Chorionic villi adaptations for efficient diffusion
Chorionic villi are finger-like projections of the placenta that maximise exchange of gases, nutrients and waste between mother and foetus by increasing the surface area for diffusion.
Remember
As pregnancy advances, the cytotrophoblast (the inner layer of separate cells) reduces. That leaves a thin barrier dominated by the syncytiotrophoblast (the outer sheet of fused cells), which shortens the diffusion distance while the blood of mother and foetus stays separate.
Memory aid
thin, big and fresh
A thin barrier, a big surface of villi, and fresh blood flowing past: the three adaptations for fast diffusion.
The chorionic villi are finger-like projections of the foetal side of the placenta, and their purpose is to maximise exchange between mother and foetus. They are arranged in groups called villous trees (so called because they look like small trees), which gives a very large surface area for diffusion.
Each villus has an outer layer of syncytiotrophoblast (a sheet of fused trophoblast cells, the cells that form the placenta), a layer of cytotrophoblast cells beneath it, and a core of connective tissue where the foetal blood vessels run.
Maternal blood (the mother's blood) from the spiral arteries (the maternal arteries that supply the placenta) bathes the outside of the villi. These arteries are high flow vessels, so the supply is rich. Foetal blood stays inside the villi. The membrane of the villi separates the two bloods and prevents them from mixing, so gases, nutrients and waste cross it, with gases such as oxygen and carbon dioxide moving by diffusion.
The placenta is built for fast diffusion in three ways: a large surface area from the villous trees, a steep concentration gradient kept up by rich blood flow, and a membrane that is as thin as possible. At the start of pregnancy the membrane is very thick. As pregnancy progresses the cytotrophoblast layer reduces, so the barrier becomes much thinner and sits close to the foetal capillaries. Of the three adaptations, the thin membrane is the one that shortens the diffusion distance.
Key facts
- Shape
- villi are finger-like projections that increase the surface area of a membrane to facilitate diffusion.
- Layers
- an outer syncytiotrophoblast, a cytotrophoblast layer beneath it, and a connective tissue core holding the foetal vessels.
- Blood flow
- maternal blood from the spiral arteries bathes the outside of the villi, while foetal blood stays inside them.
- Over pregnancy
- the membrane is very thick at the start and begins to thin as the cytotrophoblast layer reduces.
- Separation
- the membrane of the villi separates maternal and foetal blood and prevents them from mixing.
Exam traps
- A thicker villous membrane would lengthen the diffusion distance. The membrane is thick early in pregnancy, but the change as pregnancy advances is thinning of the barrier.
- Maternal and foetal blood are kept apart by the villous membrane, and exchange happens across it, so mixing of the two would be abnormal rather than an adaptation.
- Slower maternal blood flow past the villi would weaken the concentration gradient that drives diffusion, so it does not help exchange. The placenta keeps the gradient steep with rich maternal blood flow.
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