There are two syllabus points in bold (only tested in paper 2) that refer to embryonic and foetal development. The first asks you to understand the role of the placenta in supplying the developing foetus with nutrients and oxygen and the second concerns the role of amniotic fluid in protecting the developing embryo.
The placenta is in many ways a remarkable organ. It contains a mixture of maternal cells from the uterine lining and embryonic cells, but these cells from two genetically different individuals are capable of sticking together to form the placenta. The placenta is only present in the uterus once an embryo has successfully implanted a week or so after fertilisation has happened in the Fallopian Tubes. The placenta is linked to the foetus via the umbilical cord, a structure that contains an umbilical artery and vein carrying foetal blood to and from the placenta.
There is a key idea here that is very important. There is no mixing of maternal and foetal blood in the placenta. This would be disastrous for both mother and baby for a whole variety of reasons. The maternal blood is at a much higher pressure than the foetal blood and if the foetus were connected to the maternal circulatory system directly, its blood vessels would burst. The foetus and mother can have different blood groups of course and you may now that some blood groups are incompatible and can trigger clotting. So it is essential that there is never any mixing of blood. But what happens in the placenta is that mother’s blood empties into spaces in the placenta and babies’ blood is carried by the umbilical artery into capillaries that are found in finger-like projections called villi. This means there is a large surface area and a thin barrier between the two bloods and so exchange of materials by diffusion is possible.
The main function of the placenta then is to allow the exchange of materials between the foetal and maternal circulations. The developing foetus inside its mother’s uterus has no direct access to oxygen nor food molecules of course yet both are needed to allow healthy development. The foetus also needs a mechanism to get rid of the waste molecule, carbon dioxide that is being produced in all its cells all the time. Until the kidneys mature fully the foetus also has to get rid of urea, another excretory molecule that could build up to toxic concentrations unless removed from the growing foetus.
A few interesting points:
You will see that antibodies are small enough to cross the placenta. This gives the baby a passive immunity that can protect it for a short time from any pathogens it encounters.
Drugs such as alcohol and nicotine can cross the placenta. This is why it is so vital that pregnant mothers do not smoke and drink to ensure that the foetus’ development is not affected by these drugs.