Mechanisms of fetal blood volume restoration after slow fetal hemorrhage.

Previous studies have shown that blood volume returns to normal within 24-48 h after hemorrhage in the adult of several species, whereas vascular volume restoration occurs within 3-4 h after a 2-h hemorrhage of the sheep fetus. The purpose of the present study was to determine whether an increase in fetal osmolality and/or circulating plasma protein mass contribute to the rapid volume restoration in the fetus. Nine chronically catheterized fetal sheep at 132 +/- 2 days gestation (term = 145-150 days) were hemorrhaged 30.9 +/- 2.2% of their initial blood volume over 2 h. Blood volume returned to normal 3 h after the hemorrhage. No change in fetal osmolality or in the fetal-maternal osmotic gradient occurred during or after the hemorrhage. However, there was a near-perfect correlation between blood volume and circulating plasma protein mass during and after the hemorrhage. Thus the present study provides no evidence for either a fetal cellular or transplacental source of fluid during fetal posthemorrhage volume restoration because osmolality was unchanged. Instead it appears that a rapid redistribution of proteins and fluid from the fetal interstitial space most likely mediates the vascular volume restoration.