Placental transport of nutrients to the fetus.

The exchange of nutrients between placenta and fetus involves three major mechanisms: (1) direct placental transfer of nutrients from the maternal to the fetal plasma; (2) placental metabolism and consumption of nutrients; (3) placental metabolism of nutrient substrates to alternate substrate forms. Carbohydrate is transported to the fetus as glucose which is taken up from the maternal plasma by the GLUT 1 transporter and transported to the fetus by facilitative diffusion according to concentration-dependent kinetics. Protein is transported to the fetus as amino acids by specific amino acid transporter proteins. Placental lipid transport to the fetus involves direct transporter mediated transfer of certain fatty acids as well as lipid uptake from lipoproteins, metabolic alteration in the placenta, and release into the fetal plasma. Placental size, architecture, developmental and pathological processes, and interaction with the fetus cooperate with transport and metabolic mechanisms to affect placental-fetal nutrient exchange.