What structure allows fetal blood to bypass the liver?

The structure that allows fetal blood to bypass the liver is the ductus venosus. In fetal circulation, the liver is not fully functional in the same way it is after birth, so blood needs to be shunted in a way that preserves its oxygenation and nourishes the fetus efficiently.

The ductus venosus is a fetal blood vessel that connects the umbilical vein, which carries oxygen-rich blood from the placenta, directly to the inferior vena cava. This bypasses the hepatic circulation, allowing most of the oxygen-rich blood to flow directly into the heart and then to the rest of the body. After birth, the ductus venosus closes off and becomes a fibrous remnant known as the ligamentum venosum.

In contrast, the foramen ovale and ductus arteriosus serve to route blood differently, facilitating shunting between the heart’s chambers and between the pulmonary artery and aorta, respectively. The umbilical vein, though it carries oxygenated blood, does not itself function to bypass the liver directly; rather, it is a vessel supplying oxygenated blood to the fetus before it is directed through other shunts.