Cambridge O Level Biology · Syllabus 5090 · Development of Organisms and Continuity of Life
Placenta
What is Placenta?
The organ that forms in the uterus during pregnancy and acts as the exchange surface between the maternal and fetal circulations. It has a large surface area and a thin barrier, so oxygen and dissolved nutrients diffuse towards fetal blood while carbon dioxide and urea diffuse towards maternal blood; the two circulations remain separate.
This definition is part of the Development of Organisms and Continuity of Life chapter in Cambridge O Level Biology.
Common mistakes with Placenta
- “Maternal and fetal blood mix in the placenta.” Why wrong The two circulations are brought close together but remain separate; only dissolved substances cross the barrier. Say instead Substances diffuse across the placenta; blood does not cross it. The two circulations stay separate.
- “The placenta blocks every harmful substance.” Why wrong It is not a perfect barrier, and the syllabus expects you to know that. Say instead Some toxins and pathogens can cross the placenta, including some viruses, alcohol, nicotine, carbon monoxide and certain drugs.
- “Amniotic fluid supplies the fetus with nutrients.” Why wrong That is the placenta's job. The fluid's roles are protection and movement. Say instead Amniotic fluid cushions the fetus against mechanical shock and allows it to move; nutrients cross the placenta.
Questions students ask about Placenta
Do the mother's and the baby's blood mix?
No. In the placenta the two circulations are brought very close together but remain separate, divided by a thin barrier with a large surface area. Dissolved substances diffuse across that barrier — oxygen and nutrients towards fetal blood, carbon dioxide and urea towards maternal blood — but the blood itself does not cross. The umbilical cord carries only fetal blood.
Does the placenta protect the fetus from everything harmful?
No. The placenta is not a perfect barrier. Some substances that can harm the fetus are able to cross it, including some viruses, alcohol, nicotine, carbon monoxide and certain drugs. This is a point the syllabus expects you to state explicitly, and “the placenta filters out everything harmful” is a recorded error.