AJP - Renal Physiology, Vol 251, Issue 2 226-F231, Copyright © 1986 by American Physiological Society
Factors affecting renal handling of sodium, hydrogen ions, and bicarbonate by the fetus
G. J. Kesby and E. R. Lumbers
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.
