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pl. placentae [L. placenta, flat cake]
The oval or discoid spongy structure in the uterus from which the fetus derives its nourishment and oxygen.
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placental (plă-sent′ăl)
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, adj.

The placenta consists of a fetal portion, the chorion frondosum, bearing many chorionic villi that interlock with the decidua basalis of the uterus, which constitutes the maternal portion. The chorionic villi lie in spaces in the uterine endometrium, where they are bathed in maternal blood and lymph. Groups of villi are separated by placental septa forming about 20 distinct lobules called cotyledons.

Attached to the margin of the placenta is a membrane that encloses the embryo. It is a composite of several structures (decidua parietalis, decidua capsularis, chorion laeve, and amnion). At the center of the concave side is attached the umbilical cord through which the umbilical vessels (two arteries and one vein) pass to the fetus. The cord is approx. 50 cm (20 in) long at full term.

The mature placenta is 15 to 18 cm (6 to 7 in) in diameter and weighs about 450 gm (approx. 1 lb). When expelled following parturition, it is known as the afterbirth.

Maternal blood enters the intervillous spaces of the placenta through spiral arteries, branches of the uterine arteries. It bathes the chorionic villi and flows peripherally to the marginal sinus, which leads to uterine veins. Food molecules, oxygen, and antibodies pass into fetal blood of the villi; metabolic waste products pass from fetal blood into the mother's blood. Normally, there is no admixture of fetal and maternal blood. The placenta is also an endocrine organ. It produces chorionic gonadotropins, the presence of which in urine is the basis of one type of pregnancy test. Estrogen and progesterone are also secreted by the placenta.

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