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placenta |
Also found in: Dictionary/thesaurus, Medical, Wikipedia, Hutchinson | 0.04 sec. |
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placenta (pləsĕn`tə) or afterbirth, organ that develops in the uterus during pregnancy. It is a unique characteristic of the higher (or placental) mammals. In humans it is a thick mass, about 7 in. (18 cm) in diameter, liberally supplied with blood vessels. The placenta is attached to the uterus, and the fetus is connected to the placenta by the umbilical cord. The placenta draws nourishment and oxygen, which it supplies to the fetus, from the maternal circulation. In turn, the placenta receives the wastes of fetal metabolism and discharges them into the maternal circulation for disposal. It also acts as an endocrine gland, producing estrogen, progesterone, and gonadotrophin. Shortly after delivery of the fetus the placenta is forced out by contractions of the uterus. Severe hemorrhage may occur if the placenta does not emerge in its entirety or if the uterus fails to contract properly. placentaOrgan in most mammals that develops in the uterus along with a fetus to mediate metabolic exchange. The umbilical cord attaches it to the fetus at the navel. Nutrients and oxygen in the mother's blood pass across the placenta to the fetus, and metabolic wastes and carbon dioxide from the fetus cross in the other direction; the two blood supplies do not mix. Other substances (e.g., alcohol or drugs) in the mother's blood can also cross the placenta, with effects including congenital disorders and drug addiction in the newborn (see fetal alcohol syndrome); some microorganisms can cross it to infect the fetus, but so do the mother's antibodies. The placenta, weighing a pound or more at the end of pregnancy, is expelled at parturition. Some animals eat it as a source of nutrients; in some species this stimulates lactation. |
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? Mentioned in | ? References in periodicals archive | ||
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| Between 3 and 7 days afterbirth, macaque babies smacked their lips
and stuck out their tongues just as an experimenter did, the researchers
report in the September PLoS Biology. (16) Sir James
Frazer, writing in the early 20th century, noted that "even in
Europe many people still believe that a person's destiny is more or
less bound up with that of his navel-string or afterbirth. " As a narrative of affiliation,
incorporation, and adoption, the afterbirth story dramatizes the
cultural, intellectual, and psychological experience of replacing
"habits of feeling, thinking, and acting by another set of habits
which belonged to strangers . |
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