Final answer:
Oxygen is supplied to the fetus by the placenta via the umbilical vein within the umbilical cord. The fetal hemoglobin in the blood stream has a higher affinity for oxygen, facilitating its transport to the fetus, bypassing the need for the respiratory system, which is still immature.
Step-by-step explanation:
Before birth, oxygen is supplied to the fetus by the placenta. The umbilical cord, which includes one umbilical vein and two umbilical arteries, acts as the conduit for this crucial exchange. The umbilical vein carries oxygen-rich blood from the placenta directly to the fetus, supplying it with the necessary oxygen and nutrients, while the two umbilical arteries return oxygen-depleted blood from the fetus to the placenta.
During gestation, the fetal lungs are not involved in oxygenation, since there is no air to breathe within the uterus. Instead, the placenta fulfills the role of gas exchange, providing oxygenated blood through the umbilical vein. The fetal hemoglobin within this blood has a higher affinity for oxygen compared to adult hemoglobin, ensuring efficient transfer of oxygen to the fetal circulatory system.
The correct answer to the question is therefore: A. The placenta.