Metabolic Production of Fixed Acids Requires That the Kidneys Eliminate Hydrogen Ions and Conserve Bicarbonate
When fixed acids are added to the blood, for example, from protein metabolism, the hydrogen ions are buffered in part by HCO3-. Buffering results in the conversion of HCO3- to H2CO3 and CO2, which is eliminated from the lungs.
Fixed acids are produced continuously and would consume the body’s HCO3- if the kidneys were not continually regenerating HCO3-.The role of the kidneys in acid-base balance is described in Chapter 44. Large amounts of HCO3 are filtered daily through the glomerulus and subsequently reabsorbed in the renal tubule. The amount of HCO3- reabsorbed depends on the amount filtered, which is determined by the plasma concentration of HCO3-, the glomerular filtration rate, and the rate of Ht secretion by renal tubular cells. This last rate is controlled in part by the acid-base status of the body.
When Pco2 is high, the reaction:
within the renal tubules is driven to the right, producing more H* for secretion into the tubular lumen and HCO3 for return to the blood. When Pco2 is low, H* elimination and therefore HCO3" reabsorption decrease.
Ammonia, an important buffer in the distal renal tubule, is produced by the action of glutaminase on glutamine. In
acidosis, the activity of glutaminase increases, resulting in increased ammonia production, an increased buffering capacity of the renal tubular fluid and, therefore, increased ability to eliminate hydrogen ions.