Chapter 9- Human Reproduction and Birth

9.1 The human reproductive system

Gametes are sex cells (either egg or sperm). The sperm has a tail, and its nucleus is in the head. The egg also has a nucleus, and has large food stores
Eggs and sperm have only 23 chromosomes, unlike other cells which have 46
Gametes are produced through meiosis, where cells with 46 chromosomes divide to produce 2 23 chromosome cells
Fertilisation occurs where the nuclei of sperm and egg fuse. The zygote (or fertilised egg) therefore has 46 chromosomes, or 23 pairs
The zygote starts dividing into two cells with identical chromosomes, through a process called mitosis. This division forms the embryo

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The area around the cervix is covered in thick mucus, which is made more watery at ovulation in the middle of the menstrual cycle. It makes it easier for sperm to enter the uterus.
Fertilisation takes part in the oviduct, so sperm have to swim through the uterus to reach the egg
The large egg is moved al0ng by cilia on the oviduct surface
Normally, only one egg is released, and only one sperm penetrates it. But if two eggs are released and fertilised with separate sperm fraternal twins will develop
Once the zygote has started dividing, it will move down the oviduct and implant into the uterus lining. This allows the body systems of the baby to develop, as well as the placenta and the amnion.
Occasionally, at implantation, the cell splits into 2 and creates identical twins
The amnion is a sac containing amniotic fluids, which acts as a shock absorber to prevent damage to the foetus
The placenta acts as an exchange surface between mother and foetus, enabling materials to pass by diffusion without mixing bloodstreams
Oxygen and nutrients (glucose, protein) pass to the foetus, and waste (urea, CO2) pass to mother

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The umbilical cord contains blood vessels to transport foetal blood between foetus and placenta
The placenta acts as a barrier to prevent large molecules or blood vessels reaching the foetus. However, tiny viruses (eg. HIV and rubella) can pass and damage the foetus (rubella causes miscarriage, stillbirth and birth defects)
Midwives advise expectant mothers on having a healthy pregnancy (don't smoke, eat well, exercise) and attend the birth
Labour occurs in 3 stages- the muscular uterus walls contract to dilate the cervix, the uterus walls squeeze to force the baby through the birth canal and the uterus contracts after birth to push out the placenta
The first "milk" contains antibodies to protect from infection, and will provide the nutrients (protein, fat and sugar) after a few days

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