Definition of The Placenta

An organ of pregnancy that nourishes and sustains the fetus. The placenta also secretes a number of hormones that maintain the biochemical environment within the woman’s body to support the pregnancy.

The placenta develops within the first two weeks after the blastocyst implants into the endometrium of the uterus, arising from the outer layer of the blastocyst’s cells, the trophoblast. The amniotic sac, which encloses the developing fetus, and the umbilical cord also arise from the trophoblast.

ActivinChorionic adrenocorticotropin
Chorionic gonadotropinChorionic somatomammotropin
Corticotropin-releasing hormone (crh)Cortisol
EstrogensGrowth hormone–releasing hormone (ghrh)
Gonadotropin-releasing hormone (gnrh)Placental actinogen
ProgesteroneThyrotropin-releasing hormone (trh)

The placenta uniquely belongs to both the mother and the fetus. Though the maternal blood circulation delivers nutrients and oxygen to the fetal blood circulation and carries away fetal wastes, the two circulations do not normally mix with each other.

The side of the placenta that faces the fetus is the chorion. Fringelike extensions called the chorionic villi permeate the tissue of the maternal portion of the placenta. Fetal blood circulates through the chorionic villi. Arterioles (tiny arteries) and venules (tiny veins) extend from the myometrium (muscular wall of the uterus) into the spaces between the chorionic villi.

The arterioles carry maternal blood into the spaces where it circulates around the chorionic villi. Nutrients, oxygen, and wastes pass across the thin membranes that enclose the chorionic villi.

Problems that can arise with the placenta during pregnancy include

  • Placenta abruptio (also called placental abruption), in which the placenta partially or completely separates from the uterus; partial separation reduces nutrition to the fetus and complete separation is fatal to the fetus
  • Placenta accreta, in which the tissues that anchor the placenta to the wall of the uterus penetrate the myometrium too deeply, making it difficult for the placenta to separate after birth
  • Placenta previa, in which the placenta grows partially or completely across the cervix, necessitating cesarean section to prevent hemorrhage during labor

After the fetus is born a second round of contractions separate the placenta from the uterine wall and expel it through the vagina. The expelled placenta is the afterbirth.

For further discussion of the placenta within the context of the structures and functions of reproduction and sexuality, please see the overview section “The Reproductive System.”


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