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•  The placenta facilitates the exchange of materials between the mother and fetus

•  Estrogen and progesterone are secreted by the placenta once it has formed

The placenta functions as the life support system for the foetus, serving two key functions:

  • It facilitates the exchange of materials between the mother and foetus
  • It secretes hormones to maintain the pregnancy after the corpus luteum has degenerated

Structure of the Placenta

  • The placenta is a disc-shaped structure that nourishes the developing foetus
  • It is formed from the development of the trophoblast upon implantation and eventually invades the uterine wall
  • Maternal blood pools via open ended arterioles into intervillous spaces within the placenta called lacunae
  • Chorionic villi extend into these pools of blood and mediate the exchange of materials between the foetus and the mother
  • Exchanged material is transported from the villi to the foetus via an umbilical cord, which connects the foetus to the placenta
  • Upon birth, the placenta is expelled from the uterus with the infant – it is then separated from the infant by severing the umbilical cord (the point of separation becomes the belly button)

Overview of the Structure of the Placenta


Material Exchange

  • The chorionic villi extend into the intervillous space (lacuna) and exchange materials between the mother and foetus
  • Chorionic villi are lined by microvilli to increase the available surface area for material exchange
  • Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from blood in the lacunae
  • Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries
  • Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels

Exchange of Materials between Foetal and Maternal Blood

placenta exchange

Hormonal Role

  • The placenta takes over the hormonal role of the ovaries (at ~12 weeks) and begins producing estrogen and progesterone
  • Estrogen stimulates the growth of uterine muscles (myometrium) and the development of the mammary glands
  • Progesterone maintains the endometrium, as well as reducing uterine contractions and potential maternal immune responses
  • Both estrogen and progesterone levels drop near the time of birth

The Role of the Placenta in the Hormonal Control of Pregnancy

placenta hormones