ductus venosus

During fetal circulation, oxygenated blood returns from the placenta via the umbilical vein. Most of this blood bypasses the hepatic circulation through the ductus venosus, directly entering the inferior vena cava (IVC), where it mixes with deoxygenated blood returning from the lower body. This blood then preferentially streams through the foramen ovale into the left atrium (aided by the eustachian valve and crista dividens), then to the left ventricle, and out the aorta to supply the coronary arteries, brain, and upper body with the most oxygenated blood available. A smaller portion of umbilical venous blood perfuses the liver via the portal sinus. After birth, with cessation of umbilical blood flow and increased hepatic portal pressure, the ductus venosus functionally closes within days and structurally becomes the ligamentum venosum, a fibrous remnant on the liver's surface. Clinically, if the ductus venosus fails to close (patent ductus venosus), it can lead to portosystemic shunting, hepatic encephalopathy-like symptoms, or be associated with other congenital anomalies. It's also relevant in fetal ultrasound assessment (ductus venosus Doppler) for evaluating fetal well-being, particularly in the context of intrauterine growth restriction—abnormal or reversed flow in the ductus venosus a-wave suggests fetal cardiac compromise or acidemia.