Final answer:
The ductus arteriosus closes through smooth muscle contraction within a few hours after birth, redirecting blood flow away from the pulmonary trunk and into the aorta.
Step-by-step explanation:
The ductus arteriosus normally closes within a few hours after birth through smooth muscle contraction. This closure is important for redirecting blood flow away from the pulmonary trunk and into the aorta.
As the newborn takes the first breath, pressure within the lungs drops, causing the smooth muscles in the wall of the ductus arteriosus to constrict and seal off the passage. Eventually, the ductus arteriosus degenerates and becomes the ligamentum arteriosum.