Edexcel IGCSE Biology: Double Science 复习笔记 3.1.6 Pregnancy

Edexcel IGCSE Biology: Double Science 复习笔记 3.1.6 Pregnancy

Implantation & Development of the Foetus

 

  • After fertilisation in the oviduct, the zygote travels towards the uterus
  • This usually takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo
  • Once it reaches the uterus, the embryo embeds itself in the thick lining of the uterus (a process known as implantation) and continues to grow and develop
  • placenta forms where the embryo is attached
    • The placenta is an organ that ensures materials can be exchanged between the blood of the mother and the blood of the growing embryo
    • The umbilical cord joins the embryo's blood supply to the placenta for exchange of nutrients and removal of waste products

     

  • The embryo is surrounded by amniotic fluid (a fluid made from the mother’s blood plasma) that is held in by the amniotic membrane (also known as the amniotic sac)
    • The amniotic fluid protects the embryo during development by cushioning it from bumps to the mother’s abdomen

     

 

 

 

The-fetus-in-the-uterusThe foetus in the uterus

 

 

  • For the next 9 months, the embryo grows in the uterus
    • This is known as the gestation period

     

  • Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining
  • After this point, the organs are all in place, the placenta has fully formed and the embryo is now called a foetus (sometimes spelt fetus)
  • The remaining gestation time is used by the foetus to grow bigger in size

 

 

The Placenta

 

  • 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-placentaThe 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

     

 

Exam Tip

It is worth learning at least two specific substances that move in either direction across the placenta – this is a common exam question and non-specific answers such as ‘waste products’ and ‘nutrients’ will not get any marks!

转载自savemyexams

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