By the end of this lesson
- Describe placental exchange.
- Explain placental adaptations.
- Relate umbilical vessels to fetal transport.
BIG QUESTION
How can a fetus obtain oxygen and nutrients without sharing its mother’s blood directly?
A thin, large-area exchange interface keeps the two circulations close but normally separate.
Maternal and fetal blood pass close together
The placentaAn organ that lets oxygen and dissolved nutrients diffuse from the mother's blood to the fetus, and carbon dioxide and urea diffuse back, without the two bloods mixing. contains fetal capillaries in chorionic villi bathed by maternal blood spaces. A thin placental barrier separates the circulations.
The blood does not normally mix directly, which helps protect each circulation and allows different pressures.
Useful substances enter fetal blood
Oxygen and glucose move from maternal blood towards fetal blood; amino acids, mineral ions and antibodies can also cross by appropriate transport processes.
A large surface area, thin exchange barrier, rich blood supply and maintained concentration gradients increase transfer.
Wastes leave through the umbilical circulation
Carbon dioxide and urea pass from fetal blood towards maternal blood for removal by the mother’s lungs and kidneys.
The umbilical vein carries oxygenated, nutrient-rich blood to the fetus; two umbilical arteries carry deoxygenated blood and wastes towards the placenta. Some harmful substances and pathogens can cross, so the placenta is not a perfect filter.
Placental transport
Direction is stated relative to the fetus.
| Substance/blood vessel | Direction | Importance |
|---|---|---|
| Oxygen | Mother → fetus. | Aerobic respiration. |
| Glucose/amino acids | Mother → fetus. | Respiration, growth and biosynthesis. |
| Carbon dioxide | Fetus → mother. | Removal by maternal lungs. |
| Urea | Fetus → mother. | Removal by maternal kidneys. |
| Umbilical vein | Placenta → fetus. | Carries relatively oxygenated blood. |
| Umbilical arteries | Fetus → placenta. | Carry relatively deoxygenated, waste-rich blood. |
Language that earns precision
- placenta
- Temporary exchange organ linking maternal supply to the fetal circulation.
- chorionic villus
- Branching fetal placental projection containing capillaries.
- umbilical cord
- Cord containing vessels connecting fetus and placenta.
- placental barrier
- Thin layers separating maternal and fetal blood.
Common misconceptions
Claim: Maternal and fetal blood normally mix in the placenta.
Correction: They flow close together but remain in separate circulations.
Claim: The umbilical artery carries oxygenated blood because arteries always do.
Correction: Arteries carry blood away from a heart; fetal umbilical arteries carry blood towards the placenta.
Claim: The placenta blocks every harmful substance.
Correction: Alcohol, nicotine, some drugs and pathogens can cross.
Match the answer to the command word
Link large area, thin barrier and blood flow to exchange rate.
Define arteries and veins by direction relative to the fetal heart.
Recall, apply, then justify
Explain three adaptations of the placenta for exchange.
Hint
Area, distance and gradients.
Suggested marking points
- Many villi provide a large surface area.
- A thin barrier gives a short diffusion distance.
- Good maternal and fetal blood flow maintains concentration gradients.
Describe transport of oxygen, glucose, carbon dioxide and urea across the placenta.
Hint
State direction for each substance.
Suggested marking points
- Oxygen moves from maternal to fetal blood.
- Glucose moves from maternal to fetal blood.
- Carbon dioxide moves from fetal to maternal blood.
- Urea moves from fetal to maternal blood.
Explain how reduced maternal blood flow through the placenta could affect fetal growth.
Planning hint
Gradients, respiration and biosynthesis.
What a strong design includes
- Less maternal blood reaches exchange surfaces.
- Oxygen and nutrient concentration gradients may be reduced.
- Less oxygen transfers to the fetus.
- Aerobic respiration and energy release may decrease.
- Less glucose and amino acids are available for growth.
- Waste removal may also be impaired.
- Sustained reduced exchange can restrict fetal growth.