Final answer:
The placenta, which develops due to the projections of the chorionic villi, facilitates the exchange of nutrients and waste between the mother and fetus. By weeks 14-16, placentation is complete, enabling nutrition and excretion, as well as respiratory and endocrine functions between the maternal and fetal bloodstreams.
Step-by-step explanation:
Once diffusion from the endometrium becomes inadequate for the delivery of oxygen, the placenta develops from projections of the chorionic villi, which facilitates exchange of nutrients and waste by the placenta.
The development of the placenta is crucial during the embryonic period and the first several weeks of the fetal period. By weeks 14-16, placentation is complete, and the placenta is a fully functional organ. It connects to the fetus through the umbilical cord, with the umbilical arteries bringing deoxygenated blood and wastes from the fetus to the placenta, while the umbilical vein carries oxygenated blood and nutrients from the pregnant person to the fetus.
Substances like oxygen and carbon dioxide move across the placental barrier by simple diffusion, while other substances such as glucose move across by facilitated diffusion. To meet the high demands for amino acids and iron, these are actively transported across the placenta from the maternal bloodstream.