Why the Umbilical Cord Has Three Vessels
A fetus does not breathe air in the uterus, so its lungs are not the source of the oxygen carried in its blood. Oxygen and nutrients pass from the pregnant person’s blood into fetal blood across the placenta, an organ attached to the uterus. The cord links the fetus to that exchange point: one vessel carries oxygen-rich blood from the placenta toward the fetus, while two arteries carry blood back toward the placenta.
A One-Way Route Through a Shared Bundle
The vessels are separate tubes, not open channels that mix their contents. They travel together within the cord, cushioned by a slippery, jellylike connective tissue called Wharton’s jelly. That padding helps keep the vessels from being squeezed shut when the cord bends or twists. The vessels’ spiral shape adds to the cord’s flexibility, allowing it to move as the fetus moves without behaving like a rigid pipe.
The names can seem counterintuitive: arteries usually carry blood away from the heart, and veins carry it toward the heart. In fetal circulation, those names still describe direction relative to the heart—not how much oxygen the blood contains. The two umbilical arteries carry blood away from the fetal heart toward the placenta; the single umbilical vein returns oxygen-rich blood to the fetal heart.
What the Arrangement Accomplishes
This layout creates a continuous delivery-and-return circuit. The placenta loads fetal blood with oxygen and nutrients and receives waste products, including carbon dioxide, for transfer back into the pregnant person’s circulation. The cord’s protective tissue and bundled vessels help preserve that connection through ordinary movement and pressure. In a cross-section, the three vessels are visible as three distinct openings inside one soft cord—three separate routes serving one shared lifeline.
