Inulin space studies in fetal sheep.

Extracellular volume was estimated in fetal sheep by measuring the 14C inulin space in 6 in situ unanesthetized fetal lambs at gestational ages ranging from 120 to 145 days (term = 150 days). The total inulin space ranged from 979 to 1,510 ml. A positive correlation between total inulin space and gestational age (r = 0.65) was noted, such that fetal lambs of 120 days gestation had a predicted inulin space of 1,072 ml and fetal lambs near term at 145 days gestation had an inulin space of 1,422 ml. However, since fetal weight increases with gestation, inulin space expressed as a percent of body weight actually decreased with gestational age (r = -0.80), so that at 120 days gestation the fetus would have an estimated inulin space of 59% of body weight and at 145 days the fetus would have an inulin space of only 34% of body weight.