Which substance serves as a major buffer in the fetal circulatory system?

In the fetal circulatory system, plasma bicarbonate serves as a major buffer due to its role in maintaining acid-base balance. Bicarbonate ions (HCO3-) help to neutralize excess acids in the blood, which is crucial for sustaining the appropriate pH levels necessary for fetal development and function. The fetal environment can be sensitive to changes in acidity, particularly because the fetus relies on maternal blood for oxygen and nutrient exchange.

Bicarbonate ions are primarily produced in the body as a byproduct of cellular metabolism and can also be derived from carbonic acid (H2CO3), which dissociates to release hydrogen ions (H+) and bicarbonate ions. This reaction is particularly important during fetal life since it supports the exchange of gases and maintains homeostasis.

In comparison, while carbon dioxide is a byproduct of metabolism and plays a role in acid-base balance, it is not a direct buffer in the way that plasma bicarbonate is. Glucose, although vital for energy, does not contribute to buffering capacity, and oxygen primarily serves as a carrier of respiratory gases rather than having a buffering effect. Therefore, plasma bicarbonate is fundamental in managing the fetal blood's pH, making it the correct answer to the question.