Fetal vessel which bypasses the liver

While still in the uterus, the baby's lungs aren't being used. Circulating blood bypasses the lungs and liver by flowing in different pathways and through special openings called shunts. Oxygen and nutrients from the mother's blood are transferred across the placenta to the fetus through the umbilical cord.

There it moves through a shunt called the ductus venosus. This allows some of the blood to go to the liver. But most of this highly oxygenated blood flows to a large vessel called the inferior vena cava and then into the right atrium of the heart.

When oxygenated blood from the mother enters the right side of the heart, it flows into the upper chamber the right atrium. Blood then passes to the aorta. This is the large artery coming from the heart.

From the aorta, blood is sent to the heart muscle itself and to the brain and arms. After circulating there, the blood returns to the right atrium of the heart through the superior vena cava. Very little of this less oxygenated blood mixes with the oxygenated blood. Instead of going back through the foramen ovale, it goes into the right ventricle. This less oxygenated blood is pumped from the right ventricle into the pulmonary artery.

A small amount of the blood continues on to the lungs. Most of this blood is shunted through the ductus arteriosus to the descending aorta. This blood then enters the umbilical arteries and flows into the placenta. In the placenta, carbon dioxide and waste products are released into the mother's circulatory system. Oxygen and nutrients from the mother's blood are released into the fetus's blood.

The oxygen rich blood then returns to the fetus via the third vessel in the umbilical cord umbilical vein. The oxygen rich blood that enters the fetus passes through the fetal liver and enters the right side of the heart.

The oxygen rich blood goes through one of the two extra connections in the fetal heart that will close after the baby is born. The hole between the top two heart chambers right and left atrium is called a patent foramen ovale PFO. This hole allows the oxygen rich blood to go from the right atrium to left atrium and then to the left ventricle and out the aorta. Most of the circulation to the lower body is supplied by blood passing through the ductus arteriosus.

This blood then enters the umbilical arteries and flows into the placenta. In the placenta, carbon dioxide and waste products are released into the mother's circulatory system, and oxygen and nutrients from the mother's blood are released into the fetus' blood.

At birth, the umbilical cord is clamped and the baby no longer receives oxygen and nutrients from the mother. With the first breaths of life, the lungs begin to expand. As the lungs expand, the alveoli in the lungs are cleared of fluid.

An increase in the baby's blood pressure and a significant reduction in the pulmonary pressures reduces the need for the ductus arteriosus to shunt blood. These changes promote the closure of the shunt.