The ductus venosus is a special blood vessel in the fetus, responsible for 80% of the blood flow from the umbilical vein into the inferior vena cava, which supplies the right atrium of the heart. It helps supply the fetal brain with oxygenated blood. The umbilical vein, which leads from the placenta to the fetus, is different from the majority of veins in that it carries oxygenated blood, rather than deoxygenated blood. The placenta is an organ attached to the uterine wall that provides nutrients to the fetus through the mother's blood supply.
About half of the oxygenated blood from the umbilical vein enters the ductus venosus, while the other half enters the liver before proceeding to the right atrium. The heart of the fetus also features the foramen ovale, an opening between the right and left atria. Whereas in adults, blood entering the right atrium must flow through the right ventricle into the lungs before entering the left atrium, the fetal heart allows blood to bypass the lungs. From the left atrium, the blood enters the left ventricle, whence it is pumped into the aorta to supply the rest of the body, as in adults.
The ductus venosus is still open at the time of birth, but normally closes during the first week of life. In premature babies, it often takes longer to close. The remnant of the ductus venosus is the fibrous ligamentum venosum, loctated on the bottom of the liver and attached to the hepatic portal vein, which brings blood from the abdominal cavity and the spleen into the liver.
A ductus venosus that fails to close is called a portosystemic shunt (PSS) or liver shunt. PSS causes some of the blood from the intestines to enter the general circulatory system rather than going to the liver to be purified of toxins. Hence, toxins such as ammonia and uric acid become present in abnormally high levels in the bloodstream. PSS results in symptoms including vomiting, failure to gain weight, and impaired brain function, which may be manifested through seizures, drooling, and depression. Surgical repair is the best treatment for PSS, though the condition may also be treated with antibiotics if surgery is not an option.