Answer:
Umbilical vein
Explanation:
Fetal circulation is dependent on the mother's circulation system since the fetal lungs are not adequate to carry out its function. Both oxygen and nutrients are supplied from the mother's blood to the fetus. Umbilical vein is the structure which carries the oxygenated blood from the mother to the fetus. Hence it has the highest concentration of oxygen. Once the blood starts to circulate inside the fetus it keeps becoming devoid of oxygen and nutrients. It is finally returned back to the mother again for oxygenation and supply of nutrients.
