fetal circulation, the pathway of blood circulation in the fetus. Oxygenated blood from the placenta travels through the umbilical vein to the heart. The blood enters the right atrium at a pressure sufficient to direct most of the flow across the atrium and through the foramen ovale into the left atrium; thus oxygenated blood is available for circulation through the left ventricle to the head and upper extremities. The blood returning from the head and arms enters the right atrium via the superior vena cava. It flows through the atrium at a relatively low pressure. Passing the tricuspid valve, it falls into the right ventricle, from which most of it is pumped through the pulmonary artery and the ductus arteriosus into the descending aorta for circulation to the lower parts of the body. A small amount of blood in the pulmonary artery is not shunted through the ductus and is carried to the lungs. The blood is returned to the placenta through the umbilical arteries.
