Placenta is the key: Once the placental separates from the uterus and the fetus breathes outside, then the flap inside the fetus' heart called the ductus arteriosis closes. The fetal heart now becomes a four chamber heart separating unozygenated venous blood now shunted to the lungs for ozygenation prior to pumping it out the other side of the heart.
Answered 2/18/2016
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