Comparison of renal function in term fetal sheep and newborn lambs

Renal function was measured in fetal sheep, in neonates following delivery by caesarean section, and in lambs in the first week of life. The most marked changes in renal function following delivery were an increase in glomerular filtration rate from 4.59 +/- 0.27 ml/min (n = 13) to 6.94 +/- 1.00 ml/min (n = 12), a decrease in urine flow, sodium and osmolar excretion rates and an increase in urinary osmolality. Fractional sodium reabsorption increased from 95.5 +/- 0.79% (n = 13) in the fetus to 99.4 +/- 0.14% (n = 12, p less than 0.001) in lambs aged 24 h or more; fractional osmolar reabsorption rose from fetal levels of 92.3 +/- 0.84% (n = 13) to 96.5 +/- 0.52% (n = 11; p less than 0.001) in lambs aged 24 h or more. Potassium excretion and fractional potassium reabsorption did not alter after birth. Urinary osmolality increased from 175 +/- 20.4 mosm/kg H2O (n = 14) to 524 +/- 45.6 mosm/kg H2O (n = 16; p less than 0.001) in lambs aged 24 h or more, and free water clearance decreased from 0.308 +/- 0.06 ml/min (n = 13) to -0.067 +/- 0.03 ml/min (n = 15; p less than 0.001). Since the above changes occurred within 24 h of delivery, they represent rapid adjustments by the kidney to lack of a placental supply of fluid and electrolytes.