Role of the Placenta (Edexcel IGCSE Biology: Double Science)

• After fertilisation in the oviduct the zygote divides to form an embryo, and travels to the uterus
• Upon reaching the uterus the embryo implants in the uterine lining, where it continues to develop
• A structure called the placenta forms at the implantation site
  • In the placenta the mother’s blood comes into very close proximity to the blood of the fetus
  • The umbilical cord connects the embryo’s blood supply to the placenta
• The role of the placenta is to enable exchange of substances between the mother's blood and that of the fetus
  • Substances that travel from the mother's blood to the fetus include:
    • oxygen
    • nutrients, e.g. glucose, amino acids and mineral ions
  • Substances that travel from the fetus' blood to the mother include:
    • carbon dioxide
    • urea
• The placenta is an efficient exchange surface because it has:
  • a large surface area
  • a thin wall for efficient diffusion
• The placenta also acts as a barrier to toxins and pathogens, although not all are stopped from passing through, for example:
  • nicotine and alcohol can pass across the placenta
  • virus particles can pass across the barrier

The placenta allows exchange of substances between the fetus and the mother