Any acid-base disturbance resulting from an abnormality of the extracellular fluid concentration of HCO3– is termed a metabolic disorder, whereas an acid-base disturbance resulting from a change in pCO2 is a respiratory disorder.
Metabolism of carbohydrates, fats etc. within the body generates a continuous amount of H+. This is constantly being buffered by various bases, which therefore need to be regenerated.
In the case of bicarbonate – the primary buffer system in the body – it is continuously reabsorbed by the kidneys in exchange for producing acidic urine: H+ is actively secreted into the tubular lumen, forming H2CO3, then CO2 and H2O. The CO2 then diffuses back into the tubule cells recombining with water to form H2CO3 and then H+ and HCO3–. The HCO3– passes back into the blood stream whilst the H+ is actively passed back into the tubular lumen. In effect, it is the active excretion of H+ in the kidney which facilitates HCO3– reabsorption (see figure, right. Click to see a larger version).
85% of HCO3– is reabsorbed by the proximal tubule, 10% by the loop of Henle and the remainder by the distal tubule.
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