Fetal circulation is entirely different from adult circulation because the fetus lives in a fluid environment and does not use its lungs to breathe. Oxygen, nutrients, and waste elimination all depend completely on the placenta. Because the lungs are filled with fluid and the blood vessels inside them are tightly squeezed, very little blood flows through the pulmonary circuit. Instead, the fetus relies on three special natural channels—called shunts—to bypass the lungs and liver, routing blood directly to the organs that need it most, especially the brain and the heart.
Fresh, oxygen-rich blood leaves the placenta through a single umbilical vein. This bright red blood travels up toward the fetal abdomen, carrying high concentrations of oxygen and vital nutrients. As it reaches the undersurface of the liver, about half of this blood enters the liver tissue itself, while the other half bypasses the liver completely through a dedicated vascular channel. This clever routing ensures that the brain and upper body receive the highest available oxygen concentration.
The ductus venosus is a specialized vascular shunt connecting the umbilical vein directly to the inferior vena cava. Its primary job is to bypass the hepatic circulation. By dumping oxygen-rich placental blood straight into the large venous return leading to the heart, it prevents the liver from consuming too much of that precious oxygen. When the baby takes its first breath and the umbilical cord is clamped, this vessel stops carrying blood and eventually turns into a fibrous cord inside the abdomen.
The foramen ovale is a flap-like opening situated right between the right atrium and the left atrium of the fetal heart. Blood arriving from the inferior vena cava—carrying high oxygen from the ductus venosus—is directed straight toward this opening by a special fold of tissue called the septum secundum. Consequently, this oxygenated blood shoots right across into the left side of the heart, bypassing the sleeping lungs entirely and pumping out to the head and heart muscle.
The ductus arteriosus is a muscular blood vessel joining the main pulmonary artery to the descending aorta. Blood that does manage to enter the right ventricle and get pumped into the pulmonary trunk mostly escapes through this channel, avoiding the high-resistance lung tissue and entering the lower body instead. This protects the delicate lung blood vessels from high pressure while ensuring that the lower body receives a steady supply of circulating blood.
When a newborn takes its first independent breaths, the lungs expand, filling with air and causing pulmonary blood vessels to relax. This massive drop in lung resistance increases blood flow back to the left side of the heart, raising left atrial pressure and physically slamming the foramen ovale shut. Meanwhile, a sudden rise in blood oxygen levels and the loss of placental prostaglandins cause the ductus arteriosus and ductus venosus to constrict and permanently seal shut within the first few days of life.