as shunts for oxygenated blood. The function of these shunts is to bypass the lungs and maintain proper circulation to important fetal tissue. In the fetal stage, the lungs fill with fluid and collapse because the fetus is within the amniotic sac and the placenta is providing the oxygen it needs to grow. With the lung collapsed, pulmonary vascular resistance remains high during the fetal stage to prevent blood flow into the lungs. As oxygenated blood arrives at the right atrium, the eustachian valve helps direct the oxygenated blood into the foramen ovale, an opening between the right and left atrium. As the blood flows through the left atrium, it will move through the mitral valve into the left ventricle and will be pumped through the aorta into the body. The shunting of oxygenated blood from right to left atrium will supply blood high in oxygen and nutrient content to the upper extremities, which includes the critical organ: brain. Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the maternal circulation. Some of the blood entering the right atrium does not pass directly to the left atrium through the foramen ovale, but enters the right ventricle. This blood consists of oxygenated placental blood and deoxygenated blood returning from the fetal circulation. This blood is pumped into the pulmonary artery. At the pulmonary artery, it is met with high pulmonary vascular resistance as a result of collapsed lungs and pulmonary capillaries. In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus. Because the aorta has lower pressure than the pulmonary artery, most of the blood flows across the ductus arteriosus away from the lungs. Once the blood goes through the ductus arteriosus, it mixes with the blood from the aorta. This results in mixed blood oxygen saturation that supplies most of the structures of the lower half of the fetal body. After birth As the umbilical vessels are obliterated and the infant starts breathing at birth, the source of oxygen changes from the placenta to the lungs. This major trigger will facilitate the transformation from fetal to postnatal circulation in many ways. First, the ductus venosus was previously kept open by the blood flow from