Fetal circulation

Oxygenated blood leaves the placenta in a single umbilical vein, the vessel carrying the most oxygen-rich blood in the fetus. Two umbilical arteries, branches of the internal iliac arteries, return oxygen-poor blood to the placenta. This reverses the usual naming logic: in the fetus, the vein carries oxygenated blood and the arteries carry deoxygenated blood, because the names describe direction relative to the heart rather than oxygen content.

Three shunts route blood around organs that are not yet doing their adult work. The ductus venosus carries most umbilical venous blood past the liver directly into the inferior vena cava. The foramen ovale lets blood pass from the right atrium to the left atrium, bypassing the lungs. The ductus arteriosus connects the pulmonary artery to the descending aorta, so blood that does enter the right ventricle is diverted to the systemic circulation. Fetal pulmonary vascular resistance is high and the lungs are fluid-filled, so only a small fraction of cardiac output perfuses them.

The first breath reorganizes the whole system. Lung expansion drops pulmonary vascular resistance, pulmonary blood flow rises, and increased left atrial pressure presses the septum primum against the septum secundum to close the foramen ovale, which becomes the fossa ovalis. Rising arterial oxygen and falling prostaglandin levels close the ductus arteriosus, leaving the ligamentum arteriosum. The ductus venosus becomes the ligamentum venosum, the umbilical vein becomes the ligamentum teres, and the umbilical arteries become the medial umbilical ligaments. Clinically, prostaglandin E1 keeps a ductus open when a newborn depends on it, while indomethacin promotes closure.

Fetal circulation is heavily tested. USMLE Step 1 asks for shunt anatomy, adult remnants, and the physiology of closure; the CCMA exam covers it within cardiovascular blood flow anatomy; and the NCLEX frames it as newborn transition and stabilization at delivery.