The two principal structural elements that regulate the rights of transplacental transfer are 1st) the maternal facing trophoblast microvillus membrane and 2nd the fetal facing trophoblast basal membrane. The net transfer of a substance from the maternal to the fetal circulation follows the Fick equation. Functionally, the unidirectional flux of a substance is affected by 1st the rate of delivery to the placenta, 2nd the concentration difference between the mother and the fetus, 3rd the interaction of a substance with the placenta and 4th the relationship of maternal and fetal blood flows.

Some substances interact with cellular membrane proteins that enhance the rate of transfer. Glucose which is present in higher concentrations in the maternal circulation than in the fetal circulation is transferred by facilitated diffusion involving a specific carrier protein. Neutral amino acids which also are present in higher concentrations in the maternal circulation than in the fetal circulation are actively transported across the placenta along a concentration gradient.

Calcium and phosphorus which are present in higher concentrations in the fetal circulation than in the maternal circulation are actively transported against a concentration gradient. Free fatty acids which are greater in the mother than in the fetus and which are lipid-soluble, easily diffuse passively along a concentration gradient. The respiratory gases oxygen and carbon dioxide are exchanged by simple diffusion resulting from concentration differences between mother and fetus.

The fetus is able to obtain sufficient oxygen at a relatively low oxygen tension because 1st fetal blood has 50 percent more oxygen-carrying hemoglobin than maternal blood and 2nd at a given partial pressure of oxygen fetal hemoglobin can carry 20 to 30 percent more oxygen than adult hemoglobin. Gas exchange is affected by uterine and umbilical blood flows and geometry.