What is a Placenta? Functions of the Placenta in Pregnancy

The placenta is the structure that connects the fetus to the uterine wall and is the conduit for nutrition and oxygen to reach the growing fetus. Essentially the placenta is the communication point between the mother’s blood circulation and that of the developing fetus.

Formation of the Human Placenta

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The ovum (egg cell from a woman’s ovary) is fertilized by sperm (from the male after intercourse) in the fallopian tubes of a woman’s body. After fertilization, it takes the fertilized ovum several days (approximately 3 to 5 days) to reach the uterine cavity. During this period it grows to a ball of cells known as a blastocyte. While the fertilized ovum is unattached to the uterine wall, it is sustained by nutritious secretions from the fallopian tube and endometrial lining of the uterus. Once in the uterine cavity, the fertilized ovum is ready to implant on the wall of the uterus.

Trophoblastic cells on the surface of the ovum secretes proteolytic enzymes that digest the endometrial cells lining the uterine wall creating a suitable surface for implantation. Trophoblastic cords then extend from the surface of the fertilized ovum to attach to the endometrial lining. Blood vessels grow into these trophblastic cords that further develop as projections into the endometrial lining known as placenta villi or chorionic villi. Eventually these blood vessels in the placenta villi lie close to the blood sinuses around the mother’s blood vessels. The blood circulation of the mother and fetus now have a contact area to exchange nutrients, gases and waste products.

Blood from the endometrial arteries (spiral arteries) circulate around these placenta villi and exchange of nutrients and gases occur through the intervillous space. Blood carried by this villi is returned to the fetus through the umbilical vein. In this way, the fetal and maternal blood never actually mix.

Functions of the Human Placenta

The placenta has multiple functions all of which develop and sustain the fetus. These functions include :

  • Nutrition. Nutrients in the mother’s blood stream can diffuse into the fetal circulation. These nutrients are derived from the food that the mother eats, digests and absorbs and nourishes the growing fetus.
  • Gas exchange. Oxygen from the mother’s blood stream can diffuse into the fetal circulation where it is used for energy production. Carbon dioxide from the fetus diffuses into the mother’s circulation to be carried away and expelled by the lungs and kidneys.
  • Immunity. Antibodies from the mother are delivered to the fetus through the placenta, and this protects the fetus and child for a period of time after birth until the child’s own immune system matures.
  • Endocrine. The placenta produces and releases several hormones that aid the fetal growth and influence the mother’s body to sustain the fetus, prepare for childbirth and nourish the baby after birth. These hormones include human chorionic gonadotropin (hCG), human chorionic somatomammotropin and the female hormones estrogen and progesterone.

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