Role of the Placenta

When a baby is developing inside the uterus, it cannot eat, breathe or remove its own waste by itself. Instead, a special organ called the placenta does these jobs for it.

The placenta is an organ that grows in the wall of the uterus during pregnancy. It is connected to the developing fetus by the umbilical cord. The placenta sits where the mother's blood supply and the fetus's blood supply come very close together.

The placenta's one big job is exchange:

• it lets useful substances pass from the mother to the fetus, and
• it lets waste substances pass from the fetus to the mother.

Because the fetus is growing fast, it constantly needs raw materials (such as glucose, oxygen and amino acids) and constantly produces waste (such as carbon dioxide and urea) that must be removed. The placenta handles both directions at once.

Exam tip. The key word for the placenta is exchange. Describe what passes to the fetus AND what passes from the fetus — examiners want both directions.

Four useful substances pass from the mother's blood to the fetus's blood across the placenta:

• Glucose — used by the fetus's cells for respiration to release energy for growth.
• Oxygen — also needed for aerobic respiration (the fetus cannot breathe air yet).
• Amino acids — the building blocks of proteins, needed for growth and development of new tissues.
• Antibodies — pass from the mother to give the fetus some protection (immunity) against disease.

This is why a developing baby relies completely on its mother: everything it needs to grow and stay alive comes across the placenta.

These substances move from where there is more of them (the mother's blood) to where there is less of them (the fetus's blood) by diffusion — the placenta's structure (see below) makes this fast.

Exam tip. A common mark is lost by writing "food" instead of the named substances. Learn the list: glucose, oxygen, amino acids and antibodies pass mother → fetus.

As the fetus's cells respire and grow, they produce waste products. These cannot build up, so they pass from the fetus's blood to the mother's blood across the placenta. The mother's body then removes them.

Two waste substances pass from fetus to mother:

• Carbon dioxide — the waste gas made during respiration.
• Urea — the waste made when the fetus breaks down excess amino acids/proteins.

The mother's body deals with this waste for the fetus: she breathes out the carbon dioxide through her lungs, and removes the urea through her kidneys (in urine).

So the exchange works in both directions at the same time: useful substances in, waste out.

Exam tip. Keep the two directions straight. Useful substances (glucose, oxygen, amino acids, antibodies) go to the fetus; wastes (carbon dioxide and urea) go from the fetus. Swapping them around loses marks.

The placenta is very good at exchange because of two key features, plus one important safety point:

• Large surface area. A big area means lots of substances can be exchanged at once — exchange is fast.
• Thin barrier. The barrier between the mother's blood and the fetus's blood is thin, giving a short distance for substances to diffuse across.
• The two bloods do NOT mix. The mother's blood and the fetus's blood flow very close together but are kept separate by the thin barrier. Substances pass across the barrier by diffusion, but the actual blood never mixes.

The fetus is joined to the placenta by the umbilical cord, which carries blood between the fetus and the placenta in both directions.

Here is the layout of the placenta and umbilical cord:

Exam tip. The phrase examiners love is "the blood of the mother and the blood of the fetus do not mix". Saying the bloods mix is a classic error — they stay separate; only the substances cross.