The main substance that are excreted from the fetus is:-
A. Bilirubin
B. Carbon dioxide
C. Uric acid
D. Tissues
Correct Answer: B. Carbon dioxide
Explanation
Question:
The main substance that is excreted from the fetus is — ?
✅ Correct Answer: Carbon dioxide
🔍 Explanation:
The fetus depends entirely on the placenta for removal of waste products because its lungs, kidneys, and liver are not fully functional before birth.
Why carbon dioxide?
- Carbon dioxide (CO₂) is the primary waste product produced by fetal metabolism.
- It is generated continuously from cellular respiration in fetal tissues.
- CO₂ diffuses easily from fetal blood into maternal blood across the placental membrane due to a concentration gradient.
- The mother’s lungs then eliminate this CO₂ through respiration.
What about other substances?
- Urea, uric acid, and bilirubin are also waste products, but:
- They are produced in smaller amounts compared to CO₂.
- Their excretion is secondary to carbon dioxide during intrauterine life.
🧠 Key Point to Remember:
Carbon dioxide is the main waste product excreted from the fetus via the placenta.
Other Options Details
This set of options is usually asked in the context of placental exchange between mother and fetus—specifically, what can and cannot pass across the placenta.
Let’s describe each option clearly 👇
A. Bilirubin
- Bilirubin is a waste product formed from the breakdown of red blood cells.
- Unconjugated bilirubin from the fetus does cross the placenta into the maternal circulation.
- The mother’s liver metabolizes and excretes it.
- This is why the fetus does not develop jaundice in utero.
✅ Crosses the placenta
B. Carbon dioxide
- Carbon dioxide is a gaseous waste product of fetal metabolism.
- It diffuses easily across the placenta from fetal blood to maternal blood.
- The mother then eliminates it through her lungs.
✅ Crosses the placenta
C. Uric acid
- Uric acid is a nitrogenous waste product formed from purine metabolism.
- The fetal kidneys are immature, so uric acid is mainly removed via the placenta.
- It passes from fetal to maternal circulation and is excreted by the mother.
✅ Crosses the placenta
D. Tissues
- Whole tissues or cells cannot pass through the placental barrier.
- The placenta allows selective transfer of gases, nutrients, and wastes, but not solid tissues.
- This separation protects the fetus from direct mixing with maternal structures.
❌ Does NOT cross the placenta
✅ Summary
- Bilirubin – crosses
- Carbon dioxide – crosses
- Uric acid – crosses
- Tissues – do NOT cross
👉 Correct answer (does not cross placenta): D. Tissues
I hope that you liked this article.
Thanks!! 🙏 😊
Writer: Vandita Singh, Lucknow (GS India Nursing Group