The placenta allows for fetal blood and maternal blood exchange of oxygen, carbon dioxide, nutrients and waste materials. Fetal blood circulation is opposite of adult circulation. Oxygenated blood travels through veins, while deoxygenated blood travels through the arteries. The fetal deoxygenated blood travels to the placenta through
two umbilical arteries. The blood circulates through the capillaries of the  chorionic villi within the placenta where the exchange process takes place, but there is no mixing of fetal and maternal blood. Most of the vessels of the fetus carry a mixture of oxygenated and deoxygenated blood. Blood rich in oxygen and nutrients leaves the placenta to return to the fetus through a  single umbilical vein. This is the only fetal vessel that carries fully oxygenated blood.

  This oxygenated blood (saturation about 80%) that is coming from the placenta to the organs of the fetus via the  umbilical vein slowly loses its oxygen concentration by mixing with desaturated blood as it passes certain fetal vessals ("bypasses or shunts"). After mixing, the oxygen saturation is approximately 67%.
   1) As it travels past the   ductus venosus , some of the enriched blood goes to the liver, but most of it bypasses the liver and empties directly into the inferior vena cava where it mixes with deoxygenated blood returning from the lower extremities, pelvis and kidneys.

   2) As the blood from the inferior vena cava enters the right atrium, a large proportion of it is shunted directly into the left atrium through an opening called the   foramen ovale.   A small valve called the septum primum, located at the atrial septum, prevents blood from moving in the reverse direction. The blood in the left atrium mixes with a small amount of blood returning from the lungs, and then enters the left ventricle and ascending aorta. The myocardium and brain are thereby supplied with the most oxygenated blood since the coronary and carotid arteries are the first to branch from the ascending aorta, and before there is too much mixing with desaturated blood from other areas of the fetal heart.

   3) As in the normal adult heart, some blood still flows from the right atrium to the right ventricle and then through the pulmonary artery to the lungs. Since the lungs of the fetus are not functioning to oxygenate, only a small amount of blood goes there for growth and development. As the desaturated blood coming from the superior vena cava flows through the pulmonary artery from the right ventricle, there is a high resistance in the pulmonary vessels that forces most of this blood to flow through the  ductus arteriosus and into the descending aorta. Here it mixes with the blood from the proximal aorta to supply blood to the lower body (lower extremities, pelvis kidneys).

      After supplying these areas, the internal iliac arteries move the blood into two umbilical arteries  that transport the desaturated blood back to the placenta. The oxygen saturation of the returning deoxygenated blood in the umbilical arteries is only about 58%.