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Summary
Much of our view of the human placenta is inferred from animal experiments. Flow limited
transfer is a regime of transfer in which there is complete equilibration of maternal
and fetal bloods in a single pass through the placental exchange vessels. Placental
exchange of non-protein bound lipid soluble materials is flow limited and the exchange
of oxygen is very nearly flow limited. The relatively inefficient human placenta requires
greater perfusion rates than some animal placentas do for the same rate of oxygen
transfer but it offers considerable protection should one (but not both) of the placental
blood flows decrease. The evolution of the placental vascular arrangement and of the
difference in half saturation pressures of maternal and fetal hemoglobin point to
the necessity to protect the fetus against high oxygen pressures. Wash-in and wash-out
rates of non-protein bound anesthetics appear to be almost exclusively governed by
the maternal and fetal placental flow rates and the distribution volume in the conceptus.
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© 1995 Pearson Professional Ltd. Published by Elsevier Inc.
