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

• During the gestation period, the foetus develops and grows by gaining all the glucose, amino acids, fatty acids, water, oxygen, mineral ions and vitamins it needs from the mother’s blood
• In the placenta, the mother’s blood comes into very close proximity to the blood of the foetus but never mixes with it
• The foetus’s blood connects to and from the placenta by the umbilical cord
• The mother’s blood also absorbs the waste products from the foetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the foetus’s blood so that they do not build up to dangerous levels
• The movement of most molecules across the placenta occurs by diffusion due to difference in concentration gradients
  • For some substances that need to be moved, active transport may be involved

• The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion
• The placenta acts as a barrier to prevent toxins and pathogens getting into the foetus’s blood
• Not all toxin molecules or pathogenic organisms (such as viruses e.g. rubella) are stopped from passing through the placenta (this usually depends on the size of the molecule)
• This is why pregnant women are advised not to smoke during pregnancy as molecules like nicotine can pass across the placenta

The placenta

• After the baby has been born, the umbilical cord is cut
  • This does not hurt as there are no nerves in it, just two blood vessels

• It is tied off to prevent bleeding and shrivels up and falls off after a few days leaving the belly button behind
• The placenta detaches from the uterus wall shortly after birth and is pushed out due to contractions in the muscular wall of the uterus
  • This is known as the afterbirth