fetal circulation
Fetal circulation is the special way blood flows before birth, when the lungs are collapsed and filled with fluid and cannot do the job of adding oxygen. Instead, the placenta serves as the baby's lungs, and the heart uses a set of clever temporary detours to send blood toward the body and away from the dormant lungs.
Three shunts make this possible. The ductus venosus lets oxygen-rich blood from the placenta skip the liver; the foramen ovale lets blood cross from the right atrium to the left, bypassing the lungs; and the ductus arteriosus shunts blood from the pulmonary artery to the aorta, around the lungs again. Because lung pressure is high in the womb, these right-to-left detours are the normal, healthy pattern.
At birth the first breaths inflate the lungs, pulmonary pressure drops, and pressure on the left side of the heart rises. These shifts close the temporary channels over the following hours to weeks. When one of them fails to close, the leftover fetal pathway becomes a congenital defect, such as a patent ductus arteriosus or a patent foramen ovale.
Knowing that fetal circulation relies on the ductus arteriosus, clinicians use medication to keep it open in a newborn whose heart defect depends on that channel for survival.
Fetal shunts explain why some defects depend on a ductus that should close.
Understanding fetal circulation explains why some duct-dependent defects only declare themselves after birth: as the ductus arteriosus naturally closes, the newborn suddenly loses the channel its circulation was relying on.