followed by altered renal excretion of acid. There is little extracellular buffering capability for carbon dioxide (i.e. carbon dioxide moves rapidly across cell membranes and bicarbonate cannot buffer increases in carbon dioxide). Because of this, the body cannot offset an acute respiratory acidosis as well as a more chronic respiratory acidosis, where renal compensatory mechanisms kick in. This is why the factor in the equation below (Y) is low for acute and higher for chronic. Note, that the respiratory system cannot correct itself, i.e. the lungs cannot correct for a primary respiratory disturbance (unlike the kidney, which can correct a primary metabolic disturbance), unless the primary disturbance was mild and self-limiting (e.g. an anticipatory respiratory alkalosis occurs in sled dogs before a race due to excitement-induced hyperventilation). Usually, the veterinarian has to intervene to help the lungs correct a primary respiratory acid-base abnormality. The following formulae can be used as guides to determine the expected metabolic compensatory responses to primary respiratory acid-base disorders: HCO3– (expected) = HCO3–(normal) + ([pCO2(measured) – pCO2 (normal)] x Y) ± 2 where, Response to a primary respiratory acidosisIntracellular buffer response to a primary respiratory acidosis A primary respiratory acidosis is initially offset by intracellular buffers, followed by increased renal acid excretion.
Response to a primary respiratory alkalosisIntracellular buffer response to a respiratory alkalosis A primary respiratory alkalosis is initially offset by body buffers, followed by decreased renal acid excretion.
Compensation summary
The table is a graphic representation of these concepts. As can be seen, the change in the primary parameter (HCO3– for metabolic and pCO2 for respiratory) is paralleled by the compensatory response.
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How the kidneys are able to compensate for an acidosis?To restore acid-base balance, the kidneys initiate a complex set of responses that include increased ammoniagenesis and gluconeogenesis, enhanced acid and ammonium ion excretion, and a net production and release of bicarbonate ions.
How do kidneys compensate for acidosis and alkalosis?In respiratory acidosis, the kidney produces and excretes ammonium (NH4+) and monophosphate, generating bicarbonate in the process while clearing acid. In respiratory alkalosis, less bicarbonate (HCO3−) is reabsorbed, thus lowering the pH.
What happens to the kidney during acidosis?Renal tubular acidosis (RTA) occurs when the kidneys do not remove acids from the blood into the urine as they should. The acid level in the blood then becomes too high, a condition called acidosis. Some acid in the blood is normal, but too much acid can disturb many bodily functions. There are three main types of RTA.
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