The ductus arteriosus is the blood vessel in the foetus through which blood passes from the pulmonary artery to the aorta, bypassing the lungs (which do not function during intra-uterine life). The ductus normally ceases to function soon after birth and within a few weeks is reduced to a fibrous cord. Patent ductus arteriosus is a congenital defect in which this conversion does not happen. It can be surgically corrected.