How the Body Works: Circulatory Changes at Birth

During pregnancy oxygen reaches the fetus from the placenta via the umbilical vein. The fetal lungs are collapsed and inactive. Most of the oxygenated blood passes through the liver, but some flows through the ductus venosus into the general circulation. In the heart, most of the blood short-circuits the right ventricle via a hole, the foramen ovale, between the right atrium and the lift atrium. Of the blood which does reach the right ventricle, about two-thirds passes from the pulmonary artery through a special vessel called the ductus arteriosus into the aorta, and the remainder passes through the lungs. At birth the placental circulation is cut off and the carbon dioxide concentration in the baby's blood stimulates respiratory nerve cells in the brain stem to induce breathing and lung expansion. The foramen ovale, the ductus venosus and the ductus arteriosus close immediately after birth and the circulatory system becomes an independent functioning unit.