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The most important difference is that the fetus does not use lungs for oxygenation, and instead uses the placenta to get in oxygenated blood from its mother, and put out the deoxygenated blood
In simple tersm
The umbilical vein brings oxygenated and nutrient rich blood from the placenta into the right atrium. From there, it travels through the Foramen Ovale, which is a one way valve between the right atrium and the left atrium. This essentially short circuits the lungs. Return blood back to the placenta is via the umbilical artery.
At birth, when the infant starts breathing, the pressure differential between the left atrium and the right atrium increases, causing the Foramen Ovale to close, removing the short circuit. Now, blood has the full path from right atrium, to right ventricle, to lungs, to left atrium, to left ventricle, to the body, and repeating.
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