Placenta

organ of metabolic interchange between fetus and mother

Description
The placenta has a portion of embryonic origin, derived from the outermost embryonic membrane (chorion frondosum or villous chorionic sac), and a maternal portion formed by a modification of the part of the uterine mucosa (decidua basalis) in which the chorionic vesicle is implanted. Within the placenta, the chorionic villi, with their contained capillaries carrying blood of the embryonic circulation, are exposed to maternal blood in the intervillous spaces in which the villi lie; no direct mixing of fetal and maternal blood occurs, but the intervening tissue (the placental membrane) is sufficiently thin to permit the absorption of nutritive materials, oxygen, and some harmful substances, like viruses, into the fetal blood and the release of carbon dioxide and nitrogenous waste from it. At term, the human placenta is disk-shaped, about 4 cm in thickness and 18 cm in diameter, and averages about one-sixth to one-seventh the weight of the fetus; its fetal surface is smooth, being formed by the adherent amnion, with the umbilical cord normally attached near its center; the maternal surface of a detached placenta is rough because of the torn decidual tissue adhering to the chorion and shows lobular elevations called cotyledons or lobes.