In the fetus the major function of the ductus arteriosus is to divert flow away from the lungs toward the placenta, therby avoiding unnecessary circulation through the lungs and reducing the work load of the heart. Constriction of the ductus arteriosus is produced by several mechanisms that may interact with one another in producing normal postnatal closure of the ductus arteriosus. The major mechanism is the constrictor effect of an increased O2 environment. This may directly affect the ductus arteriosus muscle or may work through the release of an intermediary substance. Other vasoactive substance circulating in the blood after birth may also play some role in constriction of the ductus arteriosus. There are major species differences in the degree and type of response to the various stimuli and the exact mechanisms ultimately responsible for closure are not yet fully understood.