The Placenta (DP IB Biology)

The Placenta & the Exchange of Materials

• Mammals can be split into different categories on the basis of their mechanism for foetal nourishment

  • Placental mammals, such as humans

  • Monotremes, or egg laying mammals, such as a platypus

  • Marsupials, such as kangaroos, whose offspring develop in a pouch

• Placental mammals rely on the complex system of blood vessels specially designed to maximise exchange of substances between mother and foetus without any direct connection between them

• The placenta is also responsible for production of key pregnancy hormones oestrogen and progesterone

• The foetus is connected to the placenta via the umbilical cord and is contained within the amniotic sac filled with amniotic fluid which protects the foetus

Structure of the placenta

• The placenta is an organ primarily made up of a complex arrangement of blood vessels arranged into placental, or chorionic, villi with maternal blood flow distributed around the villi

  • Throughout the course of the pregnancy, the number of villi increases to meet the demands of the growing foetus

• Maternal blood and foetal blood never mix directly, but flow either side of a layer of cells that make up the placental barrier; there is a very short distance between the maternal and foetal blood to allow exchange of nutrients and gases

  • The mother's blood flows out of the mother's blood vessels and forms pools in the spaces surrounding the placental villi; these spaces are known as the inter-villous spaces

• The placental membrane, or barrier, provides a selectively permeable barrier which restricts the exchange of substances between mother and baby

• Substances that move across the barrier from mother to foetus include

  • Oxygen

  • Antibodies

    • Antibodies cross the placenta using a mechanism called endocytosis

  • Water

  • Glucose

  • Unwanted or harmful substances may also cross the placental barrier, including alcohol, drugs or small pathogens such as viruses

    • Bacterial pathogens are too large to cross the barrier

• Substances that move across the barrier from foetus to mother include

  • Carbon dioxide

  • Water

  • Urea

• The placenta is connected to the growing foetus by the umbilical cord

The placenta diagram

The placenta brings the maternal and foetal blood systems close but without directly mixing