A foetus needs oxygen, food and a way of getting rid of waste, all from a body that is not its own — and it has to get them without the two blood supplies ever mixing. The placenta solves that by bringing the two circulations extremely close together and letting diffusion do the rest.
📚 What you need to know
Mammals are grouped by how they nourish the foetus: placental mammals such as humans, monotremes (egg-laying, such as the platypus) and marsupials (developing in a pouch, such as kangaroos).
The placenta is made of blood vessels arranged into placental (chorionic) villi, with maternal blood flowing around them. The number of villi increases through pregnancy to meet the growing demand.
Maternal and foetal blood never mix directly. They flow either side of a layer of cells, the placental barrier, across a very short distance.
Maternal blood leaves the mother’s vessels and pools in the inter-villous spaces around the villi.
From mother to foetus: oxygen, glucose, water and antibodies (by endocytosis).
From foetus to mother: carbon dioxide, water and urea.
Harmful substances including alcohol, drugs and small pathogens such as viruses can also cross; bacteria are generally too large.
The placenta also produces the key pregnancy hormones oestrogen and progesterone, and is connected to the foetus by the umbilical cord.
Why a placenta at all
Placental mammals rely on a complex system of blood vessels specially designed to maximise the exchange of substances between mother and foetus without any direct connection between them. That is the design problem in one sentence: maximum exchange, zero mixing.
The pay-off is that the foetus can be retained inside the uterus to a much later stage of development than in mammals that do not develop a placenta. A monotreme lays an egg; a marsupial gives birth very early and continues development in a pouch. A placental mammal can keep going internally, because it has a supply line.
How the exchange surface is built
The placenta is an organ made up mostly of a complex arrangement of blood vessels arranged into villi, with maternal blood distributed around them. As pregnancy continues, the number of villi increases to meet the demands of the growing foetus — more villi means more surface area for exchange.
The mother’s blood flows out of her blood vessels and forms pools in the spaces surrounding the villi, known as the inter-villous spaces. Foetal blood runs inside the villi. Between the two sits the placental membrane, or barrier: a selectively permeable layer of cells that restricts which substances can cross.
This is the same set of adaptations you meet at every exchange surface: large surface area, short diffusion distance, and a maintained concentration gradient from the blood flow on both sides.
Why the blood must not mix. The foetus is genetically different from its mother, so mixing the two circulations could trigger an immune response. It would also expose the foetus to the mother’s blood pressure, which its vessels are not built for.
What crosses, and in which direction
Direction
Substances
Why
Mother to foetus
Oxygen, glucose, water, antibodies
The foetus cannot breathe or feed itself, and antibodies give it passive immunity
Foetus to mother
Carbon dioxide, water, urea
Waste products of respiration and of protein metabolism, removed by the mother’s lungs and kidneys
Unwanted crossings
Alcohol, drugs, small pathogens such as viruses
The barrier is selectively permeable but not perfect; bacteria are generally too large to cross
One detail worth noting: antibodies are large proteins, so they do not simply diffuse. They cross the placenta by endocytosis.
The rest of the system
The foetus is connected to the placenta by the umbilical cord and is contained within the amniotic sac, filled with amniotic fluid that protects it. And beyond exchange, the placenta has a second job you met on the previous page: it is responsible for the production of the key pregnancy hormones, oestrogen and progesterone, once hCG has handed over.
You are not expected to know the fine detail of placental structure beyond the large surface area of the villi. What you do need is which exchange processes happen there, and the fact that the placenta lets the foetus stay in the uterus far longer than in mammals without one.
Worked examples
WORKED EXAMPLE
Explain how the structure of the placenta allows efficient exchange between mother and foetus. [3]
Point 1 — surface area
The blood vessels are arranged into many villi, giving a large surface area for exchange, and the number of villi increases as the foetus grows.
Point 2 — diffusion distance
Maternal blood pools in the inter-villous spaces very close to the foetal blood, so there is only a thin layer of cells between them and a short diffusion distance.
Point 3 — concentration gradient
Blood flow on both sides continually replaces the substances being exchanged, maintaining steep concentration gradients.
3 marksThese are the same three points as for alveoli or villi in the small intestine.
WORKED EXAMPLE
Suggest why a pregnant woman is advised to avoid alcohol, while a bacterial infection of her blood is less likely to reach the foetus. [2]
Point 1 — alcohol
Alcohol molecules are small enough to cross the selectively permeable placental barrier, so they reach the foetal blood and can damage development.
Point 2 — bacteria
Bacterial cells are much larger and are generally unable to cross the barrier, although small pathogens such as viruses can.
2 marksSize is the deciding factor. Say so explicitly rather than just naming the substances.
💡 Exam tip
Say never mix directly whenever the two circulations come up. It is a standard marking point.
Give exchange answers the usual three adaptations: surface area, diffusion distance, concentration gradient.
Learn the two lists by direction. Getting urea travelling the wrong way loses the mark.
Antibodies cross by endocytosis, not simple diffusion.
Name the inter-villous spaces if describing where maternal blood sits.
Remember the placenta is an endocrine organ too, making oestrogen and progesterone.
⚠ Common mix-up
The blood supplies do not join. Nothing flows from one circulation into the other.
The umbilical cord is not the placenta. It connects the foetus to it.
The placenta does not filter out everything harmful. Alcohol, drugs and viruses cross.
Marsupials and monotremes are still mammals. They simply nourish offspring differently.
Foetal waste is not excreted by the foetus. It crosses to the mother, who removes it.
Amniotic fluid is not the same as maternal blood. It cushions the foetus, it does not feed it.
Up next: Hormone Replacement Therapy — what happens when the ovaries stop responding, and a cautionary tale about correlation and causation.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
