Placenta

In the syllabus and the exam

The placenta is taught as the organ that develops during pregnancy to support the growing fetus, and exam questions focus heavily on what it exchanges and how. Candidates must be able to state that oxygen and dissolved nutrients such as glucose pass from mother to fetus, while carbon dioxide and other waste products such as urea pass from fetus to mother, and that this exchange happens by diffusion across a large, thin surface area formed by villi projecting into the maternal blood spaces. A frequently tested and easily misunderstood point is that maternal and fetal blood do not normally mix directly; substances cross between the two separate blood supplies through the placental membrane rather than through any shared bloodstream, which is precisely what allows the mother's and baby's blood types to differ safely. Structured questions sometimes also ask why the placenta is described as an adaptation for internal development, expecting an answer that links its large surface area and thin membrane to efficient diffusion over the length of pregnancy. A common error is stating that the placenta filters blood the way a kidney does, or that maternal and fetal blood combine directly within it. Candidates may also be asked why a developing fetus is described as being protected from many harmful substances yet still vulnerable to others that cross the placenta, such as alcohol or certain drugs, which is a useful way to check whether the exchange mechanism has been properly understood rather than memorised.

Example

Describing exchange during pregnancy, a candidate writes: 'The placenta allows oxygen and nutrients to diffuse from the mother's blood into the fetus's blood, while carbon dioxide and urea diffuse the other way, without the two blood supplies ever actually mixing.'

Related terms

FertilisationMenstrual cycleDiffusion

Source:SRC-DSKP-EN