The ductus arteriosus is a normal structure in the fetal circulatory system that allows blood to bypass the fetal lungs, since the lungs are not needed for respiration while the fetus is in the womb. This channel normally closes after birth. If it remains open, it is known a patent ductus arteriosus, or PDA. Blood will flow from the aorta into the pulmonary artery. If the hole is large enough, it may lead to heart failure.