Placenta (original) (raw)

placenta

[plah-sen´tah] (pl. placentas, placen´tae) (L.)

an organ characteristic of true mammals during pregnancy, joining mother and offspring, providing endocrine secretion and selective exchange of soluble bloodborne substances through apposition of uterine and trophoblastic vascularized parts. See also . adj., adj placen´tal.

In anatomic nomenclature the placenta consists of a uterine and a fetal portion. The chorion, the superficial or fetal portion, is surfaced by a smooth, shining membrane continuous with the sheath of the umbilical cord (amnion). The deep, or uterine, portion is divided by deep sulci into lobes of irregular outline and extent (the cotyledons). Over the maternal surface of the placenta is stretched a delicate, transparent membrane of fetal origin. Around the periphery of the placenta is a large vein (the marginal sinus), which returns a part of the maternal blood from the organ.

The major function of the placenta is to allow diffusion of nutrients from the mother's blood into the fetus's blood and diffusion of waste products from the fetus back to the mother. This two-way exchange takes place across the placental membrane, which is semipermeable; that is, it acts as a selective filter, allowing some materials to pass through and holding back others.

In the early months of pregnancy the placenta acts as a nutrient storehouse and helps to process some of the food substances that nourish the fetus. Later, as the fetus grows and develops, these metabolic functions of the placenta are gradually taken on by the fetal liver.

The placenta secretes both estrogens and progesterone. After birth of the infant the placenta is cast off from the uterus and expelled via the birth canal.

placenta accre´ta one abnormally adherent to the myometrium, with partial or complete absence of the decidua basalis.

battledore placenta one with the umbilical cord inserted at the edge.

placenta circumvalla´ta one encircled with a dense, raised, white nodular ring, the attached membranes being doubled back over the edge of the placenta.

placenta fenestra´ta one that has spots where placental tissue is lacking.

placenta incre´ta placenta accreta with penetration of the myometrium.

placenta membrana´cea one that is abnormally thin and spread over an unusually large area of the myometrium.

placenta percre´ta placenta accreta with invasion of the myometrium to the peritoneal covering, sometimes causing rupture of the uterus.

placenta pre´via low implantation of the placenta so that it partially or completely covers the cervical os. Percentages are used to designate the amount of obstruction; e.g., 100 per cent is total placenta previa, and 50 per cent indicates that about half the opening is obstructed. The condition occurs with greater frequency in women who have had multiple pregnancies or are over 35. The exact cause is not known.

With the onset of any contractions and cervical dilation, or when the cervix begins to dilate at the onset of labor and the upper and lower uterine segments differentiate, the placenta is stretched and pulled from the uterine wall, producing bleeding. The bleeding usually is abrupt and painless and may stop on its own. However, if it continues it can be life-threatening for the mother since it is maternal blood that is being lost. The life of the fetus is in jeopardy because of anoxia resulting from separation of the placenta from its blood supply.

Diagnosis can be established by ultrasonography or radiologic placentography. Once diagnosis is made, treatment will depend on the gestational age of the fetus and the percentage of placenta covering the cervical os. Cesarean delivery is recommended if 30 per cent or more of the opening is obstructed by the placenta. If there is minimal bleeding that stops on its own, the fetus is not in distress, and if the gestational age is such that continuing the pregnancy is necessary for delivery of a viable fetus, the pregnancy may be continued under careful monitoring in the hospital, or at home if the mother is able to stay in bed. However, if the life of the mother or fetus is threatened by continued and excessive bleeding, delivery is indicated.

Vaginal examinations are carried out in an operating room so that if hemorrhage does occur as a result of manipulation of the uterus, a cesarean section can be done immediately to remove the placenta, stop the bleeding, and deliver the child safely.

Patient Care. Premature separation of the placenta is an emergency. The maternal signs are monitored every 15 minutes and blood loss is evaluated. Fetal heart tones also are monitored to detect fetal distress. The amount of bleeding is estimated and documented. Oxygen equipment should be at hand in the event signs of fetal distress indicate anoxia.

Postpartal hemorrhage and infection are more likely in women who have had placenta previa. Placement of the placenta in the lower segment predisposes to more bleeding because that portion of the uterus does not contract as forcefully as the upper segment. Additionally, the misplaced placenta has enlarged its bed to compensate for its poor location, so that there is a larger denuded area after delivery of the placenta. The same denuded area is also more susceptible to infection because it is located near the cervical opening where infectious organisms may enter.

