Fetal circulation

Fetal circulation routes blood between the fetus and placenta while the lungs are not yet used for gas exchange. Three shunts redirect blood around the liver and lungs before birth.

Ductus venosus: A fetal vessel that carries oxygenated blood from the umbilical vein past much of the liver into the inferior vena cava. It is the first major bypass in the route from placenta toward the fetal heart.
Fetal heart: The developing heart that pumps blood through fetal and placental circulations before birth. Its chambers receive and distribute blood through the fetal shunts.
Neonatal circulation: The pattern of blood flow established after birth as a newborn's lungs begin gas exchange. It replaces fetal flow when lung expansion changes pressure across the heart and vessels.
Persistent fetal circulation: A condition in which newborn pulmonary vascular resistance remains high, sustaining fetal-like right-to-left blood flow. It demonstrates the danger of fetal pressure patterns persisting after birth.
Foramen ovale: An opening between the right and left atria of the fetal heart that directs blood away from the lungs. It sends much of the better-oxygenated incoming blood directly to the left heart.
Cardiac shunt: An abnormal or normal passage that allows blood to flow between parts of the circulatory system. The fetal openings are physiological shunts that redirect flow around inactive organs.
Foramen ovale closure: The functional sealing of the fetal opening between the atria after birth. Rising left-atrial pressure presses its flap shut after pulmonary blood flow increases.
Hypoplastic left heart syndrome: A congenital condition in which structures on the left side of the heart are severely underdeveloped. Before birth, shunts can partly sustain circulation despite this major structural defect.
Ductus arteriosus: A fetal vessel connecting the pulmonary artery to the aorta, allowing blood to bypass the lungs. It carries blood from the right ventricle into the systemic circulation.
Oxygen saturation: The fraction of hemoglobin binding sites occupied by oxygen in blood. Fetal blood has distinct oxygen levels, shaping which streams are preferentially directed to the heart and brain.

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