The circulatory system of a fetus undergoes many changes at the time of birth. For example, in a fetus, the left and right atria of the heart are connected (all

The circulatory system of a fetus undergoes many changes at the time of birth. For example, in a fetus, the left and right atria of the heart are connected (allowing blood to bypass the lungs, which are not yet in use). Upon birth, that "hole inside the heart" (the foramen ovale) normally closes, redirecting blood flow to the lungs for oxygenation. For another example, look up the ductus arteriosus.