What is the purpose of the ductus venosus during fetal circulation?

The ductus venosus plays a crucial role in fetal circulation by allowing oxygen-rich blood from the placenta to bypass the liver. In a developing fetus, most of the oxygenated blood comes from the placenta rather than the lungs, which are not yet functional for gas exchange.

When blood enters the fetus through the umbilical vein, it travels toward the liver. The ductus venosus provides a direct pathway for some of this blood to enter the inferior vena cava, essentially enabling the majority of the oxygen-rich blood to bypass the liver where it would normally be processed. Instead, this blood is delivered directly to the heart and then pumped to the rest of the body, ensuring that the brain and other vital organs receive the oxygen and nutrients they need during development.

The presence of the ductus venosus is a critical adaptation for fetal physiology, facilitating efficient circulation during a time when the lungs are not yet in use. Once the baby is born and begins to breathe air, the ductus venosus typically closes, and blood flow through the liver increases as the lungs take over the role of oxygenating the blood.