ligamentum venosum

**Fetal function (ductus venosus):** Oxygenated blood (~80% O2 saturation) returns from the placenta via the single umbilical vein. About half to a third passes through the ductus venosus, bypassing hepatic sinusoids, into the IVC and then the right atrium. From there, it is preferentially streamed across the foramen ovale into the left atrium, supplying the brain and heart. A sphincter mechanism at the ductus venosus regulates flow.

**Postnatal closure:** After birth, umbilical cord clamping and the fall in umbilical venous flow cause the ductus venosus to close functionally within minutes to days. Fibrous obliteration follows over the next weeks, producing the ligamentum venosum. Closure is mediated by smooth muscle constriction (a sphincter response, with fall in prostaglandins and increased vasoconstriction).

**Related fetal remnants (high-yield list):** - Umbilical vein → ligamentum teres hepatis (round ligament of the liver), in the free edge of the falciform ligament - Ductus venosus → ligamentum venosum - Ductus arteriosus → ligamentum arteriosum (patency is maintained by PGE2 and closed by indomethacin or ibuprofen) - Umbilical arteries → medial umbilical ligaments (distal portions obliterate; proximal portions persist as superior vesical arteries) - Urachus → median umbilical ligament - Allantois → urachus - Notochord → nucleus pulposus - Foramen ovale → fossa ovalis

**Anatomy:** The ligamentum venosum lies in the fissure for the ligamentum venosum on the liver's visceral surface. This fissure separates the caudate lobe (posterior) from the left lateral segment (anterior). It runs from the left portal vein to the IVC (near the hepatic vein confluence). It is continuous with the lesser omentum (the gastrohepatic ligament attaches along this fissure). Together, the fissures for the ligamentum teres and venosum form the left sagittal fissure, which forms the "left limb of the H" on the liver's visceral surface. The right limb of the H is formed by the gallbladder fossa and the IVC groove.

**Clinical significance:** - **Patent ductus venosus:** A persistent shunt allows portal blood to bypass the liver. This can present with neonatal cholestasis, hypergalactosemia, hyperammonemia, hepatic encephalopathy, or hepatopulmonary syndrome. Related to congenital portosystemic shunts. Also seen in dogs (a common veterinary condition). - **Surgical landmark:** It is used in liver surgery to identify the caudate lobe and to define segmental anatomy (Couinaud). The Arantius ligament is an eponym for it. - **Portal hypertension:** The umbilical vein (ligamentum teres) may recanalize to form paraumbilical collaterals (caput medusae). The ligamentum venosum is not itself a major collateral pathway. - **Fetal hemodynamics:** Absence or abnormal flow in the ductus venosus on Doppler ultrasound (reversed a-wave) suggests fetal cardiac compromise or growth restriction. - **Prematurity:** Delayed closure of the ductus venosus is more common in preterm infants, though it usually closes spontaneously.

**Board pearl:** The ligamentum venosum is a fetal remnant of the ductus venosus. Do not confuse it with the ligamentum arteriosum (the remnant of the ductus arteriosus, which connects the pulmonary trunk to the aortic arch and is related to the left recurrent laryngeal nerve) or the ligamentum teres hepatis (the remnant of the umbilical vein).