Vaginal bleeding during pregnancy or labor is frightening for the mother. She will need reassurance and frequent explanations of what is happening to her throughout the period of monitoring and delivery. Some emotional stress can be alleviated by encouraging the mother to be aware of fetal movements and allowing her to listen to normal fetal heart sounds.

placenta reflex´a one in which the margin is thickened, appearing to turn back on itself.

placenta spu´ria an accessory portion without blood vessels connecting it with the main placenta.

placenta succenturia´ta an accessory portion with an artery and a vein connecting it with the main placenta.

pla·cen·ta

(plă-sen'tă),

Fetomaternal organ of metabolic interchange between the embryo or fetus and mother. It has a portion of embryonic origin, derived from a highly developed area of the outermost embryonic membrane (chorion frondosum), 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 disc shaped, about 4 cm in thickness and 18 cm in diameter, and averages about 1/6-1/7 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.

[L. a flat cake]

Farlex Partner Medical Dictionary © Farlex 2012

placenta

(plə-sĕn′tə)

n. pl. placen·tas or placen·tae (-tē)

a. A membranous vascular organ that develops in female eutherian mammals during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord. Following birth, the placenta is expelled.

b. A similar organ in marsupial mammals, consisting of a yolk sac attached to the uterine wall.

c. An organ with similar functions in some nonmammalian animals, such as certain sharks and reptiles.

2. Botany The part within the ovary of a flowering plant to which the ovules are attached.

pla·cen′tal adj.

pla·cen·ta

(plă-sen'tă)

Fetomaternal organ of metabolic interchange between embryo or fetus and mother. It has a portion of embryonic origin, derived from the outermost embryonic membrane (villous chorion), 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 permeable to permit the absorption of nutritive materials, oxygen, and some harmful substances, such as viruses, into the fetal blood, and the release of carbon dioxide and nitrogenous waste from it. At term, the human placenta is disc shaped, about 4 cm thick 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 usually 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.

[L. a flat cake]

placenta

The part of the early developing EMBRYO, that differentiates to form an organ attached to the lining of the womb and provides a functional linkage between the blood supplies of the mother and the fetus. This allows for the passage of oxygen and nutrients from the mother to the fetus. The placenta is connected to the fetus by the UMBILICAL CORD and is discharged from the womb after the birth of the baby. Also known as afterbirth.

Fig. 253 Placenta . Section showing blood circulation.

placenta

(in animals) the structure formed from the tissues of embryo and mother at the point of attachment of the embryo to the mother, through which the embryo is nourished. In placental mammals the embryonic blood supply may be from the ALLANTOIS (allantoic placenta) or the yolk sac (yolk sac placenta). Small molecules of oxygen and food materials pass to the embryo through the placenta, and urea and CO2 pass out from the embryo. The placenta itself produces the hormones OESTROGEN, PROGESTERONE and GONADOTROPHIC HORMONE. At birth the foetus is ejected from the uterus first, the umbilical cord broken and the placentalafterbirth is discharged last, except in abnormal circumstances. See Fig. 253 .
(in plants) a part of the wall of the ovary on which OVULES are borne.

Placenta

The organ that allows interchange between the fetus and the mother. Blood from the fetus and the mother do not directly mix, but the thin placental membrane allows the fetus to absorb nutrients and oxygen from the mother. Waste products from the fetus can exit through the placenta.

Mentioned in: Childbirth, Cytomegalovirus Infection, Erythroblastosis Fetalis, Fetal Alcohol Syndrome, Gestational Diabetes, Human Chorionic Gonadotropin Pregnancy Test, Hydatidiform Mole, Maternal to Fetal Infections, Pelvic Ultrasound, Placenta Previa, Placental Abruption, Polyhydramnios and Oligohydramnios, Preeclampsia and Eclampsia, Pregnancy, Premature Rupture of Membranes, Prenatal Surgery, Sickle Cell Disease, Thalassemia, Triple Screen

pla·cen·ta

(plă-sen'tă)

Fetomaternal organ of metabolic interchange between embryo or fetus and mother.

[L. a flat cake]