Final answer:
The ductus arteriosus is kept patent, or open, by prostaglandins. Prostaglandins help keep the smooth muscles in the wall of the ductus arteriosus relaxed, allowing blood to flow from the pulmonary artery to the aorta, bypassing the lungs.
Step-by-step explanation:
The ductus arteriosus is kept patent, or open, by prostaglandins. Prostaglandins are chemical substances that help regulate various physiological processes in the body, including blood vessel constriction and relaxation. In the case of the ductus arteriosus, prostaglandins help keep the smooth muscles in the wall of the ductus arteriosus relaxed, allowing blood to flow from the pulmonary artery to the aorta, bypassing the lungs.