Prior to birth, the placenta essentially functions as the baby’s lungs, liver, GI tract, kidneys, and more. But in the first few moments after birth, many significant changes occur.
During development, blood bypassed the lungs, taking a shortcut through the heart where it must flow in order to be pumped to the rest of the body. At birth, that shortcut closes so that blood flows to the lungs, and the bypass between the heart’s right and left atria closes.
The baby spent her first nine months in fluid, so the blood and lymph systems remove fluid from the lungs and replace it with oxygen, allowing her to take her first breath.