Pregnancy and the Placenta

Explained

The placenta, and why two blood supplies stay separate

The single most important fact about the placenta is one that surprises people: the mother's blood and the fetus's blood never mix. They flow close together, separated by a thin barrier, and substances cross between them by diffusion. Almost every mark in this topic depends on that.

What crosses, and in which direction

Substances move by diffusion, from where there is more of them to where there is less.

• To the fetus: oxygen, glucose, amino acids, water and mineral ions, along with antibodies which give some immunity.
• To the mother: carbon dioxide and urea, the wastes the fetus cannot deal with itself.

The direction always follows the concentration gradient. Oxygen is more concentrated in the mother's blood, so it diffuses towards the fetus. Carbon dioxide is more concentrated in the fetus's blood, so it travels the other way.

Why separation matters

Keeping the two supplies apart protects the fetus from the mother's blood pressure, which would damage its delicate vessels, and stops the mother's immune system attacking cells it would recognise as foreign. It also means blood groups do not need to match.

The barrier is thin and the surface area is large, which is exactly what any efficient exchange surface needs. If a question asks how the placenta is adapted, say both, and add the rich blood supply that maintains the concentration gradient.

The umbilical cord

The umbilical cord connects the fetus to the placenta and carries the fetus's own blood to and from it. The cord is the transport link. The placenta is the exchange surface. They are not the same structure and questions do distinguish them.

Amniotic fluid

The fetus develops inside the amniotic sac, surrounded by amniotic fluid. The fluid cushions the fetus against physical knocks, supports its weight, and keeps its temperature steady.

Give more than one of those if the question is worth more than one mark. Protection from bumps is the answer most candidates reach for, and temperature and support are often the marks left uncollected.

Harmful substances

Because the barrier is thin, some things cross that should not. Nicotine, alcohol and some viruses and drugs can pass from the mother to the fetus.

This is a consequence of the same feature that makes the placenta work. A barrier thin enough to let oxygen diffuse quickly cannot be selective about everything else, and saying so shows you have understood the mechanism rather than memorised a warning.