Fetal blood from the placenta is about 80% oxygenated. However, mixture with unoxygenated blood at various points reduces the oxygen content. Which of the following fetal vessels contains blood with the highest oxygen content?

a. Abdominal aorta
b. Common carotid arteries
c. Ductus arteriosus
d. Pulmonary artery
e. Pulmonary vein

The answer is b. Blood from the placenta in the umbilical cord is about 80% oxygenated. Mixture with unoxygenated blood from the vitelline veins and the inferior vena cava reduces the oxygen content somewhat. However, this stream with relatively high oxygen content is directed by the valve of the inferior vena cava directly through the foramen ovale into the left atrium. This prevents admixture with oxygen-depleted blood entering the right atrium from the superior vena cava. Thus, the oxygen-saturated blood entering the left ventricle and pumped into the aortic arch, subclavian arteries, and common carotid arteries has the highest oxygen content. The oxygen-depleted blood from the superior vena cava is directed into the right ventricle and then to the pulmonary trunk. Although a small portion of this flow passes through the lungs (where any residual oxygen is extracted by the tissue of the nonrespiring lung), most is shunted into the thoracic aorta via the ductus arteriosus and thereby lowers the oxygen content of that vessel. This occurs distal to the origins of the carotid arteries and ensures that the rapidly developing brain has the best oxygen supply. The pattern of blood supply in the fetus and the changes that occur at birth are shown in the following figures.