The ligamentum arteriosum acts as a shunt to bypass the respiratory system of a developing fetus, according to Wikipedia. AnatomyEXPERT explains that the ligamentum arteriosum is a remnant of the ductus arteriosus.
According to Wikipedia, the ligamentum arteriosum forms within the first three weeks after birth, and its development closes the shunt created by the ductus arteriosus. This allows blood to flow freely throughout the respiratory system capillaries, and it prevents the mixing of oxygenated blood with un-oxygenated blood within the respiratory system. Eventually, the ductus arteriosus becomes completely non-functional and transforms into a vestigial remnant of its former self.