Where in fetal circulation is the lowest level of oxygen concentration found?

The lowest level of oxygen concentration in fetal circulation is found in the umbilical arteries. During fetal development, oxygen-rich blood from the placenta travels to the fetus via the umbilical vein. This blood is then distributed throughout the fetal body, but as it traverses through the tissues and organs of the fetus, it becomes deoxygenated and returns to the placenta through the umbilical arteries.

These arteries are responsible for carrying deoxygenated blood away from the fetal body back to the placenta, where carbon dioxide and other waste products are exchanged for oxygen and nutrients. Therefore, the umbilical arteries show the lowest levels of oxygen concentration because they carry blood that has already delivered its oxygen to the fetal tissues and is ready for reoxygenation in the placenta.

In contrast, the umbilical veins carry oxygen-rich blood towards the fetus, and the fetal heart has a mix of oxygenated and deoxygenated blood, depending on its chambers, with the right side containing more deoxygenated blood and the left side gradually receiving more oxygenated blood. The placenta also contains maternal blood, which is rich in oxygen, thus having higher oxygen levels compared to the deoxygenated blood in the umbilical arteries.