Urinary and hemodynamic responses to blood volume changes in fetal sheep

Because of the potential importance of the fetal kidney in regulating blood volume and amniotic fluid volume, and their composition, we studied renal and cardiovascular responses to changes in blood volume in 12 chronically-catheterized fetal sheep at 110-135 days gestation. During the hour following volume expansion with 50 ml of maternal whole blood, blood pressure rose by 14%, cardiac output by 11%, renal blood flow by 68% and urine output by 36%. Packed red cell volume rose from 31.3 to 34.0%, suggesting a net fluid loss from the circulation through the kidneys, as well as through peripheral and placental capillaries. During the hour following volume depletion by haemorrhage of 70-120 ml, blood pressure fell by 17%, cardiac output by 29%, renal flow by 46%, glomerular filtration rate by 19%, and urine output by 76%, compared to the preceding phase of the study. The urine-to-plasma osmolality ratio approached unity and urine sodium and U/Pcreatinine increased sharply. Following haemorrhage, fluid rapidly entered the circulation as evidenced by falling packed red cell volume and plasma protein concentration. The data support the hypothesis that the fetal kidney, although immature in some respects, effectively responds to maintain blood volume in the fetal sheep.