IGCSE Biology / Reproduction in humans
The placenta and the developing fetus
Edexcel International GCSE Biology (4BI1), chapter 9: Reproduction in humans, specification point 3.11, 3.12. Free lesson.
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The placenta lets useful substances pass between the mother and fetus while keeping their blood supplies separate. Its structure makes exchange effective, and the amniotic fluid supports and protects the developing fetus.
How the placenta is adapted for exchange
The placenta is embedded in the wall of the uterus. It has tiny projections called villi that extend into spaces filled with the mother's blood. The villi provide a large surface area, and their walls are thin, so substances can diffuse quickly over a short distance. The umbilical cord connects the fetus to the placenta. The umbilical artery carries blood from the fetus to the placenta. The umbilical vein carries blood back to the fetus. The amniotic sac contains amniotic fluid, which supports the fetus and protects it from physical damage.
What crosses the placenta, and in which direction
Diffusion is the movement of a substance from a higher concentration to a lower concentration. The fetus uses oxygen, glucose and amino acids, so their concentrations are lower in fetal blood than in the mother's blood. Waste substances build up in fetal blood, so they move in the opposite direction. Oxygen, glucose and amino acids pass from the mother to the fetus, down their concentration gradients. Antibodies also pass from the mother to the fetus. They give the newborn some immunity during its first few months. Antibodies are large proteins and are carried across rather than…
Why the blood supplies stay separate
The mother's and fetus's blood supplies come very close together at the placenta, but they never join. Substances can still cross because the placenta has a large surface area and a short diffusion distance. The mother's blood is at much higher pressure. Keeping the supplies separate prevents this pressure from damaging delicate fetal blood vessels. The mother and fetus may have different blood groups. If their blood mixed, it could cause clotting. Many pathogens in the mother's blood cannot cross to the fetus, so separation protects the fetus from some infections.
What the placenta does not block
The placenta is not a complete filter. Some harmful substances can cross it and reach the fetus. Alcohol, nicotine and some viruses can cross the placenta, which is why they can harm the fetus.