After birth, the infant is kept alive by the mixing of oxygenated blood from the left atrium with unoxygenated blood in the right atrium. This is possible because of an opening called the Foramen Ovale (1) in the atrial septum (the muscle wall that divides the two atria). The Foramen Ovale provides one way for blood to mix, but it is often not enough. The Ductus Arteriosus (2), which connects the aorta and pulmonary artery in the fetal and newborn heart, also provides a way for deoxygenated (blue blood) to mix with the oxygenated (red blood).

The Foramen Ovale and Ductus Arteriosus are features of the fetal heart that usually close soon after birth. Though these may allow enough mixing of blood to keep an infant alive initially, Transposition of the Great Arteries would still prove fatal if measures are not taken to increase the amount of this mixing of the two circulations.

(Opposite)
1) Patent (open) Foramen Ovale
2) Patent (open) Ductus Arteriosus