Hi Richard, good question! So inside the umbilical cord there are two arteries and one vein. Unusually the vein actually carries oxygenated blood. This is transported to the circulation of the fetus via the main vein that enters the heart (the vena cava). The two arteries of the umbilical cord carry deoxygenated blood to the placenta for gaseous exchange and re-oxygenation. I hope that helps!