Factors affecting renal handling of sodium, hydrogen ions, and bicarbonate by the fetus

Renal excretion of acid and reabsorption of bicarbonate was studied in 17 chronically catheterized fetal sheep aged 121-143 days. The rates of excretion of titratable acid (0.16-6.2 mumol/min) and ammonium (1.2-9.7 mumol/min) were variable. Urinary phosphate excretion was significantly greater (P less than 0.001) than the excretion of titratable acid. Of the filtered bicarbonate load 80-100% was reabsorbed. In 9 of the 17 fetuses net acid excretion was positive. Bicarbonate, sodium, and chloride reabsorption were related to glomerular filtration rate (GFR) (P less than 0.0005). The increase of GFR results in an increase in the excretion of titratable acid (P less than 0.001), phosphate (P less than 0.0005), and ammonium (P less than 0.001). These relationships could account for the age-dependent increase in renal excretion of acid (P less than 0.0005), ammonium (P less than 0.025), and bicarbonate reabsorption (P less than 0.0005). Arterial pH affected the rates of excretion of titratable acid (P less than 0.005), ammonium ions (P less than 0.05), and net acid (P less than 0.025). It is concluded that the fetal kidneys can excrete protons and generate bicarbonate. This ability increases with age due mainly to the concomitant increase in GFR.