During fetal circulation, blood returning to the right side of the heart from the ________ has the highest oxygen saturation.

• A. Pulmonary artery
• B. Umbilical artery
• C. Umbilical vein
• D. Ductus venosus

Answer: (C)

Oxygen-rich blood from the placenta reaches the fetus via the umbilical vein, which carries the highest oxygen saturation in fetal circulation. This blood travels toward the heart and, via the ductus venosus, is directed into the inferior vena cava to reach the right atrium. The lungs are not involved in fetal gas exchange, so the pulmonary artery carries largely deoxygenated blood, and the umbilical arteries return deoxygenated blood from the fetus to the placenta. The ductus venosus serves as a shunt to deliver the highly oxygenated blood into the heart and systemic circulation, but the vessel providing that high-oxygen blood to the right heart is the umbilical vein.