Which vessel has the highest pCO2 during fetal circulation?

In fetal circulation, the umbilical artery carries deoxygenated blood from the fetus to the placenta. This blood has the highest partial pressure of carbon dioxide (pCO2) compared to other vessels in the fetal system. The reason for the elevated pCO2 in the umbilical artery is due to two main factors.

Firstly, the fetal metabolism operates in a hypoxic environment and produces carbon dioxide as a byproduct of aerobic metabolism. Consequently, the blood returning from the fetal tissues via the umbilical arteries carries a higher concentration of carbon dioxide that has accumulated due to cellular respiration.

Secondly, the blood in the umbilical arteries has not yet passed through the placenta, where it would exchange gases with maternal blood. In the placenta, carbon dioxide is released, and oxygen is taken up, resulting in lower pCO2 levels in the umbilical vein that returns oxygenated blood to the fetus.

Understanding the differences in gas composition between these vessels is crucial in fetal physiology, and the umbilical artery’s role in transporting carbon dioxide-rich blood underscores its significance in the fetus's waste disposal and gas exchange processes before reaching the placenta.