Final answer:
The ductus arteriosus closes after birth due to increased systemic pressure and becomes the ligamentum arteriosum.
Step-by-step explanation:
The ductus arteriosus closes after birth due to increased systemic pressure and becomes the ligamentum arteriosum. During fetal development, the ductus arteriosus allows blood to bypass the lungs and flow directly into the aorta. However, after birth, when the newborn takes the first breath and the lungs expand, the smooth muscles in the wall of the ductus arteriosus constrict, sealing off the passage. Over time, the muscular and endothelial components of the ductus arteriosus degenerate, leaving only the connective tissue component of the ligamentum arteriosum.