Placental transport: a function of permeability and perfusion.

Studies in ovine fetus and placenta have pointed to an interaction between the fetal liver and the placenta. The supply of amino acids and carbohydrates depends on this interaction. These studies have led to clinical studies in normal and high-risk pregnancies. The objective of the present review was to compare changes in fetal circulation, in terms of both velocimetry and actual blood flow measurements, and to couple such data with data on the placental transport of amino acids. Flow studies were carried out on the umbilical vein with measurements of time-averaged velocity and venous diameter. A similar approach was used for measurements of ductus venosus flow. Stable-isotope-labeled amino acids were used to study placental transport by the non-steady state approach. The studies of flow showed a marked reduction in umbilical blood flow even when expressed per kilogram fetal body weight in fetal-growth-restricted pregnancies. This may be coupled with an increased ductus venosus shunt, the combination leading to a marked reduction in fetal hepatic blood flow. The placental transport of some amino acids is reduced in fetal-growth-restricted pregnancies. Furthermore, nonglucose carbohydrates and polyols are found in fetal blood, some in concentrations higher than maternal concentrations. There is significant uptake of several polyols and of mannose across the umbilical circulation in normal pregnancies. In conclusion, both perfusion and permeability can now be studied in both normal and high-risk pregnancies.