Placental transport provides a means of supplying nutrients to and removing metabolites from the fetus. Transport is based on substrate exchange and net flux from mother to fetus or vice versa and can be a result of a concentration difference or of unidirectional carrier-mediated transport. Blood flow regulates delivery to and removal from the area of placental exchange, and rapidly crossing compounds are dependent on blood flow for their rate of passage. There are substantial species differences in terms of flow rates normalized for fetal weight and also in terms of vascular arrangement. The barrier can be overcome via paracellular water-filled channels or via a transcellular route. Hydrophilic molecules that are not actively transported diffuse through paracellular channels, and the placentae of rodents and primates are much more permeable than the placenta of the sheep. Many different substrates such as glucose, amino acids, electrolytes and vitamins are transported by carrier systems. Transport proteins are located in the microvillous and basal membranes of the trophoblast. Asymmetry in the kinetics of binding results in differences in influx and efflux at the interface with maternal and fetal blood, allowing directional net flux across the placenta. Immunoglobulins are believed to cross by receptor-mediated endocytosis.
