What is the primary role of the ductus arteriosus in fetal circulation?

The primary role of the ductus arteriosus in fetal circulation is to connect the pulmonary artery to the aorta. This structure is critical during fetal life, as it allows blood to circumvent the non-functioning lungs of the fetus. In the fetus, the lungs are not yet used for gas exchange; instead, oxygenated blood is received from the placenta through the umbilical vein and delivered to the fetus's heart.

By connecting the pulmonary artery to the aorta, the ductus arteriosus permits a significant portion of the right ventricle's output to flow directly into the aorta rather than into the lungs, thereby ensuring that oxygen-rich blood is delivered efficiently to the body's tissues. After birth, when the lungs become functional and the lungs expand with air, the ductus arteriosus typically closes, transforming into a ligament and redirecting blood flow in the normal adult pattern through the pulmonary and systemic circulations.