When a developing puppy is growing inside his mother, he does not need to breathe. The oxygen he requires to mature in utero comes not from his lungs but from the umbilical cord. Thus, after coursing through the body, blood does not travel through the fetus’s lungs to pick up oxygen for another pump out to the body by the heart. The lungs remain deflated, and blood bypasses them through a kind of shunt called the ductus arteriosus.
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