Figure 2. A cartoon showing the location of transporter classes within the MVM and BM of the placental syncytiotrophoblast, and the factors that determine amino acid transfer.

A, maternal, placental and fetal metabolism, blood flow and transport control the gradients which determine transporter activity. B, accumulative transporters mediate uptake until their accumulative potential is reached. Functionally this means their activity is affected by low extracellular concentrations and that higher extracellular concentrations above the V _max will no longer increase uptake. C, exchangers will mediate net influx of abundant external substrates in exchange for efflux of relatively higher abundance intracellular substrates. This means that transfer of one substrate will decrease the levels of another. Within physiological limits this will be independent of concentration. D, facilitated transporters on the BM will mediate efflux of substrates down the concentration gradient built up by other transporters. Fetal consumption will increase the concentration gradient and increase transfer to the fetus.