In the placenta, how are oxygen, nutrients and waste products exchanged between mother and fetus?

Answer: A. Oxygen and nutrients diffuse from maternal to fetal blood, and wastes diffuse from fetal to maternal blood, without the two circulations normally mixing

Placental exchange

The placenta forms from the embryo and the uterus and contains both maternal and fetal blood vessels, but the two circulations do not normally mix - exchange occurs by diffusion across the placental membrane.

Direction of exchange

• Oxygen and nutrients diffuse from maternal -> fetal blood.
• Waste products (e.g. carbon dioxide, urea) diffuse from fetal -> maternal blood.

Cord vessels

The umbilical vein carries oxygenated blood to the fetus; the two umbilical arteries carry deoxygenated, waste-laden blood back to the placenta.

Why the others are wrong

The maternal and fetal circulations do not normally mix; the fetus does not pump blood into the mother; oxygen passes from mother to fetus (not the reverse); and wastes pass from fetus to mother (not the reverse).