(Note: It will be helpful to review normal heart function before reading this.)

Every baby is born with a ductus arteriosus. This is an open passageway between the two major blood vessels (the pulmonary artery and the aorta). The pulmonary artery carries venous (blue) blood from the heart's right side to the lungs. Then it picks up a fresh supply of oxygen. The aorta carries the oxygen-rich (red) blood from the heart's left side to the rest of the body.

Normally the passageway (ductus arteriosus) between these two arteries closes within a few hours after birth. If it doesn't, some blood that should have gone through the aorta and on to nourish the body goes back to the lungs. Failure of the ductus to close is quite common in premature infants but fairly rare in full-term babies.

If the ductus arteriosus is large, a child may tire quickly, grow slowly, catch pneumonia easily and breathe rapidly. In some children, symptoms may not occur until after the first weeks or months of life. If the ductus arteriosus is small, the child seems well.

If surgery is needed, the surgeon doesn't have to open the heart to correct this abnormal circulation. The ductus arteriosus can be closed by tying it. If there's no other defect, this restores the circulation to normal.

Copied from the American Heart Association Site