Upon birth several critical changes not only occur in the arrangement of blood flow but in the anatomy of the heart itself. The changes are associated with the fact that a fetus (unborn baby) obtains its oxygen not from lungs but from mother's blood. The fetal blood passes a special organ called the placenta, in which tiny capillaries with fetal blood mix with blood vessels from the mother's circulation. The oxygen and nutrients diffuse from the mother's capillaries to fetal blood, thus replacing the function of the lungs and intestines. Therefore it is unnecessary for the unborn baby to pass all blood through the lung (albeit some supply to the lung tissue itself is needed for proper development) and its cardiovascular system somewhat resembles circulation of amphibians in which blood passes through the body and an oxygen-enriching organ in series. Specifically, a hole between the right and left atria called the foramen ovale shunts some of the blood arriving into right atrium directly into the left atrium, bypassing pulmonary circulation. Moreover, most of the blood which does get into the right ventricle does not go to the lung but flows from the pulmonary artery into the aorta through a special vessel called ductus arteriosis (see picture). Therefore most of the blood flow gets shunt from pulmonary circulation into systemic circulation and only a small amount of blood gets into the lung to feed lung tissue. When a baby is born the umbilical cord which carries blood to the placenta is cut and the baby starts to breath on its own. When the baby takes his or her first breath, it's lungs expand and resistance to the blood flow through the lung is decreased 4-5 fold. On the other hand since the umbilical cord is cut and no flow through the placenta is possible, systemic resistance increases. As a result of this resistance change, pressure in the right part of the heart decreases and in the left part increases. The blood then starts to flow from the left atrium to the right, and an unidirectional valve in the foramen ovale closes. The differences in pressure between the left and right ventricles also reverse the direction of the flow between the pulmonary artery and the aorta: blood begins to flow from the aorta to the pulmonary artery. The change in blood flow direction and the higher oxygen content of the blood leads to a gradual closure of the ductus arteriosis, which is complete several months after birth.