Measurement of net transplacental transfer of fluid to the fetal sheep.

If fetal drinking activity is prevented and it is assumed that in the latter third of gestation the fetus is capable of maintaining itself in fluid balance, then the net amount of fluid gained across the placenta by the fetus is equal to the amount of fluid lost from the fetus, by routes other than the placenta, plus fluid deposited in growing tissues minus the amount of water produced as a result of oxidative metabolism. Net transplacental transfer of fluid to the fetus over a 3 h period was measured in eight chronically catheterized fetal sheep in which drinking activity was prevented by ligating the oesophagus. Urine and lung liquid flow rates were measured. In the latter third of gestation, these are the only significant sources of fluid loss from these fetuses during the 3 h experimental period. Water produced as a result of oxidative metabolism was calculated, as was the amount of fluid deposited in growing tissues during the course of the experiment. The weight of the fetus at the beginning of the experiment and the change in weight that occurred during the experiment was calculated by measuring the weight of the fetus at death (within 30 h) and applying an equation which describes the body weight-gestation age relationship for merino sheep. Net transplacental fluid transfer was 0.40 +/- 0.09 ml min-1 kg-1 (range 0.30-0.54 ml min-1 kg-1). Fetal urine flow rate averaged 0.30 +/- 0.11 ml min-1 kg-1. It was 72.8 +/- 10.0% of the volumes used to calculate net transplacental fluid transfer to the fetus. Lung liquid flow rate was 0.079 +/- 0.039 ml min-1 kg-1. It was 20.2 +/- 9.2% of the volumes used to calculate net fluid intake. The amount of fluid deposited as a result of tissue growth was 0.023 +/- 0.001 ml min-1 kg-1; it was 5.94 +/- 1.1% of the volumes used in the equation, while the production of water as a result of metabolism was 3.9 X 10(-3) ml min-1 kg-1 (Conrad & Faber, 1977) and constituted 1.01 +/- 0.22% of the volumes used in the equation. This method of measuring net transplacental fluid transfer to the fetus can be used to measure fetal fluid intake over relatively short periods of time. It also means that the effects of disturbances in maternal fluid and electrolyte balance on fluid transfer to the fetus can be studied and quantitated.