The role of the placenta in nutrition of the human fetus.

Bidirectional transport across the placenta provides substrates required for fetal nutrition and prevents accumulation of metabolites in the fetal compartment. Transmission of compounds across the placental barrier occurs by diffusion and carrier-mediated transport. The technique of dual in vitro perfusion of an isolated cotyledon has provided important data for the human placenta. The high degree of permeability for hydrophilic molecules was recently confirmed by in vivo measurements that allow estimates of flux across the human placenta for a number of compounds. The bidirectional nature in transplacental exchange requires asymmetry in flux from the maternal to the fetal side or vice versa, which can be assured by a concentration gradient or an active transport system. There is increasing evidence that placental metabolism with active modification of the composition of nutrients released to the fetus in addition to transport plays an important role